Vegetarian diets are becoming more widespread by the day, as their health benefits are promoted enthusiastically by its ideologists. Some of these include:
- The high content of fiber
- Increased amounts of many vitamins and minerals
- Low fat content, and mostly unsaturated
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This is even more so in the case of vegan diets, when all animal products are removed from the diet. As a result of these dietary differences, vegetarians have a lower risk of cardiovascular disease, obesity, and type 2 diabetes.
The risk of many cancers is also reduced, as is that of osteoporosis. On an evidence basis, it is possible that colorectal cancer incidence is reduced, probable that type 2 diabetes and osteoporosis is reduced, and convincing proof exists that cardiovascular disease risk is reduced.
Postmenopausal women in Asian vegetarian populations were found to have significantly lower bone mineral density (BMD) at spine and hip. This was thought to be due to the low protein and calcium intake, and low vitamin D, which has been generally linked to increased bone loss and fractures in the elderly.
Calcium intake is likely to be inadequate only with a vegan diet, though not in lacto-ovovegetarians. Moreover, bone fracture risk was found to be similar in vegetarians and those who had a mixed diet including animal foods (omnivores).
Thus, the risk of fractures is higher in vegans, but this could be reverted to that of omnivores by the addition of 525 mg of calcium a day. This is explained by the positive impact of a diet plentiful in fruit and vegetables, which provides large amounts of potassium and magnesium.
This creates an alkaline milieu which inhibits bone resorption, increases the BMD and inhibits bone fractures. Vitamin K which also influences bone health positively is found abundantly in a vegetarian diet.
Vitamin D deficiency
Vitamin D intakes in vegans are only about 25% that in omnivores, and since its concentration depends upon the consumption of fortified foods and sun exposure, it is important that vegans take supplements if they are not to suffer from its deficiency. In this respect it is important to realize that vitamin D2 has a lower bioavailability than vitamin D3 which is the form found in animal foods.
Chronic deficiency of vitamin D leads to low concentrations of its 25-hydroxy metabolite and excessively high parathyroid hormone, especially at high altitudes and in cold climates, which in turn causes lowering of the BMD. One study shows that these abnormal results persist throughout the year in vegans compared to omnivores.
In the absence of fish, eggs, and seaweed, long chain fatty acids of the omega-3 family are generally deficient in the diet, in particular, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These carry important health benefits for cardiovascular and visual health, as well as for brain functioning.
The major source of these omega-3 acids is omega-3 fatty acid alpha-linolenic acid (ALA) which is a plant-based fatty acid. Because of the low percentage of conversion of this to EPA and DHA, their concentrations in serum are lower in vegetarians. The Reference Dietary Intakes are 1.6 and 1.1 g ALA/d for men and women, respectively.
Thus fortification of vegetable foods and especially consumption of microalgae in the form of supplements is advised to ensure adequate concentrations of these fatty acids, as this will provide DHA which can be converted to EPA within the body.
The brown alga called kelp is an excellent source of EPA as well. DHA supplements should be taken only in adequate amounts because they can raise the total and LDL cholesterol, prolong the bleeding time, and lead to lower levels of immunity.
Another element of concern is iron, since heme iron from animal foods is significantly higher in bioavailability than the non-heme form found in plant foods. However, this does not usually translate clinically into anemia due to iron deficiency because of the synergistic action of vitamin C which is rich in vegan diets.
Vitamin B12 concentration is lower in vegans than in lacto-ovovegetarians. This is associated with a higher incidence of B12 deficiency, which can precipitate neuropsychiatric manifestations and macrocytic anemia, as well as, in children, growth failure. Plasma homocysteine levels are also raised which can be a risk factor for cardiovascular disease as well as bone fractures due to osteoporosis.
An interesting and important point is that a “vegetarian” diet by itself doesn’t necessarily bring down the risk of heart disease compared to an omnivorous diet. This is because refined flour and sugar, as well as potatoes, are associated with a positive energy balance and obesity as well as dyslipidemia, which increase the risk of cardiovascular disease.
Thus, a healthy vegetarian diet should include high levels of whole grains, fruits and vegetables, as well as healthy oils and fats, including nuts and seeds in moderation.
The following guidelines are recommended to avoid potential nutrient shortfalls in vegetarian diets:
- B12-fortified foods such as fortified beverages from soy and rice, breakfast cereals, nutritional yeast, or supplements. It is not enough to increase the consumption of fermented soy and green leafy vegetables, or seaweed, by themselves.
- Calcium fortification is also an essential step in ensuring adequate BMD and can include the above foods as well as whole fruit juices (orange or apple) which contain calcium citrate malate, a highly bioavailable form of calcium. Fortified soy milk contains tricalcium phosphate instead with a somewhat inferior bioavailability compared to cow milk.
- Vitamin D fortification is necessary to prevent deficiency particularly in the winter. If not available, daily supplements are advised, especially in older vegans.
- Vegan sources of ALA include walnuts, soy, hemp-based beverages and flaxseed, which should be part of the daily diet. Fortified foods are also recommended. DHA supplements are recommended for pregnant and lactating women who need higher intakes.
- Zinc deficiency is rarely seen clinically in vegans, but it is recommended that whole grains and legumes or fortified foods be ingested to avoid the possibility of a shortfall.