Protein requirements may be met if the diet includes a wide variety of foods from plant sources and the energy requirements are met. There is a wide variety of plant-based sources of protein, which are of different quality. Overall, the quality of protein from plant sources is inferior compared to protein from animal sources (especially milk and eggs), and therefore it is recommended that vegetarian individuals consume protein-rich foods every day (especially legumes, nuts and seeds), combining different sources of protein. Consumption of soy and its derivatives (tofu, tempeh, meat analogues) and of pseudograins such as quinoa and amaranth may contribute to achieving an adequate and balanced intake of amino acids.

A high fibre content and the presence of some antinutrients reduces the digestibility of protein in plant-based foods compared to protein from animal sources, and therefore it is important to be particularly cautious in periods of life when protein requirements increase.2

Although the iron content of some plant-based foods may be high, its bioavailability is lower due to the high phytate content of plants and because iron is found in the non-heme form. The situation is similar when it comes to zinc. Some authors have reported decreased levels of ferritin and zinc in the vegetarian population, although the frequency of iron deficiency anaemia is low.17 The prevalence of iron deficiency varies widely, ranging from 4% to nearly 50%, depending on the population under study and the biomarker used to assess iron status.18 Previous studies have not reported differences in serum zinc levels between vegetarian and omnivorous children, although lower concentrations have been found in vegetarian adolescents.19–21 Consumption of foods rich in vitamin C in every meal is recommended to facilitate iron absorption.22

Iodised salt, marine vegetables and some cereal-based foods are the best source of iodine for vegan individuals. Dairy products and egg yolks are also good sources of iodine.

Seaweed (wakame, kombu, kelp, nori, etc), which plays a prominent role in these diets, can be an important source of iodine. However, since the iodine content of seaweed varies widely, as does the arsenic content,23,24 seaweed should be given with caution to infants and young children.

Consumption of foods fortified with calcium and vitamin D combined with regular physical activity of appropriate intensity is important in children with either vegetarian or omnivorous diets. The bioavailability of calcium in plant-based foods is influenced by their oxalate content: the higher the oxalate content, the lower the bioavailability of calcium. Thus, spinach contains high amounts of calcium, but due to the presence of oxalates, the former is consumed in the form of calcium oxalates that have low solubility and therefore are poorly absorbed,25 whereas the bioavailability of calcium is higher in other vegetables, including those in the cabbage family (green cabbage, broccoli rabe, Tuscan cabbage, Chinese cabbage), broccoli and bitter salad greens such as arugula or watercress.26

Few studies in the literature have assessed bone mass in children with vegetarian diets. Some have found an increased risk of facture in the vegan population associated with a low calcium intake.27,28

Vitamin D deficiency is frequent in Spain in vegetarian and non-vegetarian individuals alike and depends mainly on the exposure to sunlight and the consumption of supplements and fortified foods.29 Some cereals and dairy products are fortified. However, if consumption of such fortified foods does not guarantee an optimal intake, supplementation should be considered, as it is in the general population.30

Consumption of monounsaturated and polyunsaturated fats containing omega-3 fatty acids, found in fish, olive oil and sunflower oil (especially the high oleic variety), soy beans and some nuts and seeds, must predominate over consumption of unsaturated fats containing omega-6 fatty acids (mid-oleic or linoleic sunflower, corn, avocado, grapeseed or cottonseed oils). Supplementation with long-chain polyunsaturated omega-3 fatty acids (docosahexaenoic acid [DHA] and eicosapentaenoic acid [EPA]) should be considered in pregnant women and breastfed infants, infants aged less than 6 months that are not breastfed and infants aged more than 6 months obtaining less than 50% of calories from breast milk. There is evidence of a lower intake of DHA and EPA in vegetarian individuals compared to the non-vegetarian population,31,32 although the long-term effects of this difference have yet to be determined.

Vitamin B12 is not found in plant-based foods.1 Fermented foods or marine plants are not reliable sources of vitamin B12.33 Oral vitamin B12 supplementation is essential in any individual following a vegetarian or vegan diet. Even in individuals following ovo-lacto-vegetarian diets or those that habitually consume fortified foods, a weekly supplemental dose is the optimal strategy to ensure optimal vitamin B12 levels (Table 2).2

Vegetarian and vegan individuals are at higher risk of suffering nutrient deficiencies compared to omnivore individuals, but if a sufficient intake of all nutrients is ensured, pregnancy outcomes (birth weight, preterm birth) are similar in both populations. Given the increased requirements in pregnancy, nutritional management, even before conception, is particularly relevant.34 Due to their impact on psychomotor development, it is essential to guarantee an adequate intake of vitamin B12 and omega-3 fatty acids, usually by means of supplementation.35,36

Human milk is the ideal food for vegetarian and vegan infants. It is important to ensure that mothers consume vitamin B12 supplements regularly and, depending on their particular diet, also iodine and omega-3 fatty acid supplements.35–37

When breastfeeding is not an option, vegan families can use soy protein-based formula. The use of plant-based milks that are not formulated for infants, sometimes mixed with fruit or vegetable juices, has in some cases resulted in severe malnutrition, neurologic sequelae and even death.38

The age of introduction of complementary foods should be the same as in children following non-vegetarian diets. Introduction of legumes is recommended starting at age 6 months, as they are the natural substitute of meat in vegetarian diets. If the infant consumes pureed foods, some options include purees made with legumes or tofu. In ovo-lacto-vegetarian infants, eggs can occasionally substitute legumes. If the family would rather introduce solids directly, one option is to offer the infant chickpea hummus (or other legume spreads), crumbled cooked tofu, peas, lentils mixed with rice, unsweetened peanut butter spread on bits of banana or bread, or omelette. From age 6 months, infants may also sometimes be offered unsweetened soy yogurt for breakfast or as a snack, with or without fruit. It is important to choose brands fortified with calcium.

The diet must include foods rich in vitamin C in the main meals to facilitate iron absorption. Regular consumption of vegetables rich in vitamin A is also recommended (sweet potato, broccoli, carrot, squash). One easy way to do this is to substitute sweet potato for potatoes in pureed vegetable soups.

Gluten should be introduced as the same age recommended in infants without special diets.5 Use of whole grains should predominate: whole grain bread, brown rice, whole wheat pasta, couscous, millet, corn polenta or quinoa. Once the child tolerates these foods well, nuts and seeds should be introduced next, either ground or in butter form (parents should be reminded of the dangers of offering whole nuts): 1) ground almonds, hazelnuts or walnuts could be added to fruit purees or cereal porridge or to cooked rice, pasta or vegetables; 2) tahini (a ground sesame butter) can be introduced from age 6 months, either as an ingredient in chickpea hummus or by adding it to vegetable purees occasionally as a substitute of legumes; 3) almond, hazelnut or other nut butters could be added to pureed foods or smoothies or spread directly on bread.

Plant-based milks fortified with calcium may be used occasionally to prepare some dishes, but should never be used as the main liquid food of the child, at least until age 2–3 years.

Until age 1 year, spinach, chard, borage, beets, arugula and other leafy greens should be avoided due to their high nitrate content; honey and syrups (agave syrup, rice syrup, wheat glucose syrup) due to the risk of contamination with botulism spores; seaweed due to its high iodine content; and also foods with a laxative effect, such as flax or chia seeds.

From this age, children ought to follow the same diet as the rest of the family, with an emphasis on offering fresh foods rather than processed foods or fruit juices with added sugars. The main problem in this population is the lack of longitudinal data from cohorts of children and adolescents following a vegetarian diet that would allow drawing conclusions on the potential advantages or disadvantages of their consumption.39

Requirements for many nutrients increase during adolescence, especially of protein, iron, zinc and calcium. It is important to ensure that vegetarian and vegan adolescents consume legumes and their derivatives daily, as they are the food group that contributes most to the dietary intake of these four nutrients. To optimise iron absorption, we must encourage consumption of foods rich in vitamin C in the main meals while at once excluding consumption of coffee or tea during meals. The best way to ensure an adequate intake of calcium is to include the following foods in the diet: 1) 2 servings per day of milk, dairy or plant-based milks fortified with calcium; 2) at least 3–4 servings per week of greens rich in calcium with a low oxalate content; 3) 2–3 servings per week of calcium-set tofu; 4) 1 serving per day of fruit, dried fruit or seeds rich in calcium (almonds, sesame seeds, figs, chia seeds, dates, oranges…).