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Does cooking with vegetable oils increase the risk of chronic diseases?:
a systematic review
q
Carmen Sayon-Orea
1
, Silvia Carlos
1,2
and Miguel A. Martı
´nez-Gonzalez
1,3
*
1
Department of Preventive Medicine and Public Health, Facultad de Medicina-Clı
´nica, Universidad de Navarra,
Ed. Investigacio
´n, C/Irunlarrea 1, 31008 Pamplona, Navarra, Spain
2
Institute for Culture and Society (ICS), Education of Affectivity and Human Sexuality, University of Navarra,
Pamplona, Spain
3
CIBERobn Physiopathology of Obesity and Nutrition, Institute of Health Carlos III (ISCIII), Madrid, Spain
(Submitted 4 March 2014 – Final revision received 23 July 2014 – Accepted 1 August 2014)
Abstract
Overweight/obesity, CVD and type 2 diabetes are strongly associated with nutritional habits. High consumption of fried foods might
increase the risk of these disorders. However, it is not clear whether the use of vegetables oils for cooking increases the risk of chronic
diseases. We systematically searched for published studies that assessed the association between vegetable oil consumption including
fried food consumption and the risk of overweight/obesity or weight gain, T2DM or the metabolic syndrome, and CVD or hypertension
in the following databases: PubMed; Web of Science; Google Scholar. Keywords such as ‘fried food’ or ‘vegetable oil’ or ‘frying’ or
‘frying oils’ or ‘dietary fats’ and ‘weight gain’ or ‘overweight’ or ‘obesity’ or ‘CHD’ or ‘CVD’ or ‘type 2 diabetes’ or ‘metabolic syndrome’
were used in the primary search. Additional published reports were obtained through other sources. A total of twenty-three publications
were included based on specific selection criteria. Based on the results of the studies included in the present systematic review,
we conclude that (1) the myth that frying foods is generally associated with a higher risk of CVD is not supported by the available
evidence; (2) virgin olive oil significantly reduces the risk of CVD clinical events, based on the results of a large randomised
trial that included as part of the intervention the recommendation to use high amounts of virgin olive oil, also for frying foods; and
(3) high consumption of fried foods is probably related to a higher risk of weight gain, though the type of oil may perhaps modify
this association.
Key words: Fried foods: Olive oil: Obesity: Hypertension: Metabolic syndrome: Type 2 diabetes mellitus: CVD
Some chronic diseases such as overweight/obesity, CVD and
type 2 diabetes mellitus (T2DM) are closely associated with
lifestyle factors and nutritional habits
(1)
. In the last few
decades, epidemiological evidence has helped to clarify the
specific role of diet and its components in the prevention or
elevation of the risk of these non-communicable diseases.
The role that some dietary components, such as vegetable
oils, may play in the determination of the risk of these
conditions has been identified by observational studies and
intervention trials. Since the pioneering Seven Countries
q
Publication of these papers was supported by unrestricted educational grants from Federacio
´n Espan
˜ola de Sociedades de Nutricio
´n, Alimentacio
´ny
Diete
´tica (FESNAD), International Nut and Dried Fruit Council (INC), International Union of Nutritional Sciences (IUNS), Fundacio
´n Iberoamericana de
Nutricio
´n (FINUT), Centro de Investigacio
´n Biome
´dica en Red de la Fisiopatologı
´a de la Obesidad y Nutricio
´n (CIBERobn) and Centro Interuniversitario
di Ricerca sulle Culture Alimentari Mediterranee (Ciiscam). The papers included in this supplement were invited by the Guest Editors and have
undergone the standard journal formal review process. They may be cited. The Guest Editors declare that Salas-Salvado
´is a nonpaid member of the
World Forum for Nutrition Research and Dissemination of the International Nut and Dried Fruit Council. Angel Gil is President of the Fundacio
´n
Iberomericana de Nutricio
´n, which is a non-paid honorary position. Lluis Serra-Majem is the President of the Scientific Committee of the Mediterranean
Diet Foundation and Scientific Director of the CIISCAM (Centro Interuniversitario di Ricerca sulle Culture Alimentari Mediterranee), Universita La
Sapienza di Roma which are both non-paid, honorary positions. Goretti Guasch is the Executive Director and Member of the Executive Committee of the
International Nut and Dried Fruit Council, which is a paid position. Mo
`nica Bullo
´declares no conflict of interest.
*Corresponding author: Professor Dr M. A. Martinez-Gonzalez, fax þ34 948 455649, email mamartinez@unav.es
Abbreviations: HR, hazard ratios; MetS, metabolic syndrome; PREDIMED, Prevencio
´n con Dieta Mediterra
´nea; SUN, Seguimiento Universidad de Navarra;
T2DM, type 2 diabetes mellitus.
British Journal of Nutrition (2015), 113, S36–S48 doi:10.1017/S0007114514002931
qThe Authors 2015
British Journal of Nutrition
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Study, dietary SFA have been considered to be directly associ-
ated with higher serum cholesterol concentrations and with a
higher risk of CVD. In this context, high SFA intake was con-
sidered to be the primary determinant of CVD risk in Western
countries. Moreover, recommending reduction of the intake of
all types of fats was considered to be a practical approach to
reduce saturated fat intake, and this recommendation was
included in almost all the dietary guidelines during the last
two to three decades
(2)
. The high energy density and high
palatability of high-fat foods were feared to exert potentially
adverse effects on body weight and cardiovascular health. In
addition, Ornish et al.
(3)
reported a regression of coronary
atherosclerosis in a study carried out in forty-eight patients
with angiographically documented CHD, in which the inter-
vention group (twenty-eight patients) followed a low-fat diet
with ,10 % of total energy as fat. This small study contributed
to the building up of the myth that all types of fats, inclu-
ding vegetable oils, are detrimental for cardiovascular health.
This belief opened the door to the recommendation of low-
fat diets for the prevention of CVD. However, the results
obtained in large randomised trials using low-fat diets as a
paradigm of a healthy diet to prevent the development of
CVD or obesity have been disappointing
(4 – 6)
. A reduction in
fat intake is usually compensated by a proportional increase
in carbohydrate intake. In the current cultural context, most
carbohydrates are highly refined and may even be more
favourable for the development of obesity, T2DM and CVD
than fats. The isoenergetic comparison of carbohydrate v.
SFA rendered both equally detrimental for cardiovascular
health
(7)
, and when the comparison was made between n-6
PUFA (present in seed oils) and SFA, the intake of PUFA was
found to be significantly associated with a reduced risk of
CHD. In these circumstances, an alternative dietary paradigm
that is different from the low-fat diet and can be more
useful for developing and implementing programmes aimed
at achieving prolonged weight loss and improving cardio-
vascular health is the traditional Mediterranean diet. This
dietary pattern is rich in fat from vegetable oils (especially
virgin olive oil) and includes an abundance of minimally
processed plant foods (vegetables, fruits, whole grains and
legumes), low consumption of meat (especially red and
processed meats), moderate consumption of fish and wine
(which is usually consumed with meals) and frugal meals.
The high fat content of the traditional Mediterranean diet
makes it more palatable and therefore more acceptable and
easily sustainable in the long term. It is likely that a higher
intake of vegetable oils may contribute to a greater palatability
and consequently sustainability.
Vegetable oils are produced from oilseeds (i.e. sunflower),
legumes (i.e. soyabean), nuts (i.e. almond) or the flesh of
some fruits (i.e. olives). They are mainly composed of TAG
and therefore serve as sources of fat. As all sources of
fat, vegetable oils, contain different kinds of fatty acids, their
compositions vary widely, but typically one type of fatty
acid will predominate over the others; for example, the
major fatty acid in olive oil is oleic acid, a MUFA, and the
predominant fatty acid in sunflower oil is linoleic acid, a
PUFA
(8)
. Concerning vegetable oils and health, besides the
predominant fatty acid in each specific oil, another issue that
has to be taken into consideration is the culinary purpose,
because oil has different effects on health depending on the
form it is eaten (used for dressing, for cooking or for frying).
Therefore, we conducted a systematic review of the existing
evidence regarding the association of vegetable oil con-
sumption including fried food consumption with the risk of
overweight/obesity or weight gain, T2DM or the metabolic
syndrome (MetS), and CVD or hypertension.
Methods
A systematic search for published studies that assessed the
association between vegetable oil consumption including
fried food consumption and the risk of overweight/obesity
or weight gain, T2DM or the MetS, and CVD and hypertension
was performed in the following databases: PubMed; Web of
Science; Google Scholar. Keywords such as ‘fried food’ or
‘frying’ or ‘frying oils’ or ‘vegetable oil’ or ‘dietary fats’ and
‘weight gain’ or ‘overweight’ or ‘obesity’ or ‘CHD’ or ‘CVD’
or ‘type 2 diabetes’ or ‘metabolic syndrome’ were used in
the primary search. Additional published reports were
obtained through other sources. The language of publication
was restricted to English and Spanish. Studies that had a
case–control, cohort or randomised controlled trial study
design; those that considered vegetable oil consumption or
fried food consumption as the exposure of interest; and
those that investigated weight gain, overweight, obesity,
T2DM, the MetS, CVD or hypertension as the outcome were
included in the present systematic review. Reviews, editorials,
comments, letters, meeting abstracts, short communications,
protocols, non-human studies, studies that did not specifi-
cally consider vegetable oil or fried food consumption as
exposure, and studies that did not consider weight gain,
overweight, obesity, T2DM, the MetS or CVD as the outcome
were excluded.
Data extraction
The following information was extracted from each included
study: author and year of publication; study design; dietary
intake assessment method; number of FFQ items; exposure
(type of vegetable oil or fried food consumption) and catego-
risation; follow-up period; number of participants included
and characteristics of the included participants such as age, sex
and country; the outcome (weight gain, overweight, obesity,
T2DM, the MetS, and CVD or hypertension); number of
cases and controls in case– control studies, number of events
in cohort and randomised controlled trial studies; the
covariates included in the adjusted models and
b
-regression
coefficients, OR and hazard ratios (HR) were also extracted.
Results
Literature search
The results of the literature search are shown in Fig. 1. In total,
3549 records were identified in PubMed, 388 records in the
Vegetable oils and chronic diseases S37
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Web of Science and 832 in Google Scholar until April 2014;
four additional records were identified through other sources.
After exclusion of duplicate records and records that were not
published in scientific journals (n936), 3837 records were
screened. In the first instance, records were screened
only by title content and 3616 records were excluded. The
abstracts of the remaining 221 records were read and records
that did not meet the inclusion criteria were excluded, leaving
a total of twenty-three articles for inclusion in the systematic
review.
Study characteristics
The information extracted from the twenty-three studies
included in the present systematic review is summarised in
Tables 1– 3. Studies that investigated weight gain or the risk
of overweight or obesity as the outcome were conducted
in the USA
(9,10)
and Spain
(11 – 13)
; studies that assessed the
incidence of T2DM or the MetS were also conducted in the
USA
(14,15)
and Spain
(16 – 19)
; and studies that investigated
the risk of CVD and hypertension were conducted in
the USA
(20)
, Spain
(21 – 25)
India
(26)
, Costa Rica
(27)
, Norway
(28)
,
Greece
(29)
, Italy
(30)
and France
(31)
(Table 2).
Vegetable oil and fried food consumption was measured
using FFQ (in the majority of the studies), dietary history
questionnaire
(22,23)
or 24 h dietary recall
(31)
. Dietary intake
information was updated in seven studies
(9,10,14 – 16,18,25)
by
repeated measurements of dietary intake using the FFQ
during the follow-up period (Table 1).
A summary of the results discussed in the review is shown
in Fig. 2.
Association between vegetable oil or fried food
consumption and weight gain or the risk of
overweight/obesity
A total of five cohort studies
(9 – 13)
that investigated the
association between vegetable oil or fried food consumption
and weight gain or the risk overweight/obesity were included
in the present systematic review. Fried food consumption
was found to be positively associated with the risk of
becoming overweight/obese, as well as with a greater weight
gain
(9,10,13)
. On the other hand, high consumption of olive oil
was found to be not associated with a higher risk
(11)
, and a
negative association was even reported by some studies
(12)
.
The most relevant results in this context were from the study
of Mozaffarian et al.
(9)
, which using data of three separate
large US cohorts investigated the relationship between
multiple dietary changes including fried food consumption
and long-term weight gain. The following three cohorts were
included in this study: (1) the Nurses’ Health Study (NHS):
a prospective cohort study of 121 701 female registered
nurses enrolled in 1976; (2) the NHS-II: a prospective
cohort study of 116 686 younger female registered nurses
enrolled in 1989; (3) the Health Professionals Follow-up
Study (HPFUS): a prospective cohort study of 51 529 male
health professionals enrolled in 1986. All cohort participants
were followed up using mailed questionnaires every 2 years.
In total, 50 422 women from the NHS, 47 898 women from
the NHS-II and 22 557 men from the HPFUS were found to
be eligible, thereby 120 877 US women and men, all free
of obesity and chronic diseases at baseline and who had
complete data on weight and lifestyle habits, were included
4773 potentially relevant records identified:
4769 through database searching
and four through hand-searching
936 records excluded: duplicate or not
published in scientific journals
3616 records excluded after title screening
198 records excluded:
- Reviews, editorials, comments, letters, meeting
abstracts, short communications or protocols (n 65)
- Non-human studies (n 8)
- Cross-sectional studies (n 27)
- No abstract available (n 8)
- Did not consider vegetable oil consumption as
exposure (n 55)
- Did not consider weight gain, overweight, obesity,
CVD, diabetes or the metabolic syndrome as the
outcome (n 35)
3837 records screened by title content
221 abstracts screened
Twenty-three aricles included in the systematic
review
Fig. 1. Flow diagram depicting the identification and selection processes of relevant studies that investigated the association between vegetable oil or fried food
consumption and the risk of overweight/obesity or weight gain, T2DM or the metabolic syndrome, and CVD or hypertension.
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Table 1. Study design characteristics of the twenty-three epidemiological studies that investigated the association between vegetable oil or fried food consumption (FFC) and weight change, incident
overweight/obesity, type 2 diabetes, the metabolic syndrome, hypertension or CVD
Author and year
Dietary intake
assessment
method FFQ items
Type of
vegetable oil
consumption or FFC Categorisation
Validation
of nutrients
Multiple
assessments of
dietary intake
Average
duration of
follow-up (years)
BMI or weight change (continuous variable)
Taveras
et al.
(10)
(2005)
FFQ 132 FFC Never or ,1
v. 4– 7 times/week
Yes Yes 3
Mozaffarian
et al.
(9)
(2011)
FFQ NR FFC, potato
chips and French fries
Increased dietary intake Yes Yes NHS ¼20; NHS-II ¼12;
Health Professionals
Follow-up Study ¼20
Overweight/obesity
Bes-Rastrollo
et al.
(11)
(2006)
FFQ 136 Olive oil Quintiles Yes Baseline 2·4
Haro-Mora
et al.
(12)
(2011)
Frequency of
consumption of
main foods in Spain
NR Olive oil and
mixture or other
types of oils
(usually sunflower oil)
Olive oil v. others No Baseline 1
Sayon-Orea
et al.
(13)
(2013)
FFQ 136 FFC ,2, 2– 4, or
.4 times/week
Yes Baseline 6·1
T2DM
Halton
et al.
(14)
(2006)
FFQ 61 French fries Quintiles Yes Yes 20
Salas-Salvado
et al.
(16)
(2011)
FFQ 137 EVOO Control group, MedDiet
supplemented with
EVOO, and MedDiet
supplemented with nuts
Yes Yes 4
Mari-Sanchis
et al.
(17)
(2011)
FFQ 136 Olive oil Quintiles Yes Baseline 5·7
Salas-Salvado
et al.
(18)
(2014)
FFQ 137 EVOO Control group, MedDiet
supplemented with EVOO,
and MedDiet
supplemented with nuts
Yes Yes 4·1
Metabolic syndrome
Lutsey
et al.
(15)
(2008)
FFQ 66 FFC Tertiles NR Yes 9
Sayon-Orea
et al.
(19)
(2014)
FFQ 136 FFC 0– 2, .2 – 4, or
.4 times/week
Yes Baseline 8·3
CVD
Mozaffarian
et al.
(20)
(2003)
FFQ NR Fried fish ,1 time/month,
1– 3 times/month,
1 time/week, 2 times/week,
or $3 times/week
Yes Baseline 9·3
Rastogi
et al.
(26)
(2004)
FFQ 141 Cooking or frying with
sunflower oil (ref.), ghee,
vanaspati, mustard oil,
peanut oil or safflower oil
Type of oil used for
cooking or frying
Yes Baseline NA
Kabagambe
et al.
(27)
(2005)
FFQ NR Palm oil v. soyabean
oil and others (including
sunflower, maize, olive
and rapeseed oils)
Type of oil used for cooking Yes Baseline NA
Lockheart
et al.
(28)
(2007)
FFQ 190 Non-hydrogenated
vegetable oil
Tertiles Yes Baseline NA
Kontogianni
et al.
(29)
(2007)
FFQ NR Olive oil and others No use of olive oil v.
exclusive use of olive
oil and olive oil plus others
NR Baseline NA
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in the study. The relationship between changes in lifestyle
factors and weight change was evaluated every 4 years. The
dietary factors with the largest positive association with
weight change/serving per d were increases in the consump-
tion of potato chips and French fries (weight change after
4 years was 1·69, 95 % CI 1·30, 2·09 lb, and 3·35, 95 % CI
2·29, 4·42 lb, respectively). Fried food consumption at home
and away from home was also found to be significantly associ-
ated with weight gain (0·39 lb, 95 % CI 0·22, 0·51 and 0·28 lb,
95 % CI 0·08, 0·48 respectively).
The ‘Seguimiento Universidad de Navarra’ (SUN) study
assessed the association between fried food consumption
and weight gain in a sample of 9850 participants with a
mean age of 38·1 (SD 11·4) years who were followed up for
a median time period of 6·1 years
(13)
. Fried food consump-
tion was found to be not significantly associated with a
higher weight gain (
b
-regression coefficient ¼þ46, 95 % CI,
217, þ109 g; Pfor trend¼0·230) on comparing the highest
(.4 times/week) v. the lowest (,2 times/week) category
of consumption. The risk of overweight/obesity was also
analysed in this study in a sample of 6821 participants initially
free of overweight/obesity at baseline, and during the same
follow-up period, 1068 incident cases were identified, and
higher consumption of fried foods was found to be associated
with a significantly higher risk of becoming overweight/obese
(OR 1·37, 95 CI% 1·08, 1·73; Pfor trend¼0·021) when
comparing the highest (.4 times/week) v. the lowest (,2
times/week) category of consumption. In this study, analyses
stratified by the type of oil used for frying were also
conducted, in which it was found that those who used olive
oil for frying tended to gain less weight and had a lower
risk of becoming overweight/obese; however, none of these
associations was found to be statistically significant. The
results of this study supported the hypothesis that a high
frequency of fried food consumption was associated with
the risk of becoming overweight/obese among young adults
in a Spanish cohort with a high educational level.
The possibility that frying with olive oil might be healthier
and could be considered as a preferable substitute for frying
with other oils deserves further assessments
(32)
.
Some authors have shown that the impact of fried food
consumption on body weight does not depend on the place
where fried foods are prepared (home or away from
home)
(9)
, with fried foods being prepared both at home and
away from home being found to increase the risk of
overweight/obesity. Therefore, it is assumed that the frying
process (pan-frying or deep-frying, number of frying sessions,
duration and temperature of frying, etc.) is not a deter-
minant of body weight increase. However, most of the
literature regarding the health effects of frying foods on
overweight/obesity does not take into account specific aspects
of the frying process, and it is well known that the frying
technology can affect both the quality and the amount of
fats consumed
(33)
. Among the oils used for cooking, olive
oil is less prone to oxidation
(34)
and it maintains its health-
beneficial properties, even after being cooked due to its
thermal stability. It has even been shown that the antioxidants
Table 1. Continued
Author and year
Dietary intake
assessment
method FFQ items
Type of
vegetable oil
consumption or FFC Categorisation
Validation
of nutrients
Multiple
assessments of
dietary intake
Average
duration of
follow-up (years)
Bendinelli
et al.
(30)
(2011)
FFQ 188/217/154 Olive oil Quartiles NR Baseline 7·9
Samieri
et al.
(31)
(2011)
FFQ/24 h
dietary recall
NR Olive oil No use of olive oil,
moderate use or
intensive use
NR Baseline 5·3
Guallar-Castillo
´n
et al.
(22)
(2012)
Dietary history
questionnaire
662 FFC Quartiles Yes Baseline 11
Buckland
et al.
(23)
(2012)
Dietary history
questionnaire
662 Olive oil Quartiles Yes Baseline 10·4
Estruch
et al.
(25)
(2013)
FFQ 137 EVOO Control group, MedDiet
supplemented with EVOO,
and MedDiet supplemented
with nuts
Yes Yes 4·8
Hypertension
Alonso & Martinez-
Gonzalez
(21)
(2004)
FFQ 136 Olive oil Quintiles Yes Baseline 2·4
Sayon-Orea
et al.
(24)
(2014)
FFQ 136 FFC ,2, 2– 4, or .4 times/week Yes Baseline 6·3
NR, not reported; NHS, Nurses’ Health Study; T2DM, type 2 diabetes mellitus; EVOO, extra-virgin olive oil; MedDiet, Mediterranean diet; ref., reference; NA, not applicable.
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Table 2. Characteristics of the twenty-three epidemiological studies that investigated the association between vegetable oil or fried food consumption and weight change, incident overweight/obesity,
type 2 diabetes, the metabolic syndrome, hypertension or CVD
(Mean values and standard deviations)
Author and year No. of participants Age (years) Country Time of data acquisition Sex (% male) Outcome
BMI or weight change (continuous variable)
Taveras et al.
(10)
(2005) 14 355 Range: 9 – 14 USA 1996– 9 46·1 Annual change in BMI
Mozaffarian et al.
(9)
(2011) 120 877 NHS ¼52·2 (SD 7·2);
NHS-II ¼37·5 (SD 4·1);
HPFUS ¼50·8 (SD 7·5)
USA 1988– 2006 18·7 Weight change
Overweight/obestiy
Bes-Rastrollo et al.
(11)
(2006) 5356 Range: 20–90 Spain 1999–2004 41·5 Incidence of overweight or obesity
Haro-Mora et al.
(12)
(2011) 92 Range: 13 – 166 months Spain NR 80·4 Risk of increased BMI
Sayon-Orea et al.
(13)
(2013) 6821 38·1 (SD 11·4) Spain 1999– 2010 46·1 Risk of overweight or obesity
T2DM
Halton et al.
(14)
(2006) 84 555 Range: 34 – 59 USA 1980– 2000 0 Type 2 diabetes
Salas-Salvado et al.
(16)
(2011) 418 67·3 Spain 2003– 8 41·6 Type 2 diabetes
Mari-Sanchis et al.
(17)
(2011) 10 491 38·9 (SD 11·4) Spain 1999 – 2009 47·2 Type 2 diabetes
Salas-Salvado et al.
(18)
(2014) 3541 Range: 55– 80 Spain 2003–2010 38·0 Type 2 diabetes
Metabolic syndrome
Lutsey et al.
(15)
(2008) 9514 53·6 USA 1987–1998 44·1 Metabolic syndrome
Sayon-Orea et al.
(19)
(2014) 8289 35·9 (SD 10·4) Spain 1999– 2012 33·9 Metabolic syndrome
CVD
Mozaffarian et al.
(20)
(2003) 3910 $65 USA 1989 – 2000 39·0 IHD
Rastogi et al.
(26)
(2004) 1050 Range: 21 – 74 India 1999– 2000 88 IHD
Kabagambe et al.
(27)
(2005) 4222 Cases ¼58·5 (SD 11·0);
controls ¼58·2 (SD 11·3)
Costa Rica 1994– 2004 73 Non-fatal acute myocardial infarction
Lockheart et al.
(28)
(2007) 211 62·5 (SD 7·7) Norway 1995 – 7 NR First myocardial infarction
Kontogianni et al.
(29)
(2007) 1926 NR Greece NR 82·5 Non-fatal acute coronary syndrome
Bendinelli et al.
(30)
(2011) 29 689 Range: 35 –74 Italy 1993 –8 0 CHD
Samieri et al.
(31)
(2011) 7625 73·8 (SD 5·3) France 1999 – 2005 37·7 Stroke
Guallar-Castillo
´net al.
(22)
(2012) 40 757 Range: 29 –69 Spain 1992– 2004 37·7 CHD
Buckland et al.
(23)
(2012) 40 142 Range: 29 –69 Spain 1992 –2004 38 CHD
Estruch et al.
(25)
(2013) 7447 Range: 55 – 80 Spain 2003 – 2010 43·0 CHD
Hypertension
Alonso & Martinez-Gonzalez
(21)
(2004) 5573 .20 Spain 2000– 39·3 Hypertension
Sayon-Orea et al.
(24)
(2014) 13 679 36·5 (SD 10·8) Spain 1999 – 2012 37·0 Hypertension
HPFUS, Health Professionals Follow-up Study; NHS, Nurses’ Health Study; NR, not reported, T2DM, type 2 diabetes mellitus.
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Table 3. Results for association between vegetable oil or fried food consumption and weight change, incident overweight/obesity, type 2 diabetes, the metabolic syndrome, hypertension or CVD
reported by the twenty-three epidemiological studies
(Odds ratios, hazard ratios or
b
-coefficients and 95 % confidence intervals)
Author and year Design
No. of
cases
No. of
controls
No. of
events Covariates in the adjusted model
b
-Coefficient or OR/HR 95 % CI
BMI or weight change (continuous variable)
Taveras
et al.
(10)
(2005)
Cohort NA NA NA Age, race/ethnicity, baseline and follow-up menstrual status (girls),
Tanner stage, baseline height, annual change in height,
previous BMI Z-score, physical activity and sex
b
-Coefficient ¼0·21 kg 0·03, 0·39
Mozaffarian
et al.
(9)
(2011)
Cohort NA NA NA Age, baseline BMI, sleep duration, changes in physical activity,
alcohol consumption, time spent watching television,
smoking status and dietary factors
Potato chips: 1·69 lb;
French fries: 3·35 lb;
fried foods: 0·39 lb
1·30, 2·09;
2·29, 4·42;
0·22, 0·51
Overweight/obesity
Bes-Rastrollo
et al.
(11)
(2006)
Cohort NA NA 405 Age, sex, total energy intake, fibre intake, vegetable consumption,
leisure-time physical activity, smoking status, snacking
between meals, time spent watching television and baseline BMI
OR ¼1·11 0·76, 1·61
Haro-Mora
et al.
(12)
(2011)
Cohort NA NA NR Age, baseline BMI and physical activity OR ¼0·19 0·06, 0·61
Sayon-Orea
et al.
(13)
(2013)
Cohort NA NA 1068 Age, sex, time spent sitting down (h/week), baseline BMI,
smoking status, physical activity, nut intake, sugar-sweetened
beverage (SSB) consumption, fibre intake, snacking, total
energy intake, family history of obesity and fast food consumption
OR ¼1·37 1·08, 1·73
T2DM
Halton
et al.
(14)
(2006)
Cohort NA NA 4496 Age, BMI, family history of diabetes, smoking status,
postmenopausal hormone use, physical activity,
total energy intake, cereal fibre intake, trans-fat
intake and polyunsaturated fat:saturated fat
RR ¼1·16 1·05, 1·29
Salas-Salvado
et al.
(16)
(2011)
PREDIMED RCT NA NA 54 Sex, age, baseline energy intake, BMI, waist circumference,
physical activity, smoking status, fasting serum glucose
concentrations, use of lipid-lowering drugs, MedDiet score
and weight changes during the study
HR ¼0·49 0·25, 0·97
Mari-Sanchis
et al.
(17)
(2011)
Cohort NA NA 42 Age, sex, BMI, physical activity, family history of diabetes,
gestational diabetes, hypercholesterolaemia, hypertension
and total energy intake
OR ¼1·04 0·43, 2·55
Salas-Salvado
et al.
(18)
(2014)
RCT NA NA 273 Age, sex, BMI, smoking status, fasting glucose concentrations,
prevalence of dyslipidaemia, hypertension, total energy intake,
adherence to the MedDiet, physical activity,
educational level and alcohol consumption
HR ¼0·60 0·43, 0·85
Metabolic Syndrome
Lutsey
et al.
(15)
(2008)
Cohort NA NA 3782 Age, sex, race, educational level, centre and total energy intake HR ¼1·25 1·14, 1·37
Sayon-Orea
et al.
(19)
(2014)
Cohort NA NA 420 Age, sex, baseline BMI, time spent sitting down,
smoking status, physical activity, sugared soft drink intake,
fibre intake, fast food consumption, snacking between meals,
categories of MedDiet, total energy intake, and carbohydrate,
fat and Na intake
HR ¼0·93 0·72, 1·20
CVD
Mozaffarian
et al.
(20)
(2003)
Cohort NA NA 247 Age, sex, educational level, diabetes, current smoking status,
pack-year of smoking, BMI, systolic blood pressure,
LDL-cholesterol concentrations, HDL-cholesterol concentrations,
TAG concentrations, C-reactive protein concentrations
and saturated fat intake
HR ¼1·37 0·48, 3·90
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Table 3. Continued
Author and year Design
No. of
cases
No. of
controls
No. of
events Covariates in the adjusted model
b
-Coefficient or OR/HR 95 % CI
Rastogi
et al.
(26)
(2004)
Case–control 350 700 NA Age, sex, hospital, smoking status, BMI, waist:hip ratio,
physical activity, hypertension, diabetes, hypercholesterolaemia,
family history of IHD, alcohol consumption, educational level,
household income, religion, cereal intake and
green leafy vegetable intake
Cooking in sunflower
v. mustard oil: RR ¼0·46;
frying in sunflower
v. mustard oil: RR ¼0·25
0·22, 0·95;
0·11, 0·57
Kabagambe
et al.
(27)
(2005)
Case–control 2111 2111 NA Age, sex, area of residence, smoking status, alcohol consumption,
diabetes, hypertension, abdominal obesity, physical activity,
income, total energy intake and fibre intake
Other oils v. palm oil:
OR ¼1·23
0·99, 1·52
Lockheart
et al.
(28)
(2007)
Case–control 106 105 NA Age, marital status educational level, family history of
heart disease, smoking status and energy intake
OR ¼0·53 0·24, 1·18
Kontogianni
et al.
(29)
(2007)
Case–control 848 1078 NA Age, sex, BMI, smoking status, physical activity,
educational level, family history of CHD, hypertension,
hypercholesterolaemia and diabetes
No use v. exclusive use of
olive oil: OR ¼0·53
0·34, 0·71
Bendinelli
et al.
(30)
(2011)
Cohort NA NA 144 Educational level, smoking status, alcohol consumption,
height, weight, waist circumference, daily non-alcohol
energy intake, hypertension, menopausal status, total
physical activity and meat consumption
HR ¼0·56 0·31, 0·99
Samieri
et al.
(31)
(2011)
Cohort NA NA 148 Age, sex, educational level, centre, consumption of fish,
meat, pulses, raw vegetables, cereals, n-3-rich oils, butter,
goose, and duck fat, alcohol consumption physical activity,
risk factors for stroke, BMI, triacylglycerolaemia
and hypercholesterolaemia
HR ¼0·59 0·37, 0·94
Guallar-Castillo
´n
et al.
(22)
(2012)
Cohort NA NA 606 Age, sex, centre, alcohol consumption, educational level,
smoking status, physical activity, diabetes, hyperlipidaemia,
cancer, oral contraceptive use, menopause, hormone
replacement therapy, consumption of fruits, nuts,
dairy products and non-fried foods, BMI, waist
circumference and hypertension
HR ¼1·08 0·82, 1·43
Buckland
et al.
(23)
(2012)
Cohort NA NA 587 Age, sex, centre, alcohol consumption, educational level,
BMI, waist circumference, physical activity, smoking status,
alcohol consumption, energy intake, hyperlipidaemia,
hypertension, diabetes and MedDiet score
HR ¼0·85 0·68, 1·07
Estruch
et al.
(25)
(2013)
RCT NA NA 288 Sex, age, family history of premature CHD, smoking status,
BMI, waist:height ratio, hypertension at baseline,
dyslipidaemia at baseline and diabetes at baseline
HR ¼0·70 0·54, 0·92
Hypertension
Alonso & Martinez-
Gonzalez
(21)
(2004)
Cohort NA NA 161 Age, sex, BMI, leisure-time physical activity,
total energy intake, alcohol consumption,
Na intake and Ca intake
OR overall ¼0·63;
OR women ¼0·97;
OR men ¼0·46
0·36, 1·07;
0·40, 2·36;
0·23, 0·94
Sayon-Orea
et al.
(24)
(2014)
Cohort NA NA 1232 Sex, age, family history of hypertension, self-reported
hypercholesterolaemia, physical activity, smoking status,
total energy intake, alcohol consumption, energy-adjusted
Na and K intake, caffeine intake, fibre intake, olive oil intake,
fruit intake, vegetable intake, low-fat and high-fat dairy
product intake, sugar-sweetened beverage intake, fast food
and sweets consumption, and time spent watching television
HR ¼1·18 1·01, 1·38
NA, not applicable; NR, not reported; T2DM, type 2 diabetes mellitus; RR, relative risk; RCT, randomised controlled trial; MedDiet, Mediterranean diet; HR, hazard ratio.
Vegetable oils and chronic diseases S43
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in frying oils (natural or added) might protect against post-
prandial oxidative stress in obese individuals
(35)
.
Furthermore, during cooking, molecular interactions occur
between oil compounds and the different foods cooked.
Oil becomes part of the product
(36)
and can therefore alter
other food components during cooking, being able to increase
or decrease the health and protective effects of some cooked
foods. An exchange of lipid compounds between the oil used
for frying and the food (fish) being fried has been demon-
strated, although it depends on the fat content of the raw
food. Furthermore, both the type of fish and the type of oil
were found to influence the oxidation status of the lipid
fraction. Ansorena et al.
(33)
found that the type of oil used
for frying significantly affected the nutritional quality of
low-fat fish than that of fatty fish as they absorb more fat.
Zotos et al.
(37)
found frying foods in olive oil to lead to
milder changes in the lipid profile compared with frying
them in sunflower oil. They found a complete change in the
fatty acid profile of anchovy samples, but the beneficial
fatty acids EPA and DHA and the n-3:n-6 ratio remained
at satisfactory levels. They found a high reduction in the
concentrations of fish beneficial fatty acids and a tremendous
reduction of the n-3:n-6 ratio when sunflower oil was used for
frying. Frying in olive or sunflower oil was found to reduce
cholesterol concentrations and significantly increase squalene
(an antioxidant intermediate in cholesterol biosynthesis)
concentrations, particularly when using olive oil
(37)
. Naseri
et al.
(38)
reported a decrease in the food n-3 fatty acid content
and an increase in the n-6 content as a consequence of the
migration of fatty acids from fish to the frying oil. Flores-
Alvarez et al.
(36)
also showed that the type of fried food
influences the oil used for frying: oil degradation was found
to be faster for fish nuggets than for French fries, and this
degradation may influence the oil content of the specific
food type fried. This oil uptake has been shown to differ
depending on the frying time or pressure
(39)
. When extra-
virgin olive oil is used for frying French fries, fish or different
vegetables, oil absorption occurs and the food absorbs anti-
oxidant compounds that get enriched with polyphenols.
Association between vegetable oil or fried food
consumption and the risk of type 2 diabetes or
the metabolic syndrome
A total of four cohort studies
(14,15,17,19)
and two studies
(16,18)
analysing the results of the PREDIMED (Prevencio
´n con
Dieta Mediterra
´nea) study, a randomised controlled trial, that
assessed the association between vegetable oil or fried food
consumption and the risk of T2DM or the MetS were included
in the present systematic review. Fried food consumption was
found to be directly associated with the risk of T2DM
(14)
and
the MetS
(15)
. However, the results of the SUN study indicated
no association between fried food consumption and the risk
of the MetS
(19)
. The PREDIMED study found an inverse
association between olive oil consumption and the risk of
T2DM
(16,18)
, while the SUN study did not find any association
between olive oil consumption and the risk of T2DM
(17)
.
The PREDIMED study is a parallel-group, multi-centre,
randomised trial that included 7447 participants (men aged
55–80 years; women aged 60– 80 years) with no CVD at
enrolment, who had either T2DM or at least three of the
following major risk factors: smoking; hypertension; elevated
• Overweight/obesity
Bes-Rastrollo(11)†
Haro-Mora(12)†
Sayon-Orea(13)*
• Type 2 diabetes mellitus
Halton(14)*
Salas-Salvado(16)†
Mari-Sanchis(17)†
Salas-Salvado(18)†
• CVD
Mozaffarian(20)*
Rastogi(26)‡
Kabagambe(27)§
Lockheart(28)†
Kontogianni(29)†
Bendinelli(30)†
Samieri(31)†
Guallar-Castillón(22)*
Buckand(23)†
Estruch(25)†
Author
10·125 0·25 0·5 2 4
Fig. 2. OR/HR and 95 % CI for the fully adjusted model in the studies included in the present systematic review. * Fried food consumption as exposure. † Olive oil
consumption as exposure. ‡ Sunflower oil consumption as exposure. § Palm oil consumption as exposure.
C. Sayon-Orea et al.S44
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LDL-cholesterol concentrations; low HDL-cholesterol concen-
trations; overweight or obesity; a family history of premature
CHD
(40)
. The participants were randomly assigned, at a 1:1:1
ratio, to one of the following three dietary intervention
groups: a Mediterranean diet supplemented with extra-virgin
olive oil (free provision of 1 litre/week for the participant
and his/her family); a Mediterranean diet supplemented with
nuts (a free 30 g/d provision of walnuts, hazelnuts and
almonds); a control low-fat diet. The principal aim of this
study was to assess the association between adherence to
the Mediterranean diet and cardiovascular risk; however,
secondary outcomes such as T2DM were also assessed. As
part of the intervention, the participants randomly allocated
to the two Mediterranean diet groups were advised (on a
quarterly basis) to exclusively use olive oil for culinary
purposes, including for frying. Salas-Salvado et al.
(16)
analysing only one participating centre of those included in
the PREDIMED study and excluding participants with T2DM
at baseline found that a Mediterranean diet supplemented
with olive oil in comparison with the control diet reduced
the risk of T2DM (HR 0·49; 95 % CI 0·25, 0·97), and they
reported that the possible biological mechanism responsible
for this effect might be the anti-inflammatory properties of
the virgin olive oil that might be relevant to the reduction of
the risk of T2DM. Recently, the PREDIMED investigators
have published the results regarding T2DM, but using data
from all the centres
(18)
. A total of 3541 patients free of T2DM
at baseline were included, and participants in the group of
Mediterranean diet supplemented with olive oil were found
to exhibit a significantly lower risk of developing new-onset
T2DM after the follow-up period (HR 0·60; 95 % CI 0·43, 0·85).
The next outcome that was analysed in the present
systematic review was the MetS as an ancillary component
to complement the results obtained for T2DM. The MetS is a
constellation of metabolic abnormalities that increase the
risk of heart disease and T2DM
(41)
. These metabolic abnorm-
alities include hypertriacylglycerolaemia, low HDL-cholesterol
concentrations, elevated blood pressure, impaired glucose
metabolism and central obesity
(42)
. The data of 8289 partici-
pants initially free of any MetS criterion were analysed in the
SUN study and the participants were followed up for a
median time period of 8·3 years. The mean age of the partici-
pants was 35·9 (SD 10·4) years. This study concluded that
frequent consumption of fried foods was not associated with
the incidence of the MetS (HR 0·93; 95 % CI 0·72, 1·20; Pfor
trend¼0·538) by comparing the highest (.4 times/week) v.
the lowest (,2 times/week) category of consumption.
However, two of the five components of the MetS, namely
central adiposity and high blood pressure, were found to be
positively associated with fried food consumption (HR 1·09;
95 % CI 1·00, 1·18; Pfor trend¼0·040, and HR 1·18; 95 % CI
1·04, 1·34; Pfor trend¼0·017, respectively) when comparing
the highest v. the lowest category of consumption after
multivariable adjustment. In the same study, a higher (but
not statistically significant) risk of developing the MetS was
observed among participants in the highest category of fried
food consumption and who used fats other than olive oil
for frying (HR 1·26; 95 % CI 0·71, 2·23; Pfor trend¼0·446).
The risk of developing the MetS among those in the
highest category of consumption but who used olive oil for
frying was lower (HR 0·85; 95 % CI 0·63, 1·15; Pfor
trend¼0·284). Therefore, it might be possible that different
types of oils used for frying may have different effects that
could explain these findings
(19)
. In this context, as a part
of the Pizarra study
(32)
, it was assessed whether insulin resist-
ance was cross-sectionally associated with cooking oils.
Samples of cooking oil being used were obtained from the
kitchen of 538 subjects, and after administration of 75 g of
glucose, oral glucose tolerance tests were conducted. The
results of this study showed that the levels of insulin resistance
were significantly lower in subjects who used olive oil
for frying than in those who used sunflower or a mixture of
several oils.
The possible biological mechanism could be explained
by the fact that olive oil can reduce the glycaemic load of a
meal, decreasing carbohydrate absorption rate, insulin secretion
and lipogenesis. MUFA appear to be powerful stimulators
of Glucagon-like peptide-1 (GLP1) secretion after an olive
oil-enriched meal, leading to better insulin sensitivity
(12)
.
Association between vegetable oil or fried food
consumption and the risk of CVD
A total of seven cohort studies
(20 – 24,30,31)
, four case–control
studies
(26 – 29)
and one randomised controlled trial study
(25)
that investigated the association between vegetable oil or
fried food consumption and the risk of CVD were included
in the present systematic review. The most relevant finding
in this context was the contribution of the PREDIMED study;
this study concluded that a Mediterranean diet supplemented
with extra-virgin olive oil led to a relative 30 % risk reduction
in the incidence of major cardiovascular events among
individuals at high cardiovascular risk. The PREDIMED inter-
vention study recommended the use of olive oil for frying in
the two active arms of the trial.
In contrast, in some observational studies, fried food
consumption was found to be directly associated with a
higher risk of CVD
(20)
and hypertension
(24)
, whereas olive
oil consumption
(23,29 – 31)
, use of vegetable oils
(28)
and mustard
oil consumption
(26)
were found to be inversely associated with
the risk of CVD. However, palm oil consumption was reported
to be positively associated with the risk of CVD
(27)
. Olive oil
consumption was also reported to be inversely associated
with the risk of hypertension
(21)
.
The European Prospective Investigation into Cancer and
Nutrition (EPIC) is a cohort study of 520 000 European men
and women that aimed to assess the impact of dietary, lifestyle
and genetic factors on the risk of cancer and other chronic
conditions. EPIC-Spain is a part of this large European
cohort study and comprised 41 440 participants.
The association between fried food consumption and the
risk of CHD was investigated in the EPIC-Spain. A total of
40 757 healthy adults were included in this study. During a
median follow-up period of 11 years, 606 incident cases of
CHD were identified. The multivariate HR for the fourth (high-
est) quartile compared with the first (lowest) quartile of fried
Vegetable oils and chronic diseases S45
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food consumption was 1·08 (0·82, 1·43; Pfor trend¼0·74). The
results of this study did not vary when stratified by the type of
oil used for frying (olive oil/sunflower oil). In this study, only
olive oil and sunflower oil were evaluated; therefore, authors
admitted that they cannot dismiss that frying with other types
of fats may still be harmful
(22)
.
The association between fried food consumption and the risk
of hypertension was investigated in the SUN cohort, including
data of 13 679 participants (5059 men and 8620 women) free
of hypertension at baseline. The mean age of the participants
was 36·5 (SD 10·8) years. During a median follow-up period of
6·3 years, 1232 incident cases of hypertension were identified.
The adjusted HR for developing hypertension among parti-
cipants with a higher frequency of baseline fried food
consumption (.4 times/week) was 1·21 (95 % CI 1·04, 1·41)
compared with those in the lowest category of consumption
(,2 times/week) (Pfor trend¼0·020)
(24)
. This result was
consistent with that of a cross-sectional study conducted in
538 participants, in which the association between fried food
consumption (specifically of reused vegetable oils) and the
prevalence of hypertension was evaluated. The results
showed that degradation due to the reuse of vegetable oils,
especially sunflower oil, was an independent risk factor for
hypertension and interestingly the serum concentration of
MUFA was found to be negatively associated with this risk
(32)
.
The Cardiovascular Heart Study (CHS) is a population-
based, longitudinal study of CHD and stroke that included
participants aged $65 years. This study was designed to
identify factors related to the onset and course of CHD and
stroke, specifically to determine the importance of conven-
tional cardiovascular risk factors in older adults and to identify
new risk factors in this age group, especially those that may be
protective and modifiable
(43)
. In this context, Mozaffarian
et al.
(20)
tested the hypothesis that fried fish consumption
could be inversely associated with fatal CHD. This study was
conducted in 3910 adults free of known CVD at baseline
with a mean age of 72 years. Over a mean follow-up period
of 9·3 years, 247 fatal CHD cases and 363 incident non-fatal
myocardial infarction cases were identified. The consumption
of tuna or other broiled or baked fish was found to be associ-
ated with a lower risk of total CHD deaths among individuals
consuming tuna/other fish $3 times/week than among those
consuming tuna/other fish ,1 time/month (HR 0·47; 95 % CI
0·27, 0·82; Pfor trend¼0·002), and contrarily, fried fish
consumption was found to be not associated with a lower
risk of total CHD deaths (HR 1·37; 95 % CI 0·48, 3·90; Pfor
trend¼0·35) or non-fatal myocardial infarctions (HR 1·93;
95 % CI 0·91, 4·08; Pfor trend¼0·11), but rather to be
associated with trends towards a higher risk. Mozaffarian
et al.
(20)
concluded that cardiac benefits of fish consumption
vary, according to the type of fish consumed.
The frying process has been reported to increase the hypo-
cholesterolaemic:hypercholesterolaemic fatty acid ratio and
reduce the n-3:n-6 ratio
(44)
. In addition, Naseri et al.
(38)
also
found that although frying led to a reduction of the n-3:n-6
content in olive oil, the reduction was not high. Vegetable
oils contain high quantities of n-6 PUFA mainly as linoleic
acid. The increase in the consumption of linoleic acid-rich
oils (e.g. soyabean oil and sunflower oil) is even associated
with a decreased n-3 long-chain PUFA content in human
body tissue. An increased intake of vegetable oils (via
excessive use in food processing) characterises high quantities
of n-6 PUFA and a deficit in n-3 PUFA
(45)
. Strobel et al.
(45)
found that fish processing methods (breading and frying in
vegetable oils) had a negative effect, increasing the linoleic
acid content and decreasing the n-3:n-6 ratio.
Lipid oxidation is much faster in cooked foods than in
fresh foods because cooking accelerates oxidation
(39)
. Olive
oil has been reported to confer cardiometabolic health
benefits when used for frying. However, in general, frying
increases the amounts of cholesterol oxidation products
and reduces the activity of paraoxonase, which is an
enzyme that inhibits the oxidation of LDL-cholesterol
(34)
. High
concentrations of phenolic compounds in olive oil have
protective effects against CVD
(46)
.
Conclusions
From the results of the studies included in this review, we
conclude that (1) the myth that frying foods is generally
associated with a higher risk of CVD is not supported by the
available evidence; (2) extra-virgin olive oil significantly
reduces the risk of CVD clinical events and weight gain,
based on the results of a large randomised trial that included
as part of the intervention the recommendation to use
large amounts of extravirgin olive oil for culinary purposes,
also for frying foods; however, the whole Mediterranean
dietary pattern plays a more significant role rather than the
supplemental extra-virgin olive oil alone; and (3) high con-
sumption of fried foods is probably related to a higher risk
of weight gain and hypertension. However, many factors
such as the type of oil used and the frying technique
(deep-frying or panfrying), frying duration and temperature,
and use of new or reused oils for frying have to be taken
into account.
The authors’ contributions were as follows: MAM-G was
responsible for the initial plan and study design, CS-O was
responsible for the data collection, data extraction; MAM-G,
SC, and CS-O were responsible for the data interpretation,
manuscript drafting, critical revision of intellectual content
and approval of the final version of the manuscript.
Acknowledgements
None of the authors has any conflicts of interest to declare.
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