Vegetarian diets have been practiced since ancient times. The popularity of vegetarian diets in recent years has been fuelled by ethical considerations, health concerns, environmental issues, and reli-gious factors. The reason a person chooses to be vegetarian influences the pattern of foods they consume, from only eating plant foods to permitting dairy and/or egg products in the diet. When a vegetarian diet is appropriately planned, it can be nutritionally adequate for individuals through all stages of the life cycle and can promote health and lower the risk of major chronic diseases. Avoiding nutrient-dense meat or animal-based diets means paying close attention to the diet in order to ensure balanced nutrition and an adequate micronutrient intake.
The diversity of vegetarian eating patterns makes it difficult to find clear relation-ships between the diet, nutrition status, and health profile of a typical vegetarian. A well-balanced vegetarian diet can be a healthy choice for most people potentially reducing the risk factors of chronic diseases as long as the diet’s potential nutrient deficits are recognized and acted upon. The nutrition adequacy of any one parti-cular plant-based diet depends upon the appropriateness of food choices, which are often determined by the individual’s knowledge of nutrition and the availability of certain foods. Vegetarians need to incorporate into their diet foods that provide adequate levels of certain vitamins, minerals, trace elements and omega-3 fatty acids.
Vegetarians are found all over the world and are of all ages. This diversity is reflected in the reasons why people choose to eat vegetarian foods and the array of vegetarian dietary practices. Vegetarian diets are defined by what they don’t contain, and therefore there is some considerable variation between them. Vegans (or “strict vegetarians”) eat food of vegetable origin only, i.e. no meat, fish, milk or eggs. Ovo-lacto vegeta-rians eat milk and eggs, but no meat or fish. Pescetarians eat fish, milk and eggs and semi-vegetarians eat meat for less than half their meals. While diet is a personal choice, vegetarians’ dietary customs have ethical, ecological, religious, economic and/or health-related foundations.
In recent times, vegetarian diets have been scientifically evaluated and shown to provide certain advanta-ges when it comes to their effects on disease or dietary composition (1). Generally, vegetarian diets don’t just mean an absence of animal products; they contain higher amounts of fruit, vegetables, whole grains and legumes, providing a large amount of fiber, and for ovo-lacto vegetarians, more dairy and eggs. Vegetarian diets have been associated with a lower risk of cardiovascular disease and cancer (2). Additionally, vegetarians have been shown to have a lower body mass index, possibly due to the lower calorific density of vegetarian diets or the satiating effect of all the fiber and bulk. The health benefits of eating a meat-free diet are thought to be linked to both a reduced consumption of less healthy dietary components, and an increased consumption of healthier foods. For example, vegetarian diets typically contain lower amounts of cholesterol, salt, and saturated and trans fats, consistent with a heart-healthy diet (3,4). They also tend to be higher in fiber, carotenoids and phytonutrients, and have a higher antioxidant status.
Depending on the diet’s actual composition and the individual’s personal characteristics, attention is needed where entire food groups are not consumed. Nutrients obtained predominantly from animal products in omnivorous diets can be scarce in vegetarian diets. Additionally, the plant-based form of some nutrients is less bioavailable than the equivalent form found in meat. Some dietary components found in larger amounts in vegetarian diets can even limit the bioavailability of critical nutrients. For example, the phytate in whole grain cereals is known to bind minerals and make them less available to the body. In some cases, supple-ments or fortified foods can provide a useful source of important micronutrients to help protect against deficiency (5).
Vitamin A and beta-carotene
Vitamin A is a very important micronutrient because it fulfills many diverse functions throughout the body. It is well known for its effects on vision and is crucial to help maintain areas where there is a high rate of cell division, such as in the mucous membranes and the skin, which form a barrier with the outside world to keep us safe from infection. Vitamin A in its most easily absorbed form is found in meat, particularly liver, and in egg yolk and dairy products. It is therefore a nutrient of great concern to vegetarians, particularly those who do not consume dairy products or eggs (6). Vegetarian children in particular are at greater risk of vitamin A deficiency because they are actively growing (7). Iron deficiency, another common problem for vegetarians, may also influence vitamin A uptake (8).
The antioxidant carotenoid beta-carotene, found in most orange or dark green vegetables, is the main dietary source of vitamin A for vegetarians. A specialized enzyme in the intestine cleaves one molecule of beta-carotene into two molecules of vitamin A (9). The bioavailability of beta-carotene and its conversion to vitamin A can be affected by a host of different factors, depending on the food matrix, total amount eaten, and fat intake with a meal (10). The absorption and conversion of beta-carotene from vegetables is low, putting people consuming only plant-based sources of vitamin A at risk of deficiency (11). Furthermore, genetic variations can reduce vitamin A activity, with around 45% of healthy individuals unable to naturally convert beta-carotene into vitamin A (12).
Vitamin B12 is a water-soluble vitamin that is particularly important for red blood cell formation, the nervous system and DNA synthesis. Adequate levels of vitamin B12 help to maintain cognitive function and reduce levels of the amino acid homocysteine, high levels of which are associated with poor heart health. Long term deficiency of vitamin B12 may result in permanent brain damage. Deficiency of vitamin B12 is prevalent in non-meat eaters around the world, both in affluent Western countries, where a recent study found around half of vegans in the UK to be deficient, and in other places like India, China and Taiwan (2,13).
Vitamin B12 is a complex molecule that is introduced into the food chain by bacteria. Although it can be produced by the normal human microflora in the large intestine, a complex series of processes along the digestive tract is required to absorb vitamin B12 into the blood. This means that absorption of vitamin B12 produced in the colon is low. Animal products are the only main source of this vitamin for humans. The vitamin B12 in some edible algae is not bioavailable, and in eggs it is only poorly bioavailable (14). Vegans in particular have to get their vitamin B12 from supplements, because there are few plant-based foods that provide even a fair source. The elderly are particularly at risk because they have reduced rates of absorp-tion. For vegetarians, consuming supplements has been associated with reduced risk of deficiency (15).
Vitamin D3 is actually a pre-hormone that can be obtained from the diet, but in its main form, it is obtained by exposing the skin to quality sunshine. Deficiency of this vitamin is a common problem in many parts of the world, mainly because there are very few good food sources (16). Moreover, due to lifestyle changes, sun exposure may not be adequate enough to maintain necessary levels of vitamin D. Deficiency is classi-cally linked to poor bone health, frailty and falling, however more recent research shows that a sufficient level of vitamin D3 is also needed to maintain a healthy heart and immune system (17). The main dietary source of this vitamin is fish, particularly oily fish.
Vitamin D intake among vegans is particularly low (18,19). Supplementation is therefore often the most suitable way for vegans to get sufficient amounts of vitamin D. Vitamin D2 is another type of vitamin D. It can be found in mushrooms exposed to UV light the one good plant-based source of vitamin D. However, recent studies have shown that vitamin D3 is up to 87% more potent than vitamin D2, which may explain why vitamin D3 exerts a stronger effect on the prevention of fractures and falls (20,21). Vitamin D3 is also the form naturally produced by the skin via sun exposure. Vegetarian sources of vitamin D3 are widely available and provide a more effective, nature-identical source.
Essential fatty acids
Polyunsaturated fatty acids (PUFAs) fulfill many essential bodily functions: they provide energy and they form part of cell membranes and nerve cells. They are widely recommended for cardiovascular health, and they also play a role in eye and brain development. Of all the omega-3 fatty acids, the most biologically significant are the long-chain acids DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid). DHA and EPA are called “conditionally essential” fatty acids, as synthesis inside the body is extremely rare, with levels of their metabolic pre-cursor alpha-linolenic acid (ALA) tending to be low in certain populations (22).
Whereas plant seeds such as flaxseed can provide sufficient amounts of omega-3 fatty acids, conversion to DHA and EPA is low (23). One of the best sources of DHA and EPA is fish, which is not part of most vegeta-rian diets. Intake is therefore low in vegans and vegetarians in general (24). Over the past decade, the discovery of a natural, vegetarian source of DHA and EPA (algae) has made these fatty acids more available to vegetarians. DHA is the omega-3 PUFA that is particularly important for pregnant and breastfeeding wo-men, infants, and toddlers, since it is needed for proper fetal and infant brain development (25). Additio-nally, EPA and DHA have been shown to have lifelong heart health benefits.
Calcium is the most abundant mineral in the body and we need it throughout our entire lives. It not only plays an important role in building stronger, denser bones early in life and keeping bones strong and healthy later in life, it is also needed for chemical releases in the brain, muscle contractions and maintaining salt balance in bodily fluids (26). Dairy products provide a very important source of calcium; however it can also be obtained from green vegetables, nuts and calcium-set tofu. Calcium uptake by the body may be blocked by compounds in leafy greens like spinach. Vegans are at particular risk of low calcium intake, with calcium consumption in women being particularly low (6). This may explain why vegans have been found to have a 30% higher risk of bone fracture (27). Getting calcium through supplements and fortified foods can help vegans and vegetarians who do not eat dairy products to meet their requirements.
Phosphorus is an element that fulfills a large range of important functions in the body. It is needed for the formation of bones and teeth, and it plays an important role in the body’s utilization of carbohydrates and fats, and in the synthesis of protein for the growth, maintenance and repair of cells and tissues. The body’s energy currency, ATP, requires phosphorus. Phosphorus is also needed for muscle contraction and kidney function, for maintaining a regular heartbeat, and for nerve conduction (28). The main food sources are meat, dairy products, and processed foods. Although whole-grain breads and cereals contain more phos-phorus than cereals and breads made from refined flour, it exists in a stored form of phosphorus called phytic acid, which humans are unable to absorb. Fruits and vegetables contain only small amounts of phosphorus, thus vegans are at particular risk of inadequate phosphorus intake.
Iron is present in all cells and fulfills several vital functions. It carries oxygen from the lungs to the tissues in the form of the blood protein hemoglobin, it is a transport medium for energy within the cells, and it is an integral part of enzyme reactions in various tissues. Too little iron can interfere with these vital functions and lead to poor health, fatigue and eventually even death. Iron is found in many types of food, including meat, green leafy vegetables, grains and eggs. An issue faced by vegetarians is that the iron in meat, known as heme iron, is more readily absorbed than the non-heme iron found in plant sources. Additionally, vegetarian diets often contain high levels of iron absorption inhibitors, such as phytate (found in grains), tannin (found in tea and coffee), and calcium (29). Because of the low bioavailability of non-heme iron, the iron require-ment for vegetarians is 1.8 times higher than that for omnivores (30). Iron absorption can, however, be enhanced by some nutrients such as vitamins A and C (31). The enzyme phytase is capable of breaking down phytate to enhance iron bioavailability (32).
Zinc is an essential mineral in any healthy vegetarian diet plan. It is important for the skin and to ensure a healthy immune system, where it contributes to resistance to infection. The main dietary sources include meat, eggs and seafood, with good vegetarian sources being nuts, legumes and dairy products (33). Plant foods offer only a low bioavailability of zinc, with requirements being 50% greater for vegetarians. This is mainly due to the higher phytic acid content of vegetarian diets (34). Although zinc intake is comparable between vegetarians and vegans and meets normal requirements, low bioavailability puts non-meat eaters at risk of deficiency.