Interest in vegetarian diets has been growing in recent years, and in Portugal, it is estimated that the number of vegetarians is 1.2% of the population .
Vegetarian dietary patterns can be quite diverse due to food availability and the different factors that motivate their adoption, such as moral and religious issues, environmental and biodiversity protection, animal welfare, and health concerns. However, they have in common the predominant use of plant-based products (such as cereals, legumes, vegetables, fruit, oils, nuts, and seeds), and the exclusion of meat and fish, and may or may not include eggs and/or dairy products ( Table 1 ).
The vegetarian diet has also recently gained some prominence for its health effects. Generally, vegetarians tend to be more health-conscious and exhibit a lower prevalence of certain risk behaviors (such as excessive alcohol consumption and tobacco use, among others), as well as lower body mass index, total cholesterol, LDL cholesterol, triglycerides, and blood glucose levels compared to non-vegetarian populations. Several studies show a lower incidence of ischemic heart disease and, to a lesser extent, cancer in vegetarian populations; however, this protective effect is not significant in all-cause mortality and mortality from vascular diseases .
In order to maximize the benefits associated with a higher consumption of plant-based products, and to minimize any risks of nutritional deficiencies associated with different dietary patterns, a vegetarian diet must be well planned, taking into account the adequate intake and bioavailability of certain nutrients such as protein, essential fatty acids, iron, zinc, iodine, calcium, vitamin D and vitamin B12.
Protein
Regarding protein , plant-based sources generally have lower digestibility and content of essential amino acids, with limiting amounts of one or two amino acids . Cereals, for example, have a lower lysine content. Adjusting dietary choices to favor a higher consumption of legumes and the inclusion of soy-based foods can ensure adequate intake of this amino acid a slightly higher consumption than recommended is appropriate for (bearing in mind that individual protein needs vary depending on several factors, including nutritional status, age, body composition, and level of physical activity). This intake can be achieved using foods such as legumes, soy-based products (soy milk, soy yogurt alternatives, textured soy protein, tofu, and tempeh), nuts, seeds, eggs, and dairy products (in an ovo-lacto-vegetarian dietary pattern).
Essential fatty acids
Vegetarians (and all those who consume minimal amounts of oily fish) may be at a disadvantage regarding the intake of omega-3 fatty acids . Among these, we can obtain alpha-linolenic acid (ALA), an essential n-3 series fatty acid, in a vegetarian diet through the consumption of ground flax seeds, chia seeds, walnuts, and their oils . This fatty acid is converted in the human body into eicosapentaenoic acid (EPA, 20:5 n-3), which in turn is converted into docosahexaenoic acid (DHA, 22:6 n-3); the endogenous conversion of ALA into EPA and DHA is an inefficient process , with fish being the main source of these pre-formed fatty acids.
Given the recommended daily intake of ALA of 1.6 g and 1.1 g for men and women, respectively, it is prudent for vegetarians to ensure a slightly higher consumption of ALA . Choosing olive oil as an added fat, rather than other vegetable fats rich in omega-6 fatty acids, also promotes better absorption of ALA.
DHA supplements based on microalgae are already available for vegetarians with increased needs (pregnancy and lactation) or with reduced conversion capacity (hypertension or diabetes).
The bioavailability (i.e., the proportion of a given ingested nutrient that is actually absorbed and used for normal metabolic and physiological functions) of some nutrients, such as calcium, iron, and zinc, is lower in a vegetarian diet.
Iron
Regarding iron , although its consumption is similar or slightly higher in vegetarians compared to non-vegetarians, vegetarians have lower iron reserves due to the lower bioavailability of iron present in plant-based foods (non-heme iron). The amount of non-heme iron absorbed is influenced by antinutritional factors such as phytates, which, when bound to minerals, form insoluble complexes that decrease absorption. Culinary methods such as soaking, malting, and fermentation, as well as thermal processing, result in the hydrolysis of phytates and increased bioavailability . Other absorption inhibitors include polyphenols present in tea, coffee, cocoa, and red wine, as well as calcium . Vitamin C, abundant in a vegetarian diet, enhances the absorption of non-heme iron.
Vegetarians are advised to increase their iron intake , above the recommended levels, by consuming a variety of plant-based foods naturally rich in iron, such as whole grains, legumes, dark green leafy vegetables, nuts and seeds , and fortified foods such as breakfast cereals.
Zinc
Similar to iron, zinc from plant-based foods also has lower bioavailability. The main inhibitor of the absorption of this trace element is phytate .
A vegetarian diet can provide adequate amounts of zinc through the consumption of legumes, soy and soy products, whole grains, nuts, and seeds . Evidence suggests that a vegetarian diet provides a similar or slightly lower zinc content than a non-vegetarian diet, and that serum zinc levels are lower in vegans, but within normal ranges .
Calcium
Calcium intake in ovo-lacto vegetarians appears to meet, or even exceed, recommendations, while calcium intake in vegans is variable and sometimes insufficient . However, there is a variety of plant-based foods that provide calcium, such as dark green leafy vegetables like kale , Portuguese cabbage and broccoli, fortified foods like tofu (produced with calcium sulfate), plant-based drinks (1), among others.
The bioavailability of calcium from plant-based foods is primarily affected by the presence of oxalates and phytates. Foods high in oxalates include spinach, beet greens, and chard, where calcium absorption can be as low as 5%. Vegetables with lower oxalate content, such as kale, turnip greens, bok choy, and broccoli, show an absorption rate of around 50% . Approximately 30% of calcium from dairy and fortified foods is absorbed . A vegan diet can meet current calcium recommendations. However, a conscious selection of food sources is important to promote adequate nutritional intake of this micronutrient .
Iodine
iodine content in plant-based foods varies and depends on the concentration of iodine in the soil. Therefore, those who follow a vegan diet may have a higher risk of iodine deficiency .
The main sources of iodine in a vegetarian diet are dairy products (in the case of ovo-lacto vegetarians), seaweed, and also iodized salt , when used as a substitute for common salt (not exceeding the recommended salt intake of a maximum of 5g/day). It is important to note that the iodine content of seaweed is variable, and some species can provide considerable amounts of this trace element, potentially leading to toxicity if consumed regularly and in large quantities . Therefore, it is recommended that seaweed consumption not exceed 3 to 4 times per week .
Vitamin B12
Vitamin B12 is almost exclusively found in animal products, and vegetarians and vegans should consume fortified foods and/or supplements to ensure adequate nutritional intake. The ovo-lacto-vegetarian diet can provide this vitamin through eggs and dairy products; however, intake of these foods alone may be insufficient .
Foods fortified with vitamin B12 include plant-based drinks, nutritional yeast, vegetarian meat alternatives, and breakfast cereals. However, depending on the dietary availability, these foods may have little nutritional value and may be insufficient to meet the recommended daily intake of 2.4 µg for adults. Therefore, supplementation may be an option , with some guidelines recommending a daily supplement of 25 to 100 µg of vitamin B12, or 1000 µg 2 to 3 times a week, or 2000 µg once a week.
Studies evaluating vitamin B12 levels in vegetarians and vegans have found lower levels compared to non-vegetarians. Some neurological symptoms of vitamin B12 deficiency include paresthesia, decreased peripheral sensitivity, memory loss, dementia, among other symptoms, which can be irreversible. Subtle neurological damage may also occur, even in the absence of anemia, due to higher folate intake, which may hinder early detection of vitamin B12 deficiency .
Conclusion
In summary, proper meal planning essentially involves: including a variety of foods, such as cereals, legumes, nuts and seeds, fruits and vegetables (taking advantage of the wide variety of plant-based products that strongly define our food tradition); selecting vegetable fats, consuming omega-3 fatty acids from plant-based sources such as flax seeds and walnuts; and finally, including good sources of protein, calcium, iron and zinc, and ensuring adequate levels of both vitamin B12 and D (through the intake of fortified foods and/or supplements).
In Portugal, Law No. 11/2017 made vegetarian options mandatory in all public canteens (prisons, hospitals, schools, and elderly care centers). Implementation began in June 2017, giving public canteens six months to be fully prepared to serve vegetarian options. As this article demonstrates, a vegetarian diet is not simply about eliminating meat or fish from one's plate. Careful planning, combined with practical knowledge of preparing a variety of plant-based foods, is essential to optimize their nutritional value.
Regardless of whether we are vegetarians or not, it is certain that in order to continue living on our planet, we have a duty to protect it. One of the measures within each of our reach is, without a doubt, the drastic reduction in the consumption of animal products and the search for foods produced responsibly, seasonally, and close to home.
(1) In addition to soy milk, we are now seeing a greater supply of plant-based drinks, made from rice, almonds, hazelnuts, hemp seeds, coconut, oats… Some of these products are fortified with calcium and vitamins D and B12, but have a considerably low protein content. Unless these products are fortified, their contribution to the nutritional adequacy of these nutrients is minimal.
Written by
She is currently pursuing a Master's degree in Food Science and Technology at the Faculty of Sciences of the University of Porto and the University of Minho. She collaborates with clinics where she provides consultations and develops her work in the area of clinical nutrition. In 2017, she published her first vegetarian cookbook, entitled "Recipes with Passion". She is also the author of Compassionate Cuisine.
