Drinking your Greens: Green Smoothies from a Nutritional and Toxicological Point of View

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Drinking your Greens: Green Smoothies
from a Nutritional and Toxicological
Point of View
Julika Lietzow, Benjamin Sachse, Bernd Schäfer
Introduction
A smoothie can be prepared quickly and is
recognized as “natural and healthy”. There-
fore, smoothies have become a real food trend.
More than 15 million people in Germany
consume smoothies once or several times a
month [1]. The beverage has a notable role in
the raw food diet, in which the consumption
of raw and largely unprocessed plant food is
of priority.
Freshly prepared green smoothies contain
numerous nutrients - including vitamins,
minerals and dietary bers, and, therefore,
can signicantly contribute to a healthy diet.
However, to avoid health risks, some points
should be considered with regard to selecting
ingredients and preparing the smoothie. For
example, in addition to plants or parts thereof
traditionally consumed as food, also ingredi-
ents that have not been eaten raw or at all
in the past are used in smoothie recipes, and
might pose a health risk.
This review should draw attention to the
fact that the consumption of green smooth-
ies may, in principle, also lead to health
risks. In addition to the description of nu-
tritional aspects and an overview of ingredi-
ents of potential health concern, recommen-
dations for a safe preparation of smoothies
are provided.
Nutritional aspects
The term “smoothie” is not legally protected
and no binding denition exists. Therefore,
there are no consistent rules which ingredi-
ents have to be added to the drink and at what
quantities. Usually, a green smoothie consists
of fruits as well as vegetables and/or greens
that are mashed with water to a smooth
purree. Hence the name “smoothie” - derived
from the English word “smooth” [2].
Abstract
Green smoothies often contain benecial ingredients (e.g. vitamins, min-
erals and bers) and can enrich the daily diet. However, with regard to
the selection of ingredients, some points should be considered in order to
avoid health risks. First, it has to be kept in mind that a high fruit content
may result in a considerably high energy intake. In principle, only plants or
parts thereof should be used in smoothies which have also been used tradi-
tionally as foods and, therefore, are regarded as safe. Further, all ingredients
should be used with as much variety as possible in order to avoid an unbal-
anced nutrient supply and to avoid a high long-term intake of potentially
harmful substances. Special care should be taken when using self-collected
wild herbs, since, among other things, confusion with poisonous plants
may occur. It is also recommended to use ingredients as fresh as possible
and to follow the general rules of kitchen hygiene.
Keywords: Green smoothies, health risks, plant toxins, contaminants, raw
food
Citation
Lietzow J: Drinking your Greens: Green Smoothies from a Nu-
tritional and Toxicological Point of View. Ernahrungs Umschau
2022; 69(8): 126–35.
The English version of this article is available online:
DOI: 10.4455/eu.2022.024
Peer reviewed
Manuscript (original) subbmitted: 24. February 2022
Revision accepted: 02. May 2022
Corresponding author
Dr. Julika Lietzow
Bundesinstitut für Risikobewertung
Max-Dohrn-Str. 8–10, 10589 Berlin
Abteilung Lebensmittelsicherheit
julika.lietzow@bfr.bund.de

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Recipes often recommend the use of whole fruits, including the
peel and the seeds. Some recipes also include other ingredients
such as wild or culinary herbs, kernels, nuts, spices or so-called
“superfoods” (table 1) [2]. “Superfoods” are not legally dened
and are commonly understood as foods that have certain nutrient
proles, such as particularly high levels of vitamins, minerals or
dietary bers. However, the health-related claims associated with
“superfoods” are often not scientically proven [3].
Priority is often given to the health-promoting characteristics of
smoothies, especially due to the high levels of vitamins, minerals,
antioxidants and dietary bers. As promised, smoothies, among
other things, are intended to support, for example, the process
of “detoxication” – a statement that has not been supported by
scientic evidence [4].
In particular, green smoothies are also often used for weight-loss
purposes due to their high proportion of vegetables, even if this
is not always promising. The green juices can have a high energy
content because of their high levels of naturally occurring sug-
ars, especially through the added fruits. For example, a self-pre-
pared smoothie with 200 g of leaf spinach, a banana and half an
apple contains about 30 g sugar and more than 200 kilocalories
[5] (table 2). Furthermore, “Stiftung Warentest” recently noted
that the content of natural sugars in green
smoothies from retailers can vary between 5
and 11 g per 100 ml [6]. This is in the same
order of magnitude as soft drinks, which usu-
ally contain 10 g sugar per 100 ml. The Ger-
man Nutrition Society (DGE) recommends not
to exceed a maximum intake of 50 grams of
free sugars per day for adults (7). It makes lit-
tle difference whether this is natural or added
sugar.
In addition, energy-dense drinks such as fruit
and vegetable juices result in a lower com-
pensatory response to food intake than solid
foods. For example, the satiating effect is less
pronounced [8]. Fruits and vegetables contrib-
ute signicantly to a healthy and diverse diet
in daily life. The consumption of one glass of
smoothie per day can supplement the diet.
However, due to their high calorie content it is
suggested not to consume fruit and vegetable
smoothies in addition to the recommended ve
portions of fruit and vegetables per day, but to
replace one portion occasionally [9].
Potential health risks resulting
from the consumption of green
smoothies
Plants naturally contain various ingredients,
whose levels can also differ from plant part
to plant part. As a result of this diversity,
the plant ingredients are often not or insuf-
ciently characterized toxicologically. There-
fore, the safety of traditionally consumed
foods is generally substantiated by the “his-
tory of safe use” [10]. On the other hand, for
plants or parts thereof that have not been con-
sumed so far, such information is usually not
available. Thus, indications of harmful effects
often arise only in relation to the occurrence
of poisoning cases or intolerances. A particu-
lar problem: Potentially carcinogenic effects
often remain undetected as such effects usu-
ally occur decades after the harmful substance
was ingested.
Hence, a general estimation whether plants
or parts thereof pose a health risk or not is
usually not possible. In some cases, a bitter
taste can be a simply perceptible indication
for consumers concerning potentially toxic
substances. However, this may not always be
true-bitter-tasting compounds are not toxic
per se, nor do all toxins have a bitter taste.
Therefore, some points, that are described
Fruits • Bananas
• Apples, pears
• Berries (including frozen foods)
• Pineapple
• Oranges
• Oiwi
• Grapefruit
• etc.
Vegetables
and leafy
greens
• Salads (lettuces, rocket etc.)
• Kale
• Spinach, chard
• Avocado
• Chives, cress
• Carrot green, radish leaves, kohlrabi leaves, beetroot
leaves, celery leaves
• Culinary herbs (parsley, basil, sage, etc.)
• Wild herbs (dandelion, chickweed, nettle, sorrel,
etc.)
• Leaves of various trees and bushes (e.g. lime tree,
blackberry, hibiscus)
Other
Ingredients
• „Superfoods” (goji berries, chia seeds, axseed)
• Sweeteners (honey, stevia, dates, birch sugar)
• Herbs and spices (culinary herbs, wild herbs, chilli,
vanilla, cinnamon, cocoa, etc.)
• Special water (coconut water, spring water, distilled
water)
• Matcha
• Acerola
• Ginseng
• Activated charcoal
• Medicinal herbs
• Protein powder
• Plant powder, e.g. wheatgrass powder
Tab. 1: Typical ingredients for the preparation of green smoothies
[2], (Free web search)

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further below, should be taken into account when selecting the
ingredients for green smoothies.
Toxicologically relevant substances in edible plants or
parts thereof
Consumption of green smoothies can result in an intake of un-
usually large amounts of certain plant ingredients that are not
taken up through the traditional consumption of plant-based
food. In consequence, health risks may result from ingredients
which are actually not a problem for healthy individuals if con-
sumed in small quantities. Some of these examples are described
below.
Oxalic acid
Plants belonging to the families Amaranthaceae (foxtail) and Po-
lygonaceae (knotweed) are known for their high levels of oxalic
acid. The best-known representatives are spinach, mangold, am-
aranth and quinoa, as well as sorrel and rhubarb. Some of these
plants contain oxalic acid levels of more than 100, some have
more than 500 mg per 100 g fresh weight [11].
Oxalic acid is an organic acid and its salts are called oxalates. While
potassium and sodium salts are highly water-soluble, oxalic acid
forms complexes of low solubility with some other minerals, such
as calcium. Plant-based foods contain both soluble potassium ox-
alate and insoluble calcium oxalate [12].
Ingredients 1 banana 1/2 apple Baby spinach Lemon juice Total*
Quantity 125 g 75 g 200 g 30 ml
Water g 94 64 185 28 370
Calories kcal 123 46 54 6,6 229
Macronutrients
Protein (total) g 0,9 0,1 5,7 0 ,1 6,8
Fat (total) g 0,4 0,1 1, 2 0,1 1, 8
Carbohydrates (total) g 29 11 4,8 2,1 47
• Fiber g 2,5 1,6 3,2 0,1 7,4
• Sugar (total) g 20 8,9 0,8 30
of which sucrose g 5,3 1,5 0,1 6,9
of which fructose g 7, 6 5,9 0,3 14
of which glucose g 6,9 1,5 0,3 8,7
Minerals and vitamins
Calcium mg 6,3 5,3 136 1, 8 149
Sodium mg 4,0 0,5 222 0,3 227
Potassium mg 408 80 1164 31 1682
Magnesium mg 35 3,7 186 1,8 226
Phosphorus mg 28 6,0 78 2,4 114
Vitamin C mg 15 3,5 53 12 83
Iron mg 0,4 < 0,1 2,5 < 0,1 3,0
Tab. 2: Nutritional profile of a green smoothie (sample recipe)
[source: U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, 2019-2021]
* corresponds to about 2 servings (glasses), depending on the addition of liquids (e.g. 200 ml of water)
In general, foods composed of vegetables
rich in oxalic acid do not pose a health risk
to healthy people when regularly consumed.
However, when spinach, but also chard, is
used in relatively large quantities (e.g. reci-
pes with more than 200 g of leaf spinach per
smoothie) as well as in unprocessed raw form
in green smoothies, it can contribute to a very
high intake of oxalic acid.
A high chronic intake of oxalic acid or its sol-
uble salts can result, together with minerals,
such as free calcium, in the formation of com-
pounds with low solubility in the intestinal
lumen which can lead to a deciency of these
minerals by the respective persons [11].
According to the European Food Safety Au-
thority (EFSA), the dietary intake of more than
180 mg of oxalic acid per day signicantly en-
hances the urinary oxalic acid excretion (hy-
peroxaluria) [13]. After systemic absorption,
the oxalic acid can form calcium oxalate com-
plexes with low-solubility, that may crystal-
lize in the kidney at higher concentrations,
which may in turn lead to an increase of the
risk of kidney, urinary or bladder stones. A
low urine volume and a low urine pH as well

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as decits in the supply of minerals can promote the formation of
stones. The simultaneous intake of calcium-rich foods (e.g. dairy
products or calcium-rich mineral water) counteracts the systemic
absorption of oxalic acid as a consequence of the formation of
low-soluble calcium oxalate complexes in the intestinal lumen,
which are then eliminated via feces. However, as described above,
this also reduces the absorption of calcium [12].
Acute poisonings caused by the intake of foods rich in oxalic acid
are not widely known. However, the scientic literature describes
cases in which a high consumption of oxalate-rich foods (more
than 1 g oxalate intake per day), e.g. via green vegetables or juices,
over an extended period of time caused kidney damage – in most
cases in people with pre-existing renal damage [14-16].
Soaking or cooking vegetables markedly reduces the content of
oxalic acid (30-87%) because the water-soluble oxalic acid passes
into the cooking water [11]. However, smoothies are usually
made with raw ingredients, and therefore, the intake of oxalic
acid can be relatively high [17].
Nitrate and nitrite
Leafy greens such as spinach, chard or rocket belong to the ni-
trate-accumulating plants and may contain high levels of nitrate,
depending on season and cultivation area [18]. Nitrate can be con-
verted to nitrite either in the food itself or in the human body (by
nitrate-reducing bacteria in the mouth or gastro-intestinal tract,
or enzymatically). Nitrite reduces the ability to transport oxy-
gen through the red blood cells, because it oxidizes hemoglobin
to methemoglobin which is no longer capable to reversibly bind
oxygen. This may result in an inadequate oxygen delivery to the
tissues, which can be particularly dangerous for infants [19]. In
addition, nitrite may contribute to the formation of a group of
compounds known as nitrosamines, some of which are considered
carcinogenic [20].
EFSA derived an acceptable daily intake for nitrate of 3.7 mg per
kg of body weight and for nitrite of 0.07 mg per kg of body
weight [20, 21]. A daily nitrate intake of 260 mg over a lifetime is
therefore unlikely to cause adverse health effects in a person with
a body weight of 70 kg. In order to protect consumers, EU-wide
maximum levels are set for nitrate in various leafy vegetables
such as spinach and fresh lettuce [22]. Nevertheless, these foods
contribute to an overall high nitrate intake.
Nitrate levels in foods can vary depending on growing conditions
and season. For example, depending on the sunlight exposure,
eld vegetables (leafy greens/lettuces) and vegetables harvested
in summer have lower levels of nitrate compared to vegetables
harvested in the greenhouse or in winter [23]. Analysis of lettuces
and leafy greens grown in the greenhouse showed average nitrate
concentration of 2,950 mg/kg. By comparison, open-eld lettuces
and leafy greens had up to 37% lower nitrate values - the average
concentration was 1 865 mg/kg [24]. In addition, nitrogen ferti-
lization also plays a major role in terms of nitrate accumulation
in plants. In organic farming the use of mineral nitrogen fertilizer
is not allowed, therefore organic vegetables usually have lower
nitrate levels [25].
Non-digestible carbohydrates in raw cabbages
Cabbage varieties have always been used in
meals such as stews and soups or to make
sauerkraut. In addition to numerous vi-
tamins and minerals, they contain lots of
bers and secondary plant ingredients such
as the pungent mustard oil glycosides, also
known as glucosinolates. Cabbages, e.g.
kale, are also often used as an ingredient
in green smoothies due to their high nu-
trient content [26]. It is generally known,
that the consumption of cabbages may lead
to gastrointestinal complaints such as at-
ulence, diarrhea and feeling of fullness up
to abdominal pain, dependent on the intake
level. The reason for these symptoms may
be the non-digestible carbohydrates in cab-
bage (e.g. rafnose in kale) which are not
or not completely enzymatically hydro-
lyzed and reach the colon undigested or not
completely digested, and are then fermented
by bacteria. Odorless gases such as carbon
dioxide, hydrogen and methane can thereby
be formed which can lead to severe atu-
lence and are thus the cause of the symp-
toms described [27, 28].
Traditionally, various cabbage meals are
seasoned with ingredients such as savory,
dill, fennel or caraway seeds to alleviate
atulence symptoms. In addition, soaking
and cooking vegetables can reduce the level
of non-digestible bers [29, 30]. In general,
the severity of gas production after intake of
cabbages in healthy people is very different
and depends, among other things, on the in-
dividual metabolic activity and the compo-
sition of the intestinal ora [31]. Therefore,
the digestibility of (raw) cabbage differs in-
dividually and should be considered when
using cabbages in smoothies.
Goitrogens
Various vegetables of the Brassica family, in-
cluding cabbages, may contain goitrogenic
substances. These compounds can impair the
iodide uptake by the thyroid gland and the syn-
thesis of thyroid hormones. This can especially
occur when large quantities are consumed
and in cases of simultaneous iodine deciency.
However, the presence of those substances var-
ies between the different Brassica plants and is
considerably dependent on the methods of pro-
cessing and preparation [32, 33]. People with
thyroid diseases (e.g. enlargement of the thy-
roid gland) and simultaneous iodine deciency
should therefore avoid frequent and excessive
consumption of (raw) cabbages.

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Interactions between plant
ingredients and drugs
People taking drugs should consider that some plant ingredients
may lead to interactions with certain drugs – either by reducing
their effect or by enhancing the side effects. The consumption of
green smoothies can lead to relatively high intakes of such plant
ingredients.
An example of an interaction is vitamin K in leafy greens, which
can reduce the effect of some anticoagulant drugs (e.g. phen-
procoumon or warfarin). In addition, goji berries contain sub-
stances that may also interact with anticoagulant drugs such as
phenprocoumon. Another example is the St John’s Wort listed in
different recipes, which may, among other things, interfere with
the effect of oral contraceptives (“pill”). Further, furocoumarins
occurring in citrus fruits can affect the bioavailability of drugs
by inhibiting their enzymatic degradation and thus increase the
risk of adverse reactions [34]. The relevance of such interactions
in the context of the consumption of green smoothies has not
been studied systematically. However, a clinically relevant in-
teraction potential is realistic due to the potentially high intake
of secondary plant ingredients via the consumption of green
smoothies.
Use of parts of plants usually not consumed
General conclusions regarding possible health risks due to the
use of plant parts that are usually not consumed, such as leaves,
stems, rinds or pits, cannot be made. In many cases, the con-
sumption of such parts does probably not pose a health risk - but
this might not always be the case. Important to note: Parts of
plants can contain harmful substances even if other parts are safe.
Potatoes and tomatoes are given here as an example. Usually, the
potato tuber or tomato regularly consumed contains low levels
of toxic glycoalkaloids, like α-solanine, which protect the plant
against harmful organisms (pests). However, higher levels can be
found in the leaves. For example, the potato tuber usually has a
glycoalkaloid content below 150 mg/kg, whereas the leaves can
contain up to 1,000 mg/kg and owers and sprouts can even
reach several thousand mg/kg. Intake of 1 mg/kg body weight or
more can lead to acute poisoning effects, such as gastrointestinal
symptoms [35].
Risk to human health can also be related to the kernels of
certain fruits. For example, while the seed kernels of the wa-
termelon can be consumed without presenting an appreciable
risk, the kernels of almost all stone fruit species (e.g. almond,
apricot, plum, apple) contain so-called cyanogenic glycosides
such as amygdalin, that can release cyanide upon consump-
tion [36, 37]. As often, the dose makes the poison: The in-
take of few apple seeds, e.g. by eating a single apple, does
usually not represent a health problem. On the other hand,
it is not recommended to consciously consume larger quan-
tities or larger kernels of stone fruits (e.g. apricot kernels,
bitter almonds) because this might result in problematically
high intake levels. For adults, the consumption of two bitter
apricot kernels or one bitter almond is considered safe. In con-
trast, such quantities can already have serious consequences
for young children and, dependent on the dose, can lead to
clinical signs of toxicity including headache and dizziness,
up to dyspnea, convulsions, coma and
death [38]. Beside stone fruit kernels, ax
seeds contain cyanogenic glycosides, too.
However, intake levels of 15 g per meal are
considered safe because the amount of cy-
anide released can easily be detoxied by
the body [39].
Avocado pits are also frequently claimed
to be used as an ingredient in smoothies. A
well-known ingredient in all over-ground
plant parts of the avocado, including the
fruit, is persin. It is known that persin can
lead to poisonings in various domesticated
species and livestock–for humans it is con-
sidered non-toxic [40]. However, the avail-
able data are not sufcient to conclusively
assess potential risks. The authors have cur-
rently no evidence regarding the occurrence
of adverse health effects in humans when
eating avocado pits–poisoning cases have
not yet been documented.
Moreover, it should be considered that leaves
and stems used for smoothies can contain
higher levels of pesticide residues or environ-
mental contaminants than the parts of the
plant usually consumed as food. According
to the Federal Ofce of Consumer Protection
and Food Safety (BVL), for example, multi-
residues of pesticides were detected more fre-
quently in the leaves of kohlrabi or radishes
in 2020 than in the corresponding edible tu-
bers [41].
In the European Union, maximum residue
levels have been set for pesticides to ensure
the safety of consumers. However, the levels
often have only been set for plant parts usu-
ally consumed, and accordingly are moni-
tored only for these parts [42]. Organic fruits
and vegetables usually have lower levels of
pesticide residues than conventional products
[43].
Use of wild plants
Other ingredients that are often used for
green smoothies are wild herbs, wildowers
or blossoms. In this case, it is very impor-
tant to inform yourself beforehand about
the used plants. In principle, there are two
potential risks by using wild herbs and
wildowers:
• Some herbs, regarded as edible, contain
harmful substances. For example, bor-
age, coltsfoot and comfrey contain so-
called pyrrolizidine alkaloids, which are
mutagenic and carcinogenic. A safe level
of intake cannot be identied for these

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substances according to the current state of knowledge.
The intake of these substances should therefore be as low
as possible [44, 45]. Accordingly, these herbs should only
be exceptionally used in smoothies. Vulnerable popula-
tion groups such as pregnant and breastfeeding women or
young children should better avoid the use of these herbs.
• A further risk might exist with respect to the possibility of con-
fusion between certain wild herbs and known poisonous plants
(table 3). One example: Confusion of leaves of wild garlic with
those of lily of the valley or meadow saffron. Both lily of the
valley and meadow saffron contain strong toxins, which can
also lead to fatal poisoning [46]. There is also a possibility of
confusion between cows parsley and poison hemlock as well as
yarrow and poison hemlock and many other wild herbs. The
consumption of poison hemlock causes death by suffocation
already after a short time [46]. Recently, a case of poisoning has
been reported in which a 43-year-old woman showed symp-
toms of poisoning after drinking a smoothie prepared from col-
lected wild plants, likely due to the confusion of common sorrel
with the poisonous plant foxglove [47].
“Exotic” ingredients
Aloe vera leaves
The leaves of plants of the genus Aloe have been considered as
remedy for centuries, e.g. for the treatment of sunburn and skin
eczema, and, more recently, are also used as an ingredient in
smoothies. With regard to consumption it should be noted that
the juice of the leaf rind of plants of the genus Aloe contain an-
thranoids (syn. anthraquinones) which have a laxative effect at
higher doses. In addition, there is some evidence that these sub-
stances could be carcinogenic. However, the gel obtained from the
inner gel layer does not contain anthranoids [48].
Therefore, only the inside layer of the leaf, i.e. the plant gel,
should be used for Aloe vera preparations. Nevertheless, anthra-
noids can pass from the leaf bark into the gel even when care-
fully prepared, thus, it is advisable not to use self-made Aloe vera
gels [49]. From a toxicological point of view,
whole leaves of Aloe vera are generally not
suitable for consumption. Aloe vera products
from retailers have to comply with food law
and should be preferred here.
Water lentils
An exotic ingredient for green smoothies are
water lentils, commonly called duckweed.
Originally and primarily consumed in Asian
countries, the green owering plants have
also been included in various smoothie reci-
pes for some time now. The small water len-
tils have a favorable amino acid and fatty
acid composition, and otherwise consist of
more than 90% water. In addition, they can
efciently absorb essential minerals from
the water or culture medium. However,
they also accumulate toxic substances, such
as heavy metals, and therefore, are used for
wastewater treatment or phytoremediation
[50, 51].
Ideas for recipes with “self-harvested” or
“self-cultured” water lentils are commonly
found in online forums. However, usually
it is not described what exact type of the
ve known genera should be used as an in-
gredient. Due to the characteristics described
above, it is not advisable to culture water
lentils on its own or to collect them from
ponds because several environmental con-
taminants can be absorbed depending on the
water quality.
In the EU, water lentils are considered as
novel foods and, as such, require pre-mar-
ket authorization. Recently, the water len-
Edible
wild herbs
Poisonous or
inedible plants
Toxic ingredients Adverse eects
Wild garlic
(Allium ursinum)
Lily of the valley
(Convallaria majalis)
Meadow saffron
(Colchicum autumnale)
Cardiac glycosides
(e.g. convallatoxin)
Alkaloids
(e.g. colchicine)
Gastrointestinal symptoms,
cardiac arrhythmia
Painful swallowing, gastrointestinal
symptoms, seizure, paralysis
Wild chervil
(Anthriscus sylvestris),
Yarrow (Achillea
millefolium)
Hemlock
(Conium maculatum)
Alkaloids
(e.g. coniine)
Burning sensation in the mouth,
gastrointestinal symptoms, seizure,
paralysis
Comfrey
(Symphytum
ofcinale)
Foxglove
(Digitalis spp.)
Cardiac glycosides
(e.g. digoxin)
Gastrointestinal symptoms, cardiac
arrhythmia, hallucinations
Corn mint
(Mentha arvensis)
Pennyroyal mint
(Mentha pulegium)
Essential Oil
(e.g. pulegone)
Liver injury
Dandelion (Taraxa-
cum ofcinale)
Ragwor
(Senecio spp.)
Pyrrolizidine alkaloids Liver injury,
possibly carcinogenic
Tab. 3: Possibilities of confusion between edible wild plants and toxic or inedible plants [46]

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til species Wolfa globosa and Wolfa arrhiza have been au-
thorized in the EU and thus can be marketed in the future
[52].
Black Smoothies
Another trend are “black smoothies”. They are intended to detox-
ify the body and have been reported to help, for example, against
symptoms of excessive alcohol consumption [53]. In many reci-
pes, the addition of 1/2 teaspoon of activated charcoal is suggested
to prepare a “black smoothie”. This amount already corresponds
to the dose used for pharmaceutical purposes, as activated char-
coal is mainly used to treat diarrhea and certain kinds of intoxi-
cations [54]. Due to its large surface area, the charcoal can bind
various (harmful) substances within the gastrointestinal tract
which are then excreted with the feces [55].
Due to its very non-specic mode of action (adsorption of or-
ganic compounds by van der Waals forces) [56] it is assumed
that large amounts of activated charcoal in smoothies can lead
to nutrient deciency (e.g. vitamins), albeit scientic studies
regarding undesirable effects in relation to the intake of acti-
vated charcoal via foods are limited. Principally, charcoal can
also affect the efcacy of drugs (including contraceptives or
painkillers) and can lead to constipation [57].
Activated charcoal (vegetable carbon) is also used in the food
industry as food colour E153 (carbo medicinalis vegetabilis) in
some food categories [58]. However, the amounts of vegetable
carbon used here are usually much lower compared to “black
smoothie” recipes. In general, regular consumption of activated
charcoal in larger quantities is not recommended due to the ef-
fects described.
Plant-based powder mixtures
Plant-based powder (mixtures) can also be part of green smoothie
recipes and are promoted to some extent as “superfood” powder.
Examples include moringa powder, barley and wheatgrass pow-
der, spirulina powder or guarana or matcha powder.
It should be noticed that these herbal powders possibly can con-
tain high levels of certain substances, such as caffeine from guar-
ana seeds or matcha tea leaves. One teaspoon (about 1.5 g) guara na
powder can contain as much caffeine as about a cup of coffee [59,
60].
It has been shown that matcha powder may contain high levels of
aluminium and thus, can cause a relevant contribution to the al-
ready high dietary intake of aluminium. The European Rapid Alert
System for Food and Feed RASFF also reported on contamina-
tion of moringa powder or spirulina powder with heavy metals,
carcinogenic polycyclic aromatic hydrocarbons (PAH) and other
toxins [41, 61].
Microbial risks
The preparation of green smoothies using fresh fruits and vegeta-
bles and other plants or parts thereof should in any case comply
with the general rules of kitchen hygiene.
During cultivation as well as during further treatment and pro-
cessing, plant-based ingredients can be contaminated with germs.
For this reason, the plant-based foods should be washed thor-
oughly in fresh water before cutting. Freshly produced green
smoothies should also be stored at a maxi-
mum temperature of 7 °C until consump-
tion and should be used on the day of man-
ufacture. The addition of sour juices or citrus
fruits (without peel) can reduce the prolifera-
tion of microorganisms [62].
Moreover, prepackaged frozen berries from
retailers can also pose a risk for foodborne
infections. Microorganisms associated with
foods from frozen berry mixtures frequently
cause serious diseases. For example, norovi-
rus can cause severe diarrhea. Therefore, it is
recommended not to use raw frozen berries
in smoothies, and to heat them beforehand in
order to inactivate potential microorganisms
[63].
Furthermore, bacteria can convert nitrate
contained in smoothies to nitrite in case of
improper storage and/or neglect of common
hygiene practice. A high nitrate or nitrite in-
take can be dangerous especially for young
children, because of inadequate oxygen deliv-
ery (methemoglobinemia) [19, 64]. For this
reason, the leftovers of smoothies contain-
ing vegetables rich in nitrate should be cooled
down as quickly as possible in order to reduce
nitrite formation by microorganisms.
Especially sensitive consumer groups, such as
immunocompromised individuals, pregnant
and breastfeeding women and (young) chil-
dren should follow the general rules of kitchen
hygiene [65].

Ernaehrungs Umschau international | 8/2022
133
Conclusion
Green smoothies represent a signicant contribution to the overall
intake of fruits and vegetables, and can enrich the daily diet. They
often contain numerous benecial ingredients - especially when
prepared freshly. From a nutritional point of view, however, it
should be noted that especially smoothies rich in fruits contain
high levels of naturally occurring sugar resulting in a signicant
energy intake. Care should be taken regarding freshness and com-
pliance with good kitchen hygiene when using the ingredients.
It is recommended to restrict the choice on plants or parts thereof
in smoothies, which are also traditionally consumed as food and
can therefore be considered as safe. In this context, plant-based
ingredients usually eaten raw are particularly suitable. Special
caution is important when laypeople collect wild herbs, as there
is a possibility of confusion between some wild herbs and poi-
sonous plants. It is generally recommended to use varying kinds
of fruits, vegetables and other plants or parts thereof in order to
avoid an unbalanced nutrient supply on the one hand and to avoid
a long-term high intake of potentially harmful substances on the
other hand.
Conflict of interest
The authors declare that there is no conict of interest.
Dr. Julika Lietzow1
Dr. Benjamin Sachse2
Prof. Dr. Bernd Schäfer3
German Federal Institute for Risk Assessment
Max-Dohrn-Str. 8–10, 10589 Berlin
Department of Food Safety
1 julika.lietzow@bfr.bund.de
2 benjamin.sachse@bfr.bund.de
3 bernd.schaefer@bfr.bund.de
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