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International Journal of
Environmental Research
and Public Health
Review
Vegetarian Diet: An Overview through the Perspective of
Quality of Life Domains
Shila Minari Hargreaves 1, *, António Raposo 2,* , Ariana Saraiva 3and Renata Puppin Zandonadi 1
Citation: Hargreaves, S.M.; Raposo,
A.; Saraiva, A.; Zandonadi, R.P.
Vegetarian Diet: An Overview
through the Perspective of Quality of
Life Domains. Int. J. Environ. Res.
Public Health 2021,18, 4067.
https://doi.org/10.3390/
ijerph18084067
Academic Editor: Jimmy Efird
Received: 25 February 2021
Accepted: 8 April 2021
Published: 12 April 2021
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Attribution (CC BY) license (https://
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4.0/).
1Department of Nutrition, Faculty of Health Sciences, University of Brasilia (UnB), Campus Darcy Ribeiro,
Asa Norte, Brasilia, DF 70910-900, Brazil; renatapz@unb.br
2CBIOS (Research Center for Biosciences and Health Technologies), Universidade Lusófona de
Humanidades e Tecnologias, Campo Grande 376, 1749-024 Lisboa, Portugal
3
Department of Animal Pathology and Production, Bromatology and Food Technology, Faculty of Veterinary,
Universidad de Las Palmas de Gran Canaria, Trasmontaña s/n, 35413 Arucas, Spain; ariana_23@outlook.pt
*Correspondence: shilaminari@gmail.com (S.M.H.); antonio.raposo@ulusofona.pt (A.R.);
Tel.: +55-61-981863262 (S.M.H.)
Abstract:
Vegetarianism has gained more visibility in recent years. Despite the well-described effects
of a vegetarian diet on health, its influence on the quality of life of the individuals who follow it
still needs to be properly investigated. Quality of life relates to a subjective perception of well-being
and functionality, and encompasses four main life domains: physical, psychological, social, and
environmental. The adoption of a vegetarian diet, despite being a dietary pattern, could potentially
influence and be influenced by all of these domains, either positively or negatively. This review
aims to present an overview of the background, conceptualization, features, and potential effects
of vegetarianism in all quality of life domains. The choice of adopting a vegetarian diet could
have positive outcomes, such as better physical health, positive feelings related to the adoption
of a morally correct attitude, an increased sense of belonging (to a vegetarian community), and
lower environmental impact. Other factors, however, could have a negative impact on the quality
of life of those choosing to abstain from meats or other animal products, especially when they go
beyond one’s control. These include the environment, the social/cultural group in which a person
is inserted, gender-based differences, economic aspects, and a limited access to a wide variety of
plant-based foods. It is important to understand all the effects of adopting a vegetarian diet—beyond
its nutritional aspects. Not only do studies in this area provide more consistent data, but they may
also contribute to mitigating all factors that might prevent individuals from adopting a vegetarian
diet, or that may have a negative impact on the quality of life of those who already follow it.
Keywords: quality of life; quality of life domains; vegetarian diet; vegetarianism
1. Introduction
Vegetarianism has its origins in 3200 BC, when ancient Egyptian civilizations started
adopting vegetarian diets based on the belief that abstaining from meat consumption
would facilitate reincarnation [
1
]. In India, another important cradle of vegetarianism,
this practice was also associated with the fact that Hindus see cows as sacred and uphold
nonviolence principles [
2
]. Later, Greek philosophers also adopted a vegetarian diet, with
Pythagoras being a leading figure among them—indeed, for many centuries, vegetarianism
was known as the “Pythagorean” diet [
3
,
4
]. In the Christian Era, vegetarianism lost its
strength, gaining some visibility again only in the late 18th and early 19th centuries, when
Darwin’s theory of evolution challenged the Church’s views that animals had no souls,
and that their only purpose on Earth was to serve human beings [1,5].
Throughout history, the expansion of vegetarianism has been associated with religions
that preach respect for all living beings and adopt nonviolence principles, such as Hinduism,
Jainism, Sikhism, Buddhism, the Hare Krishna movement, and the Seventh-day Adventist
Int. J. Environ. Res. Public Health 2021,18, 4067. https://doi.org/10.3390/ijerph18084067 https://www.mdpi.com/journal/ijerph
Int. J. Environ. Res. Public Health 2021,18, 4067 2 of 23
Church. In addition, in the 20th and 21st centuries, science has observed several health
benefits potentially associated with the reduction in meat consumption. Such benefits have
strengthened the practice of vegetarianism around the world, and attracted more and more
followers [4].
Currently, the worldwide prevalence of vegetarianism is not uniform. Asia is the
continent with the highest prevalence, with 19 percent of the population adopting this
practice [
6
]. India, the single country with the highest prevalence in the world (almost
40 percent of the population), contributes to the results of the Asian continent [
7
]. The
prevalence in Africa and the Middle East is about 16 percent; and in Central and South
America, 8 percent. The lowest prevalence of vegetarianism is found in North America
(about 6 percent of the population are vegetarians) and Europe, where vegetarianism is
adopted by only 5 percent of the population.
Vegetarianism encompasses different types of diets, classified according to how re-
strictive they are. Generally, vegetarianism is understood as the exclusion of meat from
one’s diet, but other less restrictive eating patterns can also be classified within the scope
of vegetarianism. These include, for example, flexitarians, who consume meat sporadi-
cally, or even once a week; pescatarians, who avoid all meat, except fish and seafood; and
ovolactovegetarians, who banish all types of meat but consume products of animal origin,
such as eggs and dairy products. A strict vegetarian diet, on the other hand, excludes all
foods of animal origin. Veganism is a broader concept, which involves the adoption of a
strict vegetarian diet, as well as the exclusion of other consumer items made from animal
products, or which rely on animal exploitation, such as cosmetics and clothing items [
8
,
9
].
For didactic purposes, a strict vegetarian diet is often referred to as a vegan diet.
Different motivations can lead to adopting a vegetarian diet [
10
–
13
]. Ethical concerns
are the main reasons, building on the idea that animal slaughter for human consumption is
morally inappropriate. Another important motivation is health and the potential beneficial
effects of vegetarianism. Religions that encourage abstaining from meat consumption
and concerns about the environmental impacts of meat production are also important
motivators for adopting vegetarianism [7,9].
According to the World Health Organization (WHO), quality of life (QoL) is a subjec-
tive concept that comprehends physical, psychological, social, environmental, and spiritual
aspects [
14
,
15
]. Changes in eating patterns can influence individuals’ QoL, both positively
and negatively [
16
]. A systematic review study assessed the nutritional quality of vegetar-
ian diets, and found—based on data from 12 surveys—higher nutritional quality levels
among vegetarians than omnivores [
17
]. According to the Academy of Nutrition and
Dietetics [
18
], vegetarian diets are nutritionally adequate for all stages of life, as long as
they are well planned. However, some precautions need to be taken to minimize the risk of
nutritional deficiencies.
In view of the recent growth in the number of individuals adopting a vegetarian diet, as
well as the wider interest in the topic in recent years, it is critical to understand the different
effects of vegetarianism on one’s QoL. Therefore, this review aims to present an overview
of the background, conceptualization, features and potential effects of vegetarianism
considering all QoL domains.
2. Historical Background of Vegetarianism
Over most of their 24 million years of evolution, humans’ anthropoid ancestors were
almost exclusively vegetarian, except for the occasional ingestion of insects and larvae.
Anatomically, both humans and their ancestors present significant features that distance
them from meat-eating animals, including, for example, wide flat teeth and more mobile
jaws, which facilitate the chewing of grains and seeds, as opposed to sharp teeth and
jaw movements on a vertical axis, which are characteristic of carnivores. In addition,
carnivorous animals have shorter intestines, which enable the rapid elimination of toxins,
unlike humans and other predominantly herbivorous animals, with long intestines that
allow longer digestion, fermentation and absorption processes [19,20].
Int. J. Environ. Res. Public Health 2021,18, 4067 3 of 23
However, possibly due to other reasons linked to survival, self-defense and territorial
protection, hominids began hunting other species, which led to the introduction of meat
in the diet of Homo erectus, considered the first hunters. Humans’ ability to survive on
different types of food was an essential factor in our evolution, which allowed our species,
Homo sapiens sapiens, to adapt to the most diverse conditions and spread throughout the
planet [19,20].
During the Paleolithic era, different food types were consumed, such as wild plants,
seafood, reptiles, birds, and mammals. After the emergence of agricultural practices (about
13,000 years ago), there is no evidence that humans were essentially vegetarian, and the
domestication of animals, including for consumption, became a routine activity by that
time. However, it is speculated that many farmers lived primarily as vegetarians due to
the wider availability of crops [19].
It is not known for certain when people started voluntarily abstaining from meat.
However, the first reports date from 3200 BC in ancient Egypt, when the practice was
motivated by religious factors, based on the belief that not consuming meat would facilitate
reincarnation [
1
]. Another important region that is part of the history of vegetarianism
is India, where the practice is also linked to religious issues. Hinduism has two basic
principles among its foundations: ahimsa, or the principle of nonviolence (which includes
violence against humans and other animals); and the recognition of the cow as a sacred
animal [2].
Some of the philosophers of the pre-Christian era also contributed to the spread of
vegetarianism. The practice was adopted at that time for health reasons as well as for
religious, ecological, and philosophical reasons. It was believed that the act of killing
another living being for food would have a brutal influence on one’s mind, negatively
affecting one’s body and soul [
3
]. The supporters of vegetarianism included big names
like Plato, Prophyry, Diogenes and Plutarch. The most prominent philosopher in this field
was Pythagoras, who lived in the 6th century BC. Due to his influence, vegetarianism was
known as the “Pythagorean” diet over many centuries, a name that lasted until the middle
of the 19th century in Europe and the Mediterranean region [4,19].
In Ancient Greece, it was believed that animals could think and communicate, and
that humans should be responsible for their lives. In addition, the Greeks believed that
eating meat would be harmful to one’s health and mind [
21
]. Vegetarianism was also
present during the Roman Empire, influenced by the Greek culture. However, with the rise
of Christianity, abstaining from animal consumption lost its importance. Famous Christian
thinkers such as Saint Thomas Aquinas and Saint Augustine sought to provide rational
justifications for the exploitation and consumption of animals, spreading the idea that,
unlike animals, human beings have souls and free will, and that animals are inferior beings,
placed on Earth at the service of humans [
3
–
5
]. Only a few monks still maintained the
practice, based on the belief that meat consumption would hinder their spiritual progress
in some way because it was linked to impulsive behaviors [5].
In the 15th century, vegetarianism was advocated by Leonardo da Vinci, who believed
that there was no distinction between the murder of humans and animals. However, it was
only after the spread of Darwin’s theory of evolution that vegetarianism gained strength
again in the late 18th century and early 19th century. Darwinism refuted the idea that
human beings are fundamentally different from other animals—therefore, there were no
plausible justifications for meat consumption [
5
]. At that time, the first vegetarian societies
also began to emerge, and some Christian groups began to preach in favor of abstaining
from meat based on the belief that animals should also be worthy of pity. It was only
then that the term “vegetarianism” came to be used. Despite the general belief that it
refers to “eating vegetables”, the term actually derives from “vegetus”, a Latin word that
means “active” or “vigorous” [
22
]. An important name in the history of vegetarianism, in
addition to the various vegetarian groups and societies that emerged in the 20th century,
was Mahatma Gandhi, who contributed to its dissemination [19].
Int. J. Environ. Res. Public Health 2021,18, 4067 4 of 23
Albert Einstein believed that humanity’s evolution toward a vegetarian diet would
be fundamental for the survival of life on Earth [
21
]. In Europe, the first International
Vegetarian Union was founded in 1908, after other vegetarian societies had already emerged
in several countries. From the 1960s onwards, a greater concern with food and health,
associated with evidence of the potential benefits of a vegetarian diet for disease prevention,
contributed to the spread of vegetarianism. Religious practices that preach respect for life
and adopt nonviolence principles, such as Hinduism, Jainism, Sikhism, Buddhism, the
Hare Krishna movement, and the Seventh-day Adventist Church, were also fundamental
to this growth. Therefore, the world has seen a significant rise and expansion of the practice
since the mid-20th century [4].
In recent years, vegetarianism has gained more visibility and a greater number of
followers. Rosenfeld [
23
] describes a great expansion in the scientific literature on the
psychological and social effects of choosing a vegetarian diet. Some topics started to
attract more attention, such as motivations; barriers to adopting such diets; differences
between vegetarians and vegans; morality; and gender differences. New research lines
have emerged to explore issues associated with personal identity and social and cultural
experiences [23].
Adherence to a vegetarian diet goes beyond food. Vegetarianism can be considered a
social identity, as it reflects the motivations, feelings, and attitudes of those who choose
to adopt it [24]. The main motivations for choosing a vegetarian diet are related to ethical
and health aspects. Animal welfare is the main motivator, followed by concerns with
major environmental impacts caused by the production and consumption of food of animal
origin. Regarding health, general well-being and weight maintenance are the factors
that most motivate the adoption of vegetarianism [
23
]. In addition, religious aspects can
lead individuals to adopt a vegetarian diet, and religions such as Hinduism, Adventism
and Spiritism preach abstaining from meat. Other less frequent factors, such as aversion
to the taste of meat, food intolerances and allergies, and the influence of other people
(family members, for example) can also be considered motivators for adopting a vegetarian
diet [4,7,9,21].
There are several types of vegetarian diets commonly described in the literature. The
most consensual classification consists of four different types, namely: (1) flexitarian or
semivegetarian diet, in which people consume meat sporadically (up to once a week) or
exclude red meat, but consume white meat; (2) pesco-vegetarian or pescatarian diet, which
excludes all meats, except fish and seafood; (3) ovolactovegetarian diet, which excludes all
types of meat, but allows products of animal origin, such as dairy products and eggs; and
(4) strict vegetarianism, which excludes all products of animal origin [8,25].
In addition to these categories, other diets can be considered subclassifications of
vegetarianism, namely: (1) raw vegan diet, which is mostly based on food in its most
natural (raw) state, with an emphasis on the choice of organic and self-grown products;
(2) frugal or frugivorous diet, which is similar to the raw vegan diet, but with 70–80 percent
of the diet being composed of fruits, with a small proportion of nuts, seeds and some
vegetables; and (3) macrobiotic diets, which encompass various degrees of restriction but
are primarily composed of whole grains, soybeans, algae and some vegetables [25,26].
3. Quality of Life
According to the WHO, QoL is a multifactorial concept that includes the following
domains: physical (physical state), psychological (affective and cognitive state), social
(interpersonal relationships and social roles in the lives of individuals) and environmental
(quality of the environment in which individuals live). Conceptual, pragmatic and empiri-
cal dimensions, as well as spiritual and religious aspects, can also contribute to people’s
QoL and their ability to perform certain activities, or “functionality”. Building on that, QoL
is defined as “individuals’ perception of their position in life in the context of the culture
and value systems in which they live and in relation to their goals, expectations, standards
and concerns” [14,15].
Int. J. Environ. Res. Public Health 2021,18, 4067 5 of 23
The terms “quality of life” and “well-being” are often used to indicate how well
an individual feels. There is, however, a problem of interpretation resulting from the
subjectivity of these concepts, which may acquire a broader or more specific connotation
depending on the context. QoL can be subdivided into: the quality of the environment
in which one lives, involving the physical structure of the environment and people’s
integration in the society in which they live; physical and mental health, encompassing
a wide range of individual capacities; usefulness, which involves the feeling of “being
useful”, contributing to the welfare of other people, society, and the environment; and the
appreciation of life, which is associated with tangible (wealth, for example) and intangible
(such as life satisfaction and happiness) aspects [27].
Although it is difficult to group all these qualities into a single concept, the best
general indicator of QoL would be how happy you feel and how long you live. The
concept of “well-being”, in turn, usually denotes QoL in a wider sense, as well as a positive
subjective assessment of life, or an appreciation of life. However, sometimes the concepts
of “well-being” and “quality of life” are used interchangeably [27].
The connection between vegetarianism and QoL may be analyzed through different
perspectives [
14
,
15
]. In the context of vegetarianism, each QoL domain proposed by
the WHO (physical, psychological, social, and environmental) may be influenced by the
adoption of a vegetarian diet. The opposite may also be said, that is, specific aspects
of each domain might influence one’s decision to adopt a vegetarian diet. Moreover,
these influences could be either positive or negative. The possible connections between
vegetarianism and QoL domains are illustrated in Figure 1.
Int. J. Environ. Res. Public Health 2021, 18, x FOR PEER REVIEW 5 of 26
QoL is defined as “individuals’ perception of their position in life in the context of the
culture and value systems in which they live and in relation to their goals, expectations,
standards and concerns” [14,15].
The terms “quality of life” and “well-being” are often used to indicate how well an
individual feels. There is, however, a problem of interpretation resulting from the subjec-
tivity of these concepts, which may acquire a broader or more specific connotation de-
pending on the context. QoL can be subdivided into: the quality of the environment in
which one lives, involving the physical structure of the environment and people’s inte-
gration in the society in which they live; physical and mental health, encompassing a wide
range of individual capacities; usefulness, which involves the feeling of “being useful”,
contributing to the welfare of other people, society, and the environment; and the appre-
ciation of life, which is associated with tangible (wealth, for example) and intangible (such
as life satisfaction and happiness) aspects [27].
Although it is difficult to group all these qualities into a single concept, the best gen-
eral indicator of QoL would be how happy you feel and how long you live. The concept
of “well-being”, in turn, usually denotes QoL in a wider sense, as well as a positive sub-
jective assessment of life, or an appreciation of life. However, sometimes the concepts of
“well-being” and “quality of life” are used interchangeably [27].
The connection between vegetarianism and QoL may be analyzed through different
perspectives [14,15]. In the context of vegetarianism, each QoL domain proposed by the
WHO (physical, psychological, social, and environmental) may be influenced by the
adoption of a vegetarian diet. The opposite may also be said, that is, specific aspects of
each domain might influence one’s decision to adopt a vegetarian diet. Moreover, these
influences could be either positive or negative. The possible connections between vege-
tarianism and QoL domains are illustrated in Figure 1.
Figure 1. Connections between aspects of vegetarianism and quality of life domains. The arrows
indicate the direction of the influence, that is, whether a given domain influences or is influenced
by certain aspects of vegetarianism. The plus (+) and minus (−) symbols indicate positive and neg-
ative influences, respectively. NCD: noncommunicable diseases; VD: vegetarian diet.
3.1. Physical Domain
The physical domain refers to aspects as pain, discomfort, energy, fatigue, sleep, and
rest. Aspects that positively contribute to a general feeling of physical well-being are
therefore relevant for understanding QoL. These include better general health, lower rates
of chronic and inflammatory diseases, and lifespan [28].
Figure 1.
Connections between aspects of vegetarianism and quality of life domains. The arrows
indicate the direction of the influence, that is, whether a given domain influences or is influenced by
certain aspects of vegetarianism. The plus (+) and minus (
−
) symbols indicate positive and negative
influences, respectively. NCD: noncommunicable diseases; VD: vegetarian diet.
3.1. Physical Domain
The physical domain refers to aspects as pain, discomfort, energy, fatigue, sleep, and
rest. Aspects that positively contribute to a general feeling of physical well-being are
therefore relevant for understanding QoL. These include better general health, lower rates
of chronic and inflammatory diseases, and lifespan [28].
Int. J. Environ. Res. Public Health 2021,18, 4067 6 of 23
3.1.1. Influence of Adopting a Vegetarian Diet on the Physical Domain
Positive Influence
Following a vegetarian diet may lead to better health outcomes and a lower risk of
noncommunicable diseases, which could positively influence the QoL physical domain
(Figure 1). A nutritionally adequate diet is essential to achieving and maintaining good
overall health. A systematic review published by Parker and Vadiveloo [
17
] compared the
quality of vegetarian and nonvegetarian diets based on diet quality indexes. That review
included 12 studies and showed that vegetarians have better diet quality results than
omnivores. Furthermore, among vegetarians, vegans achieved the best results. Although
different indexes were used in the studies, several common points allowed a combined
analysis of the results. Higher consumption of fruits, green vegetables, whole grains,
and vegetable sources of protein—and lower consumption of saturated fat and sodium—
contributed to the best results found among vegetarians [17].
A cross-sectional study carried out with vegetarians in Brazil (n = 3319) observed
that vegetarians have better diet quality markers than the general Brazilian population,
according to parameters used in a national annual survey carried out by the Ministry
of Health [
29
,
30
]. It was observed that a higher proportion of vegetarians had a more
adequate daily consumption of fruits and vegetables [
29
] compared to the general Brazilian
population (38.1 percent versus 23.1 percent), based on WHO recommendations (five serv-
ings a day) [
31
]. In addition, a lower regular weekly consumption of soft drinks and
artificial juices was also observed among vegetarians (3.9 percent versus 14.4 percent). Of
the different types of vegetarians, vegans showed the best results. It was also observed
that vegetarians in Brazil follow the recommendations set out in the Dietary Guidelines for
the Brazilian Population with regard to consuming more fresh foods and fewer processed
and ultraprocessed foods [32].
Vegetarian diets, including strict vegetarianism (veganism), are considered healthy
and nutritionally adequate, and can supply people’s nutritional needs at all life stages, as
long as such diets are well planned [
18
]. Moreover, the benefits related to the prevention
and better control of chronic diseases among vegetarians have already been described, and
could also lead to positive outcomes in their QoL.
The role of intestinal microbiota in the regulation of several biological functions and
in the prevention of chronic diseases is well known, as well as the fundamental role of
the diet in the microbiota and intestinal health of individuals [
33
–
35
]. Excessive protein
consumption could alter intestinal microbiota patterns by stimulating the proliferation of
bacteria capable of fermenting amino acids. Such fermentation results in the production
of molecules responsible for increased intestinal permeability, inflammation, and even
cancer [
36
]. The consumption of vegetable sources of protein, on the other hand, is not
associated with such adverse effects, possibly because they contain carbohydrates and
fibers, which could mitigate the potentially deleterious effects observed in the intestine
caused by the ingestion of proteins [
36
]. The intake of saturated fats, present mainly in
animal foods, is another factor that contributes to an increase in systemic inflammation,
possibly through the activation of Toll-like receptors (TLR), which, once activated, trigger a
proinflammatory intestinal and systemic immune response [
37
]. The activation of TLRs
and the subsequent inflammatory cascade result in an increased risk of metabolic disorders
and chronic diseases, such as cancer, insulin resistance, and cardiovascular diseases [37].
Vegetarian diets usually have a higher content of carbohydrates and fibers, in addition
to lower levels of proteins and fats—in particular saturated fats. Studies comparing the
microbiota of vegetarians and nonvegetarians show that a plant-based diet can benefit the
diversity and profile of the bacteria that make up the intestinal microbiota. In addition to
differences observed in the microbiota, with a more favorable bacterial profile, a vegetarian
diet (with high consumption of whole foods, fruits, and vegetables) leads to increased
production of metabolites from the fermentation of prebiotics and phytochemicals by
these bacteria, which also have a positive effect on the host’s health, both at intestinal and
systemic levels, contributing to the prevention of chronic diseases [38].
Int. J. Environ. Res. Public Health 2021,18, 4067 7 of 23
Among chronic diseases, cardiovascular diseases account for 43.6 percent of deaths
worldwide [
39
]. Positive results in the control of cardiovascular disease risk factors were
observed in clinical trials that promoted lifestyle changes, including adopting vegetarian,
vegan, and plant-based diets [
40
–
43
]. A review of observational studies conducted in 2018
assessed cardiovascular risk factors in vegans. In most countries, vegetarian diets were
associated with a lower intake of energy and saturated fat, and a better cardiovascular
profile (lower body weight, LDL cholesterol levels, blood pressure, fasting glucose, and
triglycerides) [44].
A 2019 review study conducted by the Diabetes and Nutrition Study Group (DNSG)
of the European Association for the Study of Diabetes (EASD) associated vegetarian eating
patterns with a 28 percent reduction in the incidence of coronary heart disease, and a 22 per-
cent drop in mortality from such conditions. That study gathered data from systematic
reviews with meta-analyses correlating different dietary patterns and cardiometabolic out-
comes in diabetic patients [
45
]. Following a balanced vegetarian diet can reduce systemic
inflammation and the risk of diabetes, two factors that are closely linked to the onset and
progression of cardiovascular disease [46].
The consumption of refined carbohydrates, saturated fats, processed meats, and
sugary drinks increases the risk of type-2 diabetes, especially when combined with low
consumption of dietary fibers. On the other hand, a low-calorie plant-based diet has a
protective effect [47].
The prevalence of diabetes among vegetarians is 1.6 to 2 times lower than among
omnivores [
48
]. In a 24-week controlled trial with diabetics, the individuals who followed a
vegetarian diet showed greater weight loss (6.2 kg versus 3.2 kg, on average), better insulin
sensitivity (30 percent versus 20 percent), greater reduction in visceral fat and medication
use, in addition to a better hormonal profile (increased adiponectin and reduced leptin)
and better levels of antioxidants, as compared to the ones following a standard diet for
diabetes control [49].
Several factors contribute to the reduction in risks and a better control of diabetes.
The first one is vegetarians’ better weight control. It is known that both obesity and the
accumulation of visceral fat are linked to increased insulin resistance, which contributes to
the onset of diabetes [
47
]. Vegetarians’ lower intake of saturated fats [
17
] also contributes
to reducing the risk of diabetes. It has been shown that reducing the consumption of
saturated fats or replacing them with unsaturated fats may contribute to improving insulin
sensitivity [
50
]. Other factors, such as higher fiber intake [
51
], lower ferritin levels and
lower intake of heme iron [
52
] among vegetarians are also related to better insulin resistance
and lower risk of diabetes.
A vegetarian diet may also contribute to improving inflammation control. Foods of
plant origin—when consumed in their most natural form—are rich in antioxidants, which
can assist directly in the control of free radicals in the body (as in the case of antioxidant
vitamins C and E), or even through several signaling pathways that modulate our immune
response and the production of antioxidant compounds and enzymes, suppressing inflam-
matory responses [
48
,
53
,
54
]. Therefore, a plant-based diet that is rich in fruits, vegetables,
whole grains, seeds, and nuts can help to control inflammatory processes.
A vegetarian diet may also bring benefits regarding cancer prevention. In addition
to vegetarians’ better weight control results [
55
], which can be considered a protective
factor against cancer [
56
], their higher consumption of dietary fibers could have protec-
tive effects due to the modulation of the intestinal microbiota. In addition, as previously
described, excessive protein consumption can lead to an increased production of inflam-
matory metabolites by the intestinal microbiota [
36
], and the consumption of saturated
fats (found mainly in foods of animal origin) is capable of activating Toll-like receptors in
immune system cells. This stimulates the production of proinflammatory cytokines [
37
],
and all these factors together can create a cancer-promoting environment.
In addition to the most common chronic diseases mentioned above, adopting a vegetar-
ian diet can help to prevent and treat other inflammatory diseases. A healthier microbiota,
Int. J. Environ. Res. Public Health 2021,18, 4067 8 of 23
higher consumption of antioxidants and lower consumption of potentially inflammatory
compounds, in addition to better weight control, are important factors that positively
contribute to the health of vegetarians. In fact, how long an individual has been following
a vegetarian diet may have an important influence on their results—which depend on con-
tinuous exposure to this type of dietary pattern. In a study that evaluated only individuals
who had been on a vegetarian diet for at least 15 years (n = 45), lower levels of oxidative
stress markers were observed compared to omnivorous individuals (n = 30) [57].
Furthermore, promising results have already been achieved with the adoption of a
vegetarian diet by individuals suffering from fibromyalgia, for example, including improve-
ments in pain symptoms, QoL, sleep quality, and anxiety depression [
58
]. In autoimmune
diseases, such as rheumatoid arthritis, a diet rich in fruits, vegetables, whole grains and
legumes—and low in animal foods—can help to control some of the symptoms [
59
]. A
vegetarian diet could also be a beneficial tool to prevent other autoimmune diseases, such
as multiple sclerosis [60], due to its role in the health of the intestinal microbiota [61].
Several factors related to lifestyle may influence the emergence of diseases and how
long an individual can live. Habits such as regular physical activities, stress control,
good personal relationships, and a balanced diet have a positive impact on longevity [
62
].
A more detailed analysis of the dietary patterns followed by the world’s longest-living
populations, who live in regions known as Blue Zones, can help us understand important
food-related aspects that might contribute to improving people’s health and life expectancy.
The five regions considered Blue Zones are: Loma Linda (California—United States),
Nicoya (Costa Rica), Sardinia (Italy) Ikaria (Greece), and Okinawa (Japan). In all of them,
individuals adopt a predominantly plant-based diet, with sporadic meat consumption (on
average five times a month, in small portions). On the other hand, the consumption of
legumes is frequent in all of them, being part of their daily diet, in addition to vegetables,
tubers, cereals, fruits, and other regional foods, including dairy products [63].
The increased consumption of fruits and vegetables—rich in phytochemicals—may
contribute to longevity through several mechanisms. The control of low-grade inflamma-
tion provided by antioxidant protection can prevent cell structure damage, slowing down
the aging process [
64
]. On the other hand, prioritizing the consumption of proteins from
animal sources could have a negative impact on one’s life expectancy. The profile of the
amino acids found in these foods, with a higher content of methionine and branched-chain
amino acids, leads to greater stimulation of IGF-1 and mTOR, in addition to greater cell
proliferation. This contributes to the cellular senescence process and, consequently, to
aging [65–68].
These potential health benefits of consuming a mostly or strictly plant-based diet
can contribute to better physical health and well-being, resulting in better QoL. In fact, a
cross-sectional study conducted with a total of 4628 individuals in the United Kingdom
(with a wide range of diseases and conditions) showed that people who were ill had
lower QoL scores than those feeling well. Post hoc comparisons indicated higher differ-
ences in the physical domain, especially among patients with musculoskeletal conditions
(arthritis/arthroplasty, chronic pain), and those with cardiovascular disease awaiting a
heart transplant [
69
]. Therefore, a diet that helps to prevent chronic and inflammatory
diseases could also reduce the negative effects of these conditions on people’s QoL.
Negative Influence
Despite the potential health benefits from adopting a vegetarian diet, special attention
should be given to the adequacy of iron, zinc, vitamins B12 and D, calcium, iodine, omega-3,
and protein in adults [
70
], and especially in infants [
71
]. Low intake of such nutrients could
lead to nutritional deficiencies and impair an individual’s health [
70
,
72
], with a negative
impact on their QoL.
Vitamin B12 deficiency should be highlighted, as this nutrient can only be found in
animal-origin foods. Vegetarians (especially vegans) have been shown to have lower levels
of serum vitamin B12. In addition, increased homocysteine levels [
73
–
75
] are observed, a
Int. J. Environ. Res. Public Health 2021,18, 4067 9 of 23
metabolite that is elevated due to deficiency of vitamin B12 (and other nutrients), and which
is associated with increased inflammation. B12 deficiency and increased homocysteine can
lead to neurological problems, anemia and developmental delay in children, in addition to
increasing the risks of cardiovascular disease, dementia, osteoporosis and death [
73
,
75
].
For this reason, it is necessary to monitor and supplement vitamin B12 levels among this
groups, and possibly encourage the intake of fortified foods.
Iron, an essential mineral used for hemoglobin formation and oxygen transport in
the body, also needs to be carefully adjusted. Vegetarians have been shown to have lower
serum ferritin levels, a protein responsible for storing iron in the body. Lower levels of
iron could increase the risk of developing anemia [
76
], which might also be caused by
vitamin B12 deficiency [
75
]. In this scenario, an inadequately planned vegetarian diet
could negatively affect aspects related to “energy and fatigue” in the physical domain of
QoL [28].
Bone health should also be addressed when considering the potential negative effects
of a vegetarian diet. A systematic review published in 2019 showed that vegetarians
and vegans had lower bone mineral density than omnivores, and vegans also had higher
fracture rates. Such results were unlikely explained only by lower calcium intake, as bone
health encompasses many complex mechanisms and depends on different nutrients [
77
].
A recent cross-sectional study also found lower bone health in vegans when compared
to omnivores (measured using quantitative ultrasound—QUS) [
78
], which reinforces the
need for proper diet planning and careful bone health monitoring among vegetarians.
3.1.2. Influence of the Physical Domain on the Adoption of a Vegetarian Diet
Positive Influence
Seeking health improvement is one of the reasons why people chose to adopt a
vegetarian diet [
7
]. According to Hopwood et al. [
79
], health was the most common reason
why nonvegetarians considered adhering to a vegetarian diet. Vegetarianism is currently
being more widely studied, and a growing number of scientific papers about the topic
have been published over the past few years [
80
]. Consequently, the topic has received
more attention from the media, and more information is reaching the general population.
As more people are informed about the health benefits of adopting a vegetarian diet, the
need or desire to improve their health might serve as a trigger. A study conducted in
Germany with 329 vegans showed that more than two-thirds of them (69.6 percent), despite
having more than one motive for following the diet, included health and well-being among
them [81].
In this sense, following a vegetarian diet is both the cause and consequence of the
positive outcomes related to the physical domain. People who seek health improvement
may be prone to adopting a vegetarian diet; and, once they do it, the physical benefits may
serve as further motivation for maintaining their new diet.
3.2. Psychological Domain
The psychological domain is related to positive or negative feelings, self-esteem
and body image/appearance, and thinking/learning/memory/concentration. Different
aspects of vegetarianism can either influence or be influenced by psychological factors
(Figure 1) [28].
3.2.1. Influence of Adopting a Vegetarian Diet on the Psychological Domain
Positive Influence
Avoiding meat and other animal products can enhance positive feelings arising from
the fact that person is adopting an attitude that confirms their beliefs. The positive psycho-
logical impact goes beyond the individual sphere, as it can also increase social connections
with others adopting similar ideas and behaviors. According to Rosenfeld and Burrow [
24
],
being a vegetarian goes beyond the choice of a dietary pattern, as it gives individuals a
new social identity, which influences their way of thinking, behaving, and socializing. The
Int. J. Environ. Res. Public Health 2021,18, 4067 10 of 23
adoption of a plant-based diet can have a positive effect on well-being and contentment,
which could positively impact someone’s QoL [82].
The different motivations for adopting vegetarianism are also able to influence in-
dividuals psychologically. Those who adopt vegetarianism for ethical reasons tend to
create more aversion to meat due to the association between its consumption and animal
suffering. Such individuals also exclude more animal foods and tend to adopt stricter
diets than those who become vegetarians for health or environmental reasons [
23
]. That
does not necessarily implicate a negative outcome, though. As it has been shown by
Cruwys et al. [
83
], vegetarians and vegans are more likely to report no barriers to diet
adherence (25.2 percent of vegans and 15.6 percent of vegetarians) when compared to
individuals following a gluten-free, paleo, or weight-loss diet. Indeed, both vegans and
vegetarians had higher diet adherence when compared to the other groups, which might
be connected to positive psychological effects related to the social identification within the
vegetarian/vegan community.
Negative Influence
Potentially negative outcomes of vegetarianism in the psychological domain could
be related to mental health impairment. The data related to the effect of vegetarianism
on mental health are conflicting. Adopting a vegetarian diet was positively associated
with a better mood in a cross-sectional study with Seventh-day Adventists [
84
]. A study
of South Asians living in the United States found that the likelihood of depression was
43 percent lower among vegetarians [
85
]. However, a contrary association has also been
observed: in the United Kingdom, a positive association of depressive symptoms was
found in men, even after adjusting for confounding factors such as nutritional deficiencies
and sociodemographic data [
86
]. Similar results were found among adolescents in a study
conducted in Turkey, in which higher levels of anxiety, as well as eating disorders, were
observed. That study raises the possibility that a vegetarian diet might be adopted among
young people as a way of limiting food intake, and that it might be related to preexisting
eating disorders [87].
Discrepant results have already been observed in a study that evaluated mental health
in representative population samples from Germany, Russia, and the United States, in
addition to samples from students in China and Germany. An increase in anxiety and
depression was observed only in the sample from China, but the result was considered
mild since a vegetarian diet would explain only 1 percent of the variance in cases of
depression and anxiety. In addition, the motivations that led Chinese students to adopt
a vegetarian diet differed from those of the other groups studied, being more related to
cultural and economic factors [
88
]. A study with Chinese elderly people also found a
positive association between adopting a plant-based diet and depression compared to a
meat-based diet. However, the correlation was observed only in men [89].
A French cohort’s cross-section study carried out a separate analysis by types of
vegetarian diets, and identified a positive association between depressive symptoms and
a fish diet and an ovo-lacto-vegetarian diet. However, no association was found with a
vegan diet, which contradicts the idea that a stricter diet (excluding more or all animal
products) would lead to more severe symptoms of depression [
90
]. The authors claim that
differences in motivation (between vegans and other vegetarians) may have contributed to
this group’s lack of association. In addition, the same study found a positive association
between depressive symptoms and the exclusion of items from the diet, both for foods
of animal and vegetable origin. That is, the more items excluded (not types of food, but
number of products excluded), the greater the symptoms. Such a result could indicate
that the higher levels of depression found in vegetarians in several studies could reflect an
increase in risk related to diet restriction, and not necessarily to vegetarianism itself [90].
Another point that needs to be considered is that studies on depression in vegetarians
are predominantly transversal, and therefore do not enable the determination of a cause-
and-effect relationship. A study that evaluated mental disorders and adopting a vegetarian
Int. J. Environ. Res. Public Health 2021,18, 4067 11 of 23
diet in the previous 12 months (through interviews with a population sample in Germany)
also found a positive association between the two variables. However, the time difference
between the beginning of both suggests that mental disorders preceded the change in diet,
thus refuting the hypothesis that vegetarianism might cause mental disorders [91].
A systematic review study carried out by Medawar et al. [
92
] points out that, despite
several health benefits related to adopting a vegetarian diet, its effect on mental health
has yet to be properly studied. It is possible that nutritional deficiencies, such as lower
levels of vitamin B12, contribute to worsening the nervous system’s health. On the other
hand, a diet that favors a more balanced intestinal microbiota, such as a vegetarian diet,
positively contributes to the maintenance of neurological functions due to its importance
in modulating the gut-brain axis [
92
]. In a meta-analysis study published in 2016, it has
also been observed that the consumption of fruits and vegetables is inversely associated
with the risk of depression [
93
]. Vegetarians consume more fruits and vegetables than
omnivorous individuals [
17
], and also tend to have better health markers and lower risk
of other chronic diseases [
94
]. In view of this, the conflicting results on the relationship
between vegetarianism and depression may reflect a lack of standardization with regard to
diet quality and adequate intake (or supplementation) of nutrients in some of the studies,
as well as the possibility already raised of reverse causality.
3.2.2. Influence of the Psychological Domain on the Adoption of a Vegetarian Diet
Positive Influence
The main reason individuals decide to adopt a vegetarian diet is because of ethi-
cal/moral reasons [
7
,
9
], which is related to compassion and empathy towards the animals.
Since some people feel that eating animal products is wrong, abstaining from their con-
sumption could contribute to a better psychological state. Adopting a vegetarian diet can
bring about positive feelings, such as altruism and a sense of purpose, while the pursuit of
such guilt-free peace of mind could also positively influence one’s choice to adopt a vegetar-
ian diet. A study conducted by Antonetti and Maklan [
95
] showed that experiencing either
guilt or pride could change consumers’ behavior and their intention to purchase more
sustainable products. Building on that, feeling guilty about eating animal products could
lead to a behavioral change, and feeling proud of doing it could reinforce the maintenance
of a vegetarian diet.
Moreover, some individuals adopt a vegetarian diet due to spiritual or religious
reasons [
7
]. Spirituality is a concept related to people’s quest for the meaning in life and
a connection to a higher or sacred power. On the other hand, religiousness is related to
the degree in which an individual believes, follows, and practices a religion, which might
influence how one chooses to live their lives [
96
]. An individual who follows a religion that
preaches abstention from animal products might feel encouraged to adopt a vegetarian diet.
Good adherence to the diet could, in this case, be a positive psychological reinforcement,
as it would be in line with their own beliefs. As it has already been demonstrated, high
levels of spirituality and religiosity are associated with better social, psychological, and
environmental QoL outcomes [96].
Negative Influence
Despite the positive outcomes related to the adoption of a vegetarian diet, some
challenges can be found. For many, the barriers to adopting vegetarianism outweigh
the possible benefits, and may prevent them from taking that step. Studies corroborate
the evidence that attachment to the taste of meat constitutes an obstacle to adopting
vegetarianism [
97
,
98
]. In addition, other barriers may be considered, such as the fear that a
vegetarian diet could be nutritionally inadequate or monotonous, or that it may not favor
satiety; the belief that preparing vegetarian meals is harder; difficulties in finding options
when eating in restaurants; living with people who eat meat; and a lack of knowledge
about meat-free eating [
97
,
99
–
101
]. Especially among men, meat is considered a “comfort
food”, and its intake is associated with strength, muscle building, and masculinity. These
Int. J. Environ. Res. Public Health 2021,18, 4067 12 of 23
beliefs represent a barrier to reducing meat consumption, as demonstrated by a study
with soldiers from Norway who evaluated their perception of the implementation of the
“Meatless Monday” program [
97
]. The program is a worldwide campaign, adopted in
more than 40 countries, which aims to make people aware of the advantages of reducing
meat consumption [102].
These results are in line with older studies conducted by Lea et al. [
103
,
104
]. Having
a taste for meat was considered the main barrier for the adoption of a vegetarian diet,
but other important factors have also been described, such as, for example, difficulties
in changing one’s eating pattern; the fact that family and friends may still eat meat; little
knowledge about the subject; and difficulties in finding vegetarian options when eating
out [103].
Moreover, according to another study from Lea et al. [
104
], some of the factors that
prevent or hinder the adoption of a plant-based dietary pattern are related to one’s family
(family members or close people do not adopt this eating pattern); convenience (difficulty
finding options or preparing food); health (fear of iron, protein and other nutrient deficien-
cies); cost and lack of options for eating out; and lack of information about vegetarianism.
The low prevalence of adopting a plant-based diet among the participants demonstrates
that several factors discourage its adoption—even though it is a more flexible dietary
pattern than a vegetarian diet.
All these barriers interconnect with the social domain, as they are influenced by the
social context in which an individual is inserted. Nevertheless, the negative psychological
effects refer to how individuals react to these fears or barriers, which might negatively
affect their choice of adopting a vegetarian diet. As described by Schmitt et al. [
105
], the
perception of discrimination, both about an individual and a group, has an impact on well-
being, with potential psychological consequences (contributing to mental stress, anxiety,
depression) and affecting other aspects, such as self-esteem, humor, and satisfaction with
life [105].
3.3. Social Domain
The social domain related to QoL includes personal relationships and social sup-
port [
28
]. In fact, having good social connections is essential for mental health and well-
being, positively influencing one’s QoL. In this case, the consequences of adopting a
vegetarian diet have to be analyzed based on the social and cultural group in which an
individual is inserted, as well as the attitudes of close people towards vegetarianism.
3.3.1. Influence of Adopting a Vegetarian Diet on the Social Domain
Positive Influence
Unlike other dietary patterns, vegetarianism goes beyond the definition of one’s food
choices. Rather, it is defined as a social identity, which consists of how a person identifies
themselves in terms of the social group in which they believe to belong. A study conducted
with young vegan women revealed that not only did they identify with the diet, but they
also passionately engaged in a “vegan lifestyle”. The choice of becoming a vegan had
positive effects in many different ways, including social relationships, and identification
and sense of connection with the vegan subculture [
106
]. Therefore, the choice of following
a vegetarian diet can enhance one’s connection with other people who share the same
life philosophy [
107
], strengthening social bonds and positively influencing one’s QoL
(Figure 1).
Negative Influence
Many of those who decide to adopt vegetarianism suffer rejection from others and are
victims of stereotyping and discrimination. Such negative attitudes towards vegetarians
and vegans are known as “vegaphobia” or “veganophobia”, a term already spread in the
scientific literature. A possible explanation for the discrimination against vegetarians and
vegans is related to the cognitive dissonance suffered by individuals who eat meat. In this
Int. J. Environ. Res. Public Health 2021,18, 4067 13 of 23
context, cognitive dissonance refers to the contradiction experienced by individuals who
like animals and feel compassion for them, but, at the same time, consume meat. Therefore,
individuals who eat meat may discriminate against vegetarians not out of fear or dislike,
but because they represent an affirmation that eating meat is not necessary and is, therefore,
unjustified [108].
In order to avoid conflict and embarrassment, many vegetarians prefer to omit their
dietary choice. In fact, social aspects are so relevant that the greatest reason why vegetarians
make exceptions and eat meat is due to pressure from friends, family, and coworkers.
According to Rosenfeld and Tomiyama [
109
], in a qualitative study that evaluated dieters’
motivations to break their diet, 51 percent of individuals reported having already eaten
meat after adopting vegetarianism. In general, their justifications do not involve missing
meat itself, but rather an attempt to avoid uncomfortable situations in a social context. The
fear of being rude or offending some family culture or tradition, the need to make a good
impression, or the fear of being stigmatized are some of the most important factors that
lead vegetarians to stop following their diets momentarily. Such a study reinforces the
idea that vegetarianism goes far beyond a dietary choice, creating a social identity that
influences the entire context in which an individual is inserted [109].
The negative consequences of a vegetarian identity usually have a stronger impact
on vegans than vegetarians because the former suffer more rejection and are viewed more
negatively by omnivores [
23
]. Such discrimination comes not only from nonvegetarian
people, but also from the media, as demonstrated by Cole and Morgan [
110
] in a study that
evaluated how veganism was reported in UK newspapers. Such a study concluded that
the media tends to present vegans as sentimentalists, fanatics and extremists, in addition
to mocking veganism and considering it impossible to maintain in practice.
3.3.2. Influence of the Social Domain on the Adoption of a Vegetarian Diet
Positive Influence
Vegetarians and vegans also showed more adherence to their diet when compared
to individuals who follow a paleo, gluten-free, or weight-loss diet. Social identification
was an important predictor of adherence in both quantitative and qualitative analyses.
According to Cruwys et al. [
83
], vegetarians and vegans described their diet not as an
individual choice, but as a manifestation of their social ethics. Ethical and moral concerns
were considered the most important facilitators of diet adherence, and a lack of adherence
would go against the group’s moral code. Feeling part of a social group can also positively
influence how strictly one sticks to a dietary pattern. The sense of belonging and the
in-group social reinforcement could make it easier for individuals to maintain their dietary
patterns, provided they feel supported by the group.
Vegetarians that have a close circle of vegetarian contacts (friends, family or coworkers)
have been shown to have higher QoL than those who do not [
13
]. In this case, they can
be positively influenced by their social environment. Moreover, just as the social context
in which vegetarians are inserted may influence their adherence to the diet, individuals
who eat meat may also be influenced by living with vegetarians. In their study, Geerts,
Backer, and Erreygers [
108
] described some characteristics of meat-consuming individuals,
with emphasis on the fact that meat consumption is considerably lower among those living
with vegetarians in the same household. In addition, discrimination against vegetarians
was less common among individuals who had vegetarians in their household or circle of
friends. Thus, greater acceptance and lower levels of veganophobia among meat consumers
(resulting from their close contacts with vegetarians) may have a positive influence on
other individuals’ feeling more comfortable when adopting a vegetarian diet.
Negative Influence
Cultural aspects are relevant predictors of meat consumption. The consumption of
different species of animals varies between cultures. Animals considered suitable for
consumption in some countries may not be seen in the same way by individuals of other
Int. J. Environ. Res. Public Health 2021,18, 4067 14 of 23
nationalities. As demonstrated by Ruby [
111
], in countries considered individualistic (such
as the United States and Canada), a feeling of disgust is the primary attitude of certain
individuals when faced with the idea of eating certain animals. On the other hand, in more
collectivist nations, such as China and India, cultural norms influence individual emotions
and the sense of morality, being the greatest predictor for not consuming meat.
Moreover, gender differences may also influence one’s choice of eating or avoiding
animal products. Meat consumption is usually seen as a symbol of masculinity and
dominance over other species in several cultures where meat is considered a proper food
for men [
23
,
97
]. In addition, men tend to eat less fruits and vegetables; care less about
the nutritional properties of the food they eat; and agree more with the belief that a
healthy diet needs to include meat [
7
,
112
]. According to Rosenfeld and Tomiyama [
98
],
men are more resistant to adopting a vegetarian diet, mainly because they believe that a
meatless diet would not be tasty. In addition, women are more likely to believe that meat
consumption is harmful to the environment and that adopting vegetarianism is a plausible
and healthy choice [
113
]. In fact, large population studies such as the Epic-Oxford [
114
]
and the Adventist Health Study 2 [
115
] identified a higher proportion of females among
vegetarians, with 78 percent and 65 percent of the sample consisting of women.
Such gender differences may influence the adoption of vegetarianism depending on
the sociocultural context in which an individual is inserted. A study by Ruby et al. [
116
]
with participants from Argentina, Brazil, the United States, and France (countries that
are among the largest consumers of beef in the world) revealed that men consume beef
more frequently and enjoy the taste of it more, while women show more negative attitudes
towards the consumption of red meat, such as disgust. The same study also demonstrated
that there are cultural differences related to the acceptance of vegetarianism. American
women showed greater admiration for vegetarianism, while French women were the ones
who admired vegetarians the least. Participants from Brazil and Argentina, considering the
entire sample, demonstrated more positive attitudes toward beef consumption, followed
by participants from France and, finally, from the United States [116].
3.4. Environmental Domain
The environment in which an individual is inserted also exerts an important influence
on their QoL. Living in a safe and healthy environment, with proper social care and an
efficient transport system, opportunities for acquiring new information and skills, as well as
recreation/leisure areas, are all relevant factors. Moreover, having good financial resources
can positively contribute to a good QoL. On the other hand, factors that have a negative
impact on the environment, such as pollution and climate change, could also negatively
affect one’s QoL [28].
3.4.1. Influence of Adopting a Vegetarian Diet on the Environmental Domain
Positive Influence
Following a more sustainable diet, which will contribute to a healthier environment,
could positively influence QoL (Figure 1). In general, plant-based diets are more sustain-
able than those based on animal foods, as they require fewer natural resources for food
production and have a lower impact on the environment. An omnivorous diet is estimated
to require 2.9 times more water, 2.5 times more energy, 13 times more fertilizers, and
1.4 times more pesticides than a vegetarian diet [
117
]. In addition, meat and dairy produc-
tion contribute 80 percent of all gas emissions from food production, and 24 percent of total
greenhouse gases coming from food. Livestock production uses about 70 percent of all
agricultural land globally, and consumes 29 percent of all water spent on agriculture [
118
].
Regarding the analysis of different types of diets, the data from 34 articles gathered in
a systematic review showed that the more a diet is plant-based, the more sustainable it is.
The vegan diet was considered the most sustainable of all, with the lowest greenhouse gas
emissions and the least environmental impact, especially when based on locally produced
foods and with a lower consumption of ultraprocessed meat substitutes. Ovolactovege-
Int. J. Environ. Res. Public Health 2021,18, 4067 15 of 23
tarian diets have a greater environmental impact than vegan diets, and it has been shown
that 40 percent of greenhouse gases from ovolactovegetarian diets are attributed to the
consumption of dairy products [118].
The production of animal-origin food is very inefficient in terms of energy, as it
requires the use of many resources (water, energy, land, food) to keep animals alive. The
animals themselves use much of the energy and nutrients in the form of food to maintain
their metabolism, whereas only a small part of it is actually stored and converted into
food for humans in the form of meat. This amount of energy wasted during production,
standardized through the rate of the conversion of energy into protein, varies considerably
from one animal to another. Whereas 4 calories from fossil fuels are required for each
calorie of chicken protein that is produced, 40 calories are required for the production of
1 calorie of beef protein. For pork and dairy production, the rate is 14 fuel calories for each
calorie of protein. In the case of eggs, the value is similar to that of beef (39 calories). On
average, the energy used to produce each gram of animal protein (25 kcal/g) is 11 times
greater than that used to produce vegetable proteins (2.2 kcal/g) [119].
In general, in the case of plant-origin foods, the higher the protein concentration, the
greater the energy efficiency (which means that such foods need less energy to provide
greater amounts of protein, as they are more concentrated in protein). Such an association
does not exist for foods of animal origin, as their energy demand is very high—in fact, a
decline in energy efficiency is observed as protein concentration increases (that is, foods
with a higher protein concentration are those that demand more energy) [120,121].
According to Aleksandrowicz et al. [
122
], the change from a typical Western diet to
more sustainable food patterns could reduce greenhouse gas emissions and land use related
to food production by up to 80 percent, in addition to a 50 percent reduction in water use.
In that study, all diets involved reducing or replacing animal foods with others of plant
origin (such as, for example, vegetarian, vegan, Mediterranean and pescatarian diets), in
addition to replacing the consumption of ruminant animals with monogastric animals [
122
].
Similar results were observed in a study by Rosi et al. [
12
] in Italy, which showed that
vegetarian diets (ovolactovegetarian and vegan) had a lower ecological footprint in the
three aspects assessed: CO
2
production, water consumption, and land use. Corroborating
these data, a global analysis of different dietary strategies to reduce the environmental
impact and improve health estimated that, in developed countries, the replacement of
animal foods with plant-origin foods could reduce the number of premature deaths by up
to 12 percent, and greenhouse gas emissions by up to 84 percent [123].
3.4.2. Influence of the Environmental Domain on the Adoption of a Vegetarian Diet
Positive Influence
Environmental issues are part of the motivations that lead individuals to reduce meat
consumption or adopt a vegetarian diet. The concept of sustainability applied to food refers
to a diet that, in addition to being nutritionally adequate and healthy, respects biodiversity
and ecosystems, is accessible, culturally accepted, and contributes to preserving natural
resources [124].
A motivation to live in a healthier and more sustainable environment may positively
influence people to adopt and maintain a vegetarian diet, as it has already been proved that
a more plant-based diet has a lower environmental impact when compared to animal-based
diets [
122
]. Individuals who are naturally engaged in sustainability and environmental
issues are more likely to have positive feelings related to a sense of altruism achieved
from adopting a vegetarian diet. The possibility of protecting their own environment
and contributing to a better world can bring a sense of purpose in life [
125
], which could
positively influence diet adherence and QoL.
Negative Influence
Adopting a vegetarian diet may depend on other factors beyond an individual’s will.
Economic aspects, both at the global level (economic situation of the country) and the
Int. J. Environ. Res. Public Health 2021,18, 4067 16 of 23
individual level (income and social status), could influence food choices. In general, the
lower the income, the greater its influence on food. People with higher income suffer less
from fluctuations in food prices and are more demanding in their choices. Likewise, in
poorer countries, the consumption of certain foods is highly influenced by their prices,
which does not occur with the same intensity in developed countries [
126
]. The influence
of economic aspects on the nutritional quality of a diet is quite variable. For example,
it has been shown that increased income leads to a higher intake of fruit. However, the
same increase might lead to eating out more often, or consuming more processed foods,
in addition to eating more meat and fewer legumes [
126
]. Moreover, a cross-sectional
study carried out in the United States showed that lower income levels were associated
with poorer quality of food—in particular, lower consumption of fruits and vegetables and
higher consumption of sugary drinks and frozen desserts [127].
The economic context is one of the factors that may influence the adoption of vegetari-
anism. On the one hand, the price of animal-origin foods may cause individuals to reduce
their consumption. A study carried out in Canada found that an increase in meat price
led 37.9 percent of individuals to reduce or eliminate their consumption. Still, as it is a
food that is part of local culture, individuals value meat consumption more than any other
food group. Therefore, despite economic issues, cultural aspects may also be considered an
important barrier to reducing meat consumption [
128
]. In Australia, it has been shown that
price increases are the biggest motivators for reductions in meat consumption, a factor that
was considered more relevant than health, religious, ethical, and environmental aspects,
among others [129]. Therefore, understanding the economic context in which individuals
live is essential for understanding the motivations that lead them to reduce their meat
consumption and possibly adopt vegetarianism.
Reducing meat consumption also depends on access to various plant-origin foods,
which is also limited by economic issues. In Brazil, for example, the consumption of fruits
and vegetables is influenced by prices and family income, with the cost burden being indi-
cated as the primary barrier [
130
]. Data from the Brazilian Household Budget Survey (POF)
showed that individuals from lower income groups spend a higher percentage of their
budget on food. Families with a monthly income of up to BRL 1908.00 spend 22.6 percent
of their household budget on food, compared with only 7.6 percent among families whose
monthly income exceeds BRL 23,850.00 [
131
]. One of the barriers to adopting a vegetarian
diet is the perception that it would be more expensive [
98
]. However, a vegetarian diet
could be considered cheaper than an omnivorous diet, since meat is often the most expen-
sive food item. In Brazil, a national survey from 2017–18 revealed that over 20 percent of
all household food expenses were spent on “meats, viscera and fish”, a percentage higher
than to any other food item [
131
]. Still, a vegetarian diet could become more expensive
when more meat-substitute foods (which are less accessible) are consumed [132].
Another factor that could hinder the adoption of a healthy vegetarian diet is the
logistics involving access to fresh fruits and vegetables. As they are perishable foods and
are usually eaten fresh (unlike meats and other foods, which are often frozen and stored
for longer), many types of fruits and vegetables require more frequent trips to the market,
and adequate storage to minimize losses. Therefore, the consumption of fresh fruits and
vegetables could be affected by people’s lack of time to purchase these foods frequently,
and by losses resulting from inadequate storage. In other words, the perishability of fruits
and vegetables could generate a cost increase. In addition, especially among low-income
individuals, a more restricted access to fresh food is a factor that negatively influences
its consumption [
133
]. Moreover, lower education levels could also negatively influence
one’s decision to adopt a vegetarian diet, as a positive association has been demonstrated
between higher educational levels and the adoption of a vegetarian diet [
114
,
134
]. In view
of this, educating individuals to make healthier and more economically viable choices
could encourage more people to adopt vegetarianism. Public policies that help reduce
prices and facilitate access to fruits, vegetables, and other plant-origin foods could also
help more people to reduce their meat consumption.
Int. J. Environ. Res. Public Health 2021,18, 4067 17 of 23
4. Vegetarians’ Quality of Life
A vegetarian diet’s effect on QoL was assessed in a cross-sectional study carried
out with runners. A convenience sample was selected from German-speaking countries,
namely Germany, Switzerland and Austria, and a total of 281 individuals (158 vegetarians
and 123 omnivores) participated in the study. The instrument used to assess QoL was the
WHOQOL-BREF, which was applied virtually to the study subjects. The results showed
that all participants scored high on QoL, regardless of the type of diet adopted, with no
difference between groups. Therefore, it was concluded that runners have high levels of
QoL, and that a vegetarian diet was as good as an omnivorous diet for this population
segment [135].
In Brazil, a specific questionnaire to evaluate the QoL of vegetarians was developed
and validated, since other studies used only general questionnaires or others that were
not specific to vegetarians [
13
]. The responses showed that vegetarians have satisfactory
levels of QoL (average scores between 70 and 80 on a 100-point scale). Among the different
types of vegetarians, vegans were the ones with the highest scores. Other factors that had
an influence on participants’ QoL included their age, how long they had been following a
vegetarian diet, and whether they had other vegetarians in their close circle of contacts [
13
].
In a clinical trial conducted with diabetic patients, the effect of a vegetarian diet on
their QoL and eating behavior was compared to a standard diet used to treat type 2 diabetes.
QoL was assessed using the Obesity and Weight-Loss QoL questionnaire (OWQOL) and
Weight-Related Symptom Measure questionnaire (WRSM). Both diets led to positive effects
on QoL and mood, but the effect was stronger in the group that followed a vegetarian diet,
demonstrating that such a dietary pattern can have positive effects not only on the physical
health, but also on the mental health of patients with type 2 diabetes [136].
Older studies [
137
–
139
] show similar results, with positive QoL outcomes when in-
dividuals were exposed to a vegetarian diet. Katcher, Ferdowsian, Hoover, Cohen, and
Barnard [
137
] developed a workplace study in a US-based company as part of a health pro-
motion program, in which volunteers adopted a vegan diet for 22 weeks. At the beginning
and the end of the period, individuals answered the Food Acceptability Questionnaire—
FAQ (SF) and the Work Productivity and Activity Impairment questionnaire (WPAI). The
responses to the questionnaires showed that individuals who adopted the vegan diet re-
ported improvement in general health, physical fitness, mental health, vitality and overall
satisfaction with the diet, in addition to the reduced cost of food items. However, they
reported more difficulty in finding options when eating out. Still, the vegan diet was
effective in improving the participants’ QoL. QoL was also assessed in a study conducted
at a health institute in the United States that offers a raw vegan diet to visitors and guests.
Participants who remained at the institute for at least a week and who would maintain the
raw vegan diet after leaving the institute were selected. A QoL analysis was performed
at the beginning of the study and 12 weeks after the intervention, with a questionnaire
that evaluated individual satisfaction with taste, food cost, convenience (ease of buying,
planning and preparing food), and self-care perception. Individuals who followed the raw
vegan diet for 12 weeks were compared to those who did not. There was an improvement
both in the parameters of general QoL (assessed by SF-36), as well as in the QoL associated
with changes in the diet, cost aspects and the perception of self-care. This shows the positive
effect that this type of food can have in QoL, when used as a clinical treatment [138]
A study conducted in the United States by Barnard, Scialli, Bertron, Hurlock, and
Edmonds [
139
] assessed the acceptability of a low-fat vegan diet in women. The study was
carried out with 35 nonmenopausal women divided into two groups: one adopting the
diet for a period equivalent to two menstrual cycles, and the other group not following
any diet, with a crossover design. The low-fat vegan diet had high adherence and good
acceptability, although the participants reported that maintaining the diet required more
effort. They also reported weight loss and improved sleep, digestion and energy levels,
which can positively contribute to improving QoL.
Int. J. Environ. Res. Public Health 2021,18, 4067 18 of 23
5. Summary of Knowledge and Future Directions
Adopting a vegetarian diet can have a positive influence on all four QoL domains. Bet-
ter health outcomes and lower rates of noncommunicable diseases have a positive impact
on the physical domain. Positive feelings associated with doing something good, together
with a feeling of belonging or stronger in-group bonds created with the vegetarian commu-
nity, have a positive effect on the psychological and social domains, respectively. Finally,
the lower environmental impact of vegetarian diets benefits the environmental domain.
On the other hand, negative effects on QoL might also result from adopting a veg-
etarian diet. Despite better overall health, a nonbalanced vegetarian diet could lead to
nutritional deficiencies that would be detrimental to health, affecting the physical domain.
As vegetarians are still a minority group, rejection and stigmatization from nonvegetarians
may have a negative impact on the social domain. The psychological and mental effects of a
vegetarian diet are not clear, although some studies point to an increased risk of depression.
Several aspects of different QoL domains can also have an impact on one’s decision
whether or not to adopt a vegetarian diet. Improving one’s health can be an important
motivator to try a vegetarian diet. Ethical/moral and religious/spiritual reasons are
important psychological aspects that can lead to the adoption of vegetarianism, while an
attempt to reduce one’s environmental impact can motivate someone to adopt such a diet.
Becoming part of a social group and achieving a sense of belonging can also be a trigger
for someone to become vegetarian.
Just as some individuals might feel motivated to follow a vegetarian diet for a number
of different reasons, others might feel discouraged due to psychological, social, or envi-
ronmental factors. A fear of being stigmatized or excluded from their social group could
hinder one’s intention of becoming a vegetarian. Moreover, cultural aspects that enhance
meat consumption could have the same effect, together with the connection that people
make between meat and masculinity. Finally, since the adoption of an alternative dietary
pattern also relies on environmental factors, such as food availability and economics, indi-
viduals may face difficulties when adopting a vegetarian diet if they lack a good supply of
plant-based food options.
6. Conclusions
In conclusion, vegetarianism can either influence or be influenced by different QoL
domains. The choice of adopting a vegetarian diet can have positive consequences, such as
better physical health, positive feelings related to the adoption of a morally correct attitude,
an increased sense of belonging (to a vegetarian community) and lower environmental
impact. On the other hand, factors that go beyond an individual’s control, such as the
environment and social/cultural group in which they are inserted, as well as gender-based
differences, economic aspects, and limited access to a wide variety of plant-based foods,
can negatively impact the QoL of those choosing to abstain from meats or other animal
products. Despite the low number of studies on vegetarianism and quality of life, the
existing evidence points toward a more positive impact. It is important to understand
all the effects of adopting a vegetarian diet—beyond its nutritional aspects. Not only do
studies in this area provide more consistent data, but they may also contribute to mitigating
all factors that might prevent individuals from adopting a vegetarian diet, or that may
have a negative impact on the quality of life of those who already follow it. Further studies
are necessary to understand how strongly these connections between QoL domains and
vegetarianism can influence the individuals who adopt this dietary pattern.
Author Contributions:
Conceptualization, S.M.H. and R.P.Z.; methodology, S.M.H. and R.P.Z.;
investigation, S.M.H. and R.P.Z.; writing—original draft preparation, S.M.H. and R.P.Z.; writing—
review and editing, S.M.H., A.R., A.S. and R.P.Z.; visualization, S.M.H., R.P.Z., A.R.; supervision,
R.P.Z. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Int. J. Environ. Res. Public Health 2021,18, 4067 19 of 23
Informed Consent Statement: Not applicable.
Data Availability Statement: The study did not report any data.
Acknowledgments:
The authors acknoledge the “Programa de Pós Graduação em Nutrição Humana
da Universidade de Brasília (PPGNH/UnB)” and Luiz Eduardo S. Hargreaves for the support.
Conflicts of Interest: The authors declare no conflict of interest.
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