As I progress through my nutritional studies, we are asked to write plenty of academic essays and other not-so-fun (although very interesting) stuff. So I decided to share one of my first evidence-based works. Evidence overwhelms - get ready!
In the last decade, the percentage of vegans has increased by 350% (1).
Up to 2% of British adults were reported to eat largely plant-based diets (2)
Veganism has become a popular dietary approach, characterised by a complete abstinence of animal-based foods, such as meat, dairy, eggs and honey and consisting of plant foods like grains, vegetables, fruits and nuts.
Fully plant-based eaters scored highest on the Healthy Eating Index (3). Omnivores, however, had the lowest grade. Vegans also demonstrated a 75% risk reduction for hypertension and 47%–78% for type 2 diabetes (4) However, if not planned correctly, #vegandiet may result in a reduction of caloric intake and nutritional deficiency in macro- and micronutrients. The EPIC-Oxford study with the largest sample of vegans worldwide showed a lower intake of saturated fatty acids (SFA), vitamins B12 and D, calcium, zinc, although featuring a higher intake of fibre, magnesium, iron, folic acid in vegans compared to omnivores (5).
In general, animal-based foods provide high-quality protein, heme-iron, cholecalciferol, docosahexaenoic acid (DHA), vitamin B12, creatine, taurine, carnosine – all the compounds not present in plant-based foods (6). They are also recognised as a superior source of protein due to a complete composition of essential amino acids with high digestibility (>90%) and bioavailability (7).
The Digestible Indispensable Amino Acid Score (DIAAS) for animal protein is usually ≥1, outperforming plant proteins (legumes: 0.6; cereals: 0.3–0.5) and only approximated by soy (0.8–0.9) (8). Protein found in legumes is limited in amino acids methionine and cysteine; cereals lack lysine and tryptophan, whereas vegetables, nuts and seeds are short of methionine, cysteine, lysine, and threonine.
However, this may be considered a limiting factor only for vegans who would solely eat rice or beans for a substantial period of time (9). Importantly, the limited data collected so far in humans, contrary to historical findings in rats, has shown that the differences in the digestibility of both protein sources are only a few per cent. For pea and wheat flour the figures were 89–92%, similar to those found for eggs (91%) or meat (90–94%), and slightly lower than those reported for milk protein (95%).
Leroy and Barnard (2020) reported that over 90% of vegans were deficient in vitamin B-12, compared with 1 in 10 omnivores. This vitamin is detrimental to neurological and cognitive health when intake is low, which is also associated with stroke, Alzheimer’s and Parkinson’s diseases (10). A total intake of vitamin B12 from the diet between 4 and 7 µg/d is associated with its normal plasma level.
Nevertheless, bioavailability from different animal products remains unclear. The main plant-based sources are broccoli, asparagus, #tempeh and edible algae like Chlorella, fermented foods and #nutritionalyeast, although many of them contain only traces of vitamin B12 and should not be considered as the main daily intake (11).
Although contamination of food with microorganisms, insects, and synthesis by gut bacteria may also contribute significant amounts of #B12 to the diet of even the strictest vegetarian, about 50% of the vegan dieters from the EPIC-Oxford study (12) showed vitamin B12 deficiency, therefore, supplementation is highly recommended for this dietary group.
Low concentrations of #VitaminD, related to both calcium absorption and bone mineralization, have been documented in vegan societies (13), especially in winter or spring. Vitamin D3 (cholecalciferol) can originate from plants or animals, whereas vitamin D2 (ergocalciferol) is produced by the action of ultraviolet radiation. Therefore, mushrooms treated under ultraviolet light can be an important source of vitamin D, as well as fortified cereals and milk substitutes (14).
#VitaminK2 found in animal products may play a protective role for bone and cardiovascular health, beyond the effects associated with plant-associated vitamin K-1 (15).
Another risk for plant-based dieters is #irondeficiency which could contribute to #anaemia, possibly leading to fatigue, alterations in neurobiological systems provoking impaired cognitive functioning. Both, iron and ferritin levels in the blood are lower in vegans than in non-vegans and may not reach the UK daily RNI levels of 8.7 mg and 14.8mg for men and women respectively.
Two-thirds of body iron is derived from heme because much of the non-heme iron from plant sources is not bioavailable due to chelators that precipitate the iron, making it unavailable for absorption. This can be counteracted by consuming ascorbic acid in citrus fruits. Other sources are: legumes, whole grains, dark-green leafy vegetables and nuts (16).
Iron metabolism, in turn, is dependent on #zinc acting as a catalyst and is not as easily absorbed from plant sources compared to animal products. Zinc-rich plant foods are nuts, whole grains, soybeans, cabbage, radish and watercress. Therefore, frequent blood tests and supplements may be required to avoid nutrient deficiencies or nutrient overdose-related toxicity (17).
#Calcium absorption may also be impaired due to oxalate as well as phytate and fibre found in vegetables. The best absorption is provided by low-oxalate vegetables, including broccoli, kale and bok choy, including plant-based milk substitutes (18).
Previously regarded as anti-nutritional compounds, #phytochemicals are now increasingly associated with beneficial effects which regulate blood glucose levels, improve lipid profile and reduce the risk of certain cancers (19). Polyphenols are found in grapes, berries, and nuts, indole-3-carbinol in cruciferous vegetables such as sprouts and cauliflower, isoflavones are found in legumes, and lycopene is present in tomatoes.
Although having no nutritional value, they can affect various metabolic pathways of the body thus providing anti-inflammatory and antioxidant benefits (20). They increase Bifidobacterium and Lactobacillus abundance with their anti-pathogenic, anti-inflammatory effects and cardiovascular protection.
#Iodine is one of the nutrients in vegan diets that tends to be overlooked. A systematic review (21) with a total of 127,094 adult participants concluded that a vegan diet was associated with an increased risk of #iodinedeficiency, compared to omnivorous. Therefore, people following vegan diets are recommended to consume seaweed, dark leafy greens, and iodised salt to mitigate the risk.
#Selenium deficiency may also prevent optimum nutritional levels as its availability in crops strongly depends on the mineral status of the soil, whereas animal products are less dependent.
A vegan compared to an omnivorous diet has generally been considered to be less energy dense as it consists of less fat and protein and more starch and fibre resulting in a lower total energy intake. As for protein, plant-based foods are reported to be the leading source globally, comprising 57% of daily protein intake, followed by meat (18%) and dairy (10%).
In contrast, the main source of #protein in the British diet is animal-based, contributing to nearly two-thirds of total daily protein intake, which mostly meet the RNI of 0.75 g/kg of body weight (22). This recommendation may vary depending on various factors, such as age and health status.
#Bioavailability of plant proteins is lower compared to animal sources due to a higher content of dietary fibre and phytochemicals, e.g., trypsin inhibitors, phytates, saponins or tannins. Plants contain a lower content of leucine (<8% of total protein) in comparison to animal-originated foods (approx. 8–14%), with corn being an exception (12%).
Relative or absolute #deficiencies of choline, carnosine, anserine, creatine, taurine, carnitine, and glutathione may occur with avoidance of #animalproducts. However, some plants are still a relatively good source, if consumed in larger volumes: dried seaweed (4.95 g/100 g), and roasted pumpkin seeds (2.39 g/100 g).
Katz and colleagues (2019) have also argued that the need for #essentialaminoacids can be met without any animal protein intake when an abundant variety of sources (e.g., combining cereals and pulses) are consumed. A gradual decline in muscle mass from the third decade of life has been reported, with a 30–50% decrease between the ages of 40 and 80. Therefore, an estimated consumption of 25–30 g of high-quality protein is required for the stimulation of 24-h muscle protein synthesis (MPS) and prevention of muscle loss with ageing.
#Essentialfattyacids, among which are EPA and DHA, play a crucial role in normal brain functioning and maintaining optimum health. They are abundantly present in oily fish and some animal foods. However, their shorter chain precursor, ɑ-linolenic acid (ALA), which is available in plants, is known to be poorly converted in vivo: 5–10% for EPA and 1–5% for DHA.
On the other hand, the low content of saturated fat in a plant diet leads to weight loss, improved lipid profile, and reduced blood pressure, which is associated with the prevention of coronary heart disease (23). Plant foods contain small amounts of monounsaturated (MUFAs) and polyunsaturated fatty acids (PUFAs), therefore, omega-3 PUFAs can be obtained from most vegetable oils, cereals, walnuts, chia seeds, linseed and hemp. PUFAs have been shown to have antithrombotic and antiarrhythmic effects and improve serum lipids, inflammation and blood pressure.
Both types of fatty acids favour beneficial Bifidobacteria in human gut microbiota (24). The human gut #microbiota appears to be altered with a greater impact in omnivores than in vegans.
Numerous studies have shown that saturated fatty acids (SFAs), abundantly present in animal foods, increase LDL cholesterol resulting in a higher risk of cardiovascular diseases, while there is evidence that vegan diets lowered total and LDL cholesterol by 0.36 mmol/L and 0.34 mmol/L, respectively. Thus, the replacement of SFAs with PUFAs, MUFAs and high-quality carbohydrates may decrease the total level of cholesterol (25).
One of the big #advantagesofvegandiet, unlike omnivore, is its plentiful #fibre content, which is resistant to digestion in the small intestine. It gets fermented in the large intestine and produces short-chain fatty acids (SCFAs), associated with beneficial effects (cardiovascular disease prevention, body weight management, immunity, and colorectal cancer prevention) (26). These substances also reduce proinflammatory cytokine production, concentrations of serum triglycerides, total cholesterol, and LDL-cholesterol.
Interestingly, an increase in SCFAs is observed when omnivores consume a Mediterranean diet rich in fruit, legumes and vegetables (27). Sources include whole grains, vegetables, fruits, and legumes.
Despite all the advantages of both diets described above, they also have their limitations which must be carefully considered should a person be willing to maintain optimum nutrition levels.
As previously mentioned, consumption of animal-fat diets can be a major driving factor in CVD through the increase of cholesterol levels, whereas animal protein intake may contribute to the development of a bowel disease by decreasing beneficial bacteria in the gut that metabolises dietary plant polysaccharides (28).
This change activated inflammation (induces proinflammatory cytokines such as IL-1, IL-6, and TNF-α) and led to metabolic disorders. For instance, International Agency for Research on Cancer (IARC, 2018) determined that a daily 50g portion of processed meat increased the risk of colon cancer by 18 per cent.
On the other hand, the presence of plant cell walls, #enzymeinhibitors and #tannins as well as food processing and heat treatment in vegan diets may represent inhibitory factors in plant protein digestibility. The current UK diet has at least 10 times more n6 fatty acids than n3. Also, trans-fat intake from vegetable oils is associated with adverse effects on health (29).
Vegan diets, being restrictive dietary patterns, may lead to unintended negative consequences such as #malnutrition, and impaired social life and well-being (30). #Disorderedeating in vegans (7.9%) has been shown to be higher than in omnivores (3.6%) (31).
#Vegansubstitute foods may contain roughly the same number of calories and saturated fat as omnivore options. Although fortified with vitamins and minerals, the content was shown to be lower compared to animal-derived alternatives.
More sodium, wheat gluten, #soyprotein isolate and other chemical derivatives may likely be used as a replacement for animal-based ingredients. Although, not all soy products are equal. Whole food soy sources, such as fermented natto and #tempeh, can be a good #meatsubstitute. They are absorbed more efficiently and don’t seem to block mineral absorption like other soy products (32).
There have been a lot of attempts to promote either dietary approach, however, some of the research they relied on may be seen as controversial and biased due to its complex nature. Although randomised control trials have shown the benefits of vegan diets, the comparison group has not typically followed an optimal omnivorous diet.
According to Han et al. (2019), there is little evidence that severe restriction of animal foods beyond a well-formulated omnivorous diet would reduce the prevalence of overweight, obesity and other health issues.
Vegan diets might not be adopted by everyone due to biological differences and preferences.
A thoughtfully constructed vegan diet, with supplements or fortified foods, however, may be more adequate and diverse than the current diets of many omnivores.
The saddest conclusion of the debate? Either diet can be technically followed without eating any actual whole foods.
A highly-processed diet lacking unprocessed foods is a problem for achieving optimum nutrition, regardless of whether the person is an omnivore or a vegan.
Whatever you choose to eat, make sure it is healthy for YOU as our bodies' responses vary greatly to be copied/pasted blindly.
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