Fleshing Out the Menu: How Critical Are Animal-Sourced Nutrients?

Today's food system, heavily reliant on starchy staples, continues to face the persistent global challenge of micronutrient deficiencies. Optimal health and nutrient intake are best achieved through a diet that appropriately combines nutrient-dense foods from both plant and animal sources. Research suggests that diets providing at least one-third of calories or half of protein from animal sources substantially enhances nutritional adequacy. Below these thresholds, as seen in predominantly plant-based diets like the EAT-Lancet diet, deficiencies in key vitamins and minerals, such as iron, zinc, calcium, and vitamins A, D, and B12, become harder to avoid. A large-scale intervention trial, MyPlanetDiet, demonstrated that a low-emission diet falls short in meeting micronutrient needs compared to a control diet richer in dairy, meat, and eggs but lower in legumes, nuts, and seeds. To prevent nutritional gaps in plant-centered diets, sufficient consumption of nutrient-rich foods like fish, shellfish, eggs, beef, and dairy while moderating high-phytate foods (e.g., whole grains, pulses, and nuts) is advisable.

Plant-only diets pose a specific challenge. Understanding the nutritional adequacy of vegan diets, particularly for vulnerable populations, is limited by two key factors: 1) scarce data on the long-term metabolic effects of such diets and 2) the tendency to conflate veganism with less restrictive vegetarian diets, which likely have different metabolic impacts. Moreover, genetic variations, particularly in fat metabolism and brain function, further influence an individual's ability to thrive on plant-based diets long-term. Be that as it may, studies frequently show that vegan, and to a lesser extent vegetarian, populations are at higher risk of deficiencies in micronutrients typically found in animal-sourced foods (iron, zinc, calcium, iodine, choline, long-chain omega-3 fatty acids, and vitamins A, D, and B12), potentially increasing rates of bone fractures, sarcopenia, anemia, and depression. Individuals who heavily restrict or exclude animal-sourced foods must therefore prioritize nutrient-dense plant foods and carefully supplement their diets. Yet, even well-planned vegan meal plans, such as those promoted by Forks Over Knives, can lead to multiple deficiencies. Conversely, individuals on carnivorous diets, particularly those limiting offal, dairy, pork, fish, or eggs, may also have to pay attention to certain micronutrient levels (in particular magnesium, potassium, and vitamins B1, B9, C, and E).

Long-chain omega-3 fatty acids, EPA and DHA, are vital for immunity, brain development, mental health, memory, and cardiovascular, neuronal, and retinal health. Fish, meat, and eggs are primary sources, with oily fish being particularly rich. Algal oil is a viable DHA source for those avoiding animal foods, though many supplements are oxidized beyond safe levels. Alpha-linolenic acid (ALA), found in plants, is the precursor to EPA and DHA but converts inefficiently (0.2–21% for EPA, 0–9% for DHA). Omnivores therefore have higher blood/tissue DHA due to preformed dietary DHA, unlike vegetarians, even with equal or higher ALA intake. 

A minimum of 5 mg/day DHA is needed for brain health, with >1 g/day EPA+DHA suggested for neurocognitive benefits and 0.5–4 g/day for cardiovascular health. Most Western populations fall below the recommended 250–400 mg/day, except in regions like Japan and Scandinavia with high fish consumption. Low EPA/DHA blood levels are widespread, particularly in Western and low/middle-income countries. At-risk groups include infants, children, and adolescents in dryland agriculture regions. Vegan diets typically lack DHA, and lacto-ovo-vegetarian diets provide only ~20 mg/day. Achieving 250 mg/day EPA/DHA requires 2–12 g ALA daily, but U.S. vegetarians/vegans consume <1 g ALA/day, leading to significantly lower EPA/DHA levels (30% lower in vegetarian males, 50% lower in vegans compared to omnivores).

Vitamin A (retinol) is essential for cell differentiation, tissue growth, immunity, and eye and skin health. Deficiency increases infection susceptibility, particularly in young children, while it also impairs night vision and can cause corneal damage or even blindness. Retinol is found in animal-sourced foods only, especially liver, but also eggs, cheese, and meat. Plants contain carotenoids (vitamin A precursors), but their absorption (5–65%) and conversion to retinol (4:1 to 28:1) are often inefficient. The conversion of carotenoids to retinol varies due to genetic factors (e.g., BCO1 mutations reduce conversion by 32–69% in about 40% of Europeans), carotenoid type, dose, dietary context, and fat intake. 

Low-fat diets (with <10% energy from fat) or fat-free consumption of carotenoid-rich plants impairs absorption. Absorption thus fails when carotinoid-rich plants are consumed with a fat-free salad dressing, while the type of fat and emulsification in the dressing also matter. Moreover, carotenoids that do not function as substrates (e.g., canthaxanthin, zeaxanthin) interfere with retinol conversion (by inhibiting carotene dioxygenase), whereas protein-energy malnutrition hampers retinol circulation (by affecting plasma levels of retinol-binding protein). Vegans and vegetarians, particularly those with poor converter phenotypes, can sometimes be at higher risk of vitamin A deficiency in case they need to rely on poor or inefficient carotenoid sources. In such cases, monitoring retinol status is advisable.

Vitamin B12 is a critical nutrient for neural health, DNA production, and red blood cell formation. Deficiencies can lead to irreversible nerve damage. The vitamin is found in animal-sourced foods only. 

Despite advice to supplement, many vegetarians and vegans remain deficient in B12, which is particularly worrying in the case of pregnant and lactating women, infants, and children. Concerns about lower B12 levels are justified based on population-level observations, and deficiencies are often overlooked or misdiagnosed. 

Among the B vitamins, other than vitamin B12, vitamins B1 (thiamine), B2 (riboflavin), and B3 (niacin) stand out as potentially limiting in diets that contain low amounts of animal source foods. While these vitamins are present in various plants, they may become a concern in inadequately developed diets, especially for populations heavily dependent on cereals and living in poverty. 

Thiamine, which is found in a variety of foods of plant and animal origin, is essential for energy metabolism, and its deficiency leads to beriberi. While beriberi is now rare, it can occur in people on low-nutrient diets high in refined carbohydrates or with high alcohol intake. 

Riboflavin is crucial for oxidation and reduction reactions in metabolism, especially energy metabolism. While plant foods contain riboflavin, dairy is a major source, providing sufficient riboflavin for about half of the global population. Vegans may sometimes have lower riboflavin status, and deficiency in pregnant women can lead to consequences for newborns. 

Niacin deficiency, known as pellagra, was historically associated with diets low in animal source foods. Niacin can also be obtained from tryptophan in the diet, but its availability may be limited in protein-poor diets. Niacin deficiency can lead to neurological conditions and impaired immunity.

Vitamin K2 participates in the synthesis of blood clotting factors in the liver, as does vitamin K1. However, its biological relevance is more important, given its role in brain function and bone health, and in the prevention of cardiometabolic diseases. 

Vitamin K2 is found in animal source foods and in some fermented plants. Full-fat dairy products serve as rich sources. Some vegetable fats and oils may potentially interfere with vitamin K2-dependent processes. More research is needed to fully understand the specific benefits of vitamin K2.

Adequate intake of calcium and vitamin D is crucial for neuromuscular regulation, bone health, immune system function, and disease prevention. Excellent sources of calcium include fish with edible bones and dairy products. Plant-derived calcium may have reduced bioavailability, although good options exist. Vitamin D is best obtained through sun exposure, complemented by the intake of oily fish, liver, and eggs. 

Vegans may require supplementation as vegan-friendly D2 supplements are less effective than D3 supplements from animal sources. Both calcium and vitamin D deficiencies are common in various populations, which may need particular care in vegans and vegetarians, and may lead to lower bone mineral density and increased risk of fractures.

Iron is essential for blood health, the endocrine system, and neurodevelopment. Excellent sources include red meat and liver, whereas plants have reduced bioavailability. Even with higher total iron intake, vegetarian diets may result in lower iron status. In high-income countries, iron-deficiency is on the rise, especially among females, due to dietary changes, such as a decline in red meat intake.

Avoiding animal-sourced foods requires specific practices like sprouting, fermentation, and soaking legumes, as well as consuming vitamin C-rich foods and drinks (while minimizing tea and coffee) to improve iron absorption. Adequate levels of vitamin A and riboflavin are also necessary for iron mobilization and haemoglobin synthesis. Interestingly, the absorption of non-haem iron from plants can be enhanced when consumed with meat or fish.

Zinc is required for a healthy immune system, correct DNA synthesis, healthy growth during childhood, fertility, and wound healing. Zinc deficiency results in compromised immunity, delayed puberty, and cardiometabolic risk. 

The best sources are meat, poultry, and fish, while vegetarian diets can reduce absorption by up to one-third. When plants are combined with animal-derived foods, absorption improves. Diets low in animal-sourced foods and rich in fibre and phytates cause low zinc status, even at recommended intake levels. Even in omnivorous populations, zinc intake levels can be below the estimated average requirement.

Iodine is essential for thyroid hormone synthesis, which may be compromised in diets that contain high levels of goitrogenic foods or have become more Westernized. Fish, seafood, seaweed, eggs, and dairy are good dietary sources. Dietary change to more plant-based diets could aggravate iodine adequacy, especially in regions without iodine fortification programmes. 

Deficiency leads to goitre, low metabolic rates, and mental retardation. This is particularly critical for lactating females. Vegetarians and vegans in such countries are at increased risk of low iodine status and deficiency, and may need iodine supplements to ensure adequate intake. 

Selenium is involved in antioxidant defences, thyroid function, and immunity. Good sources are seafood, organ meats, dairy, nuts, and grains. Selenium availability in food relies on the content in soils, which affects plants more than animals. 

Intake may be low in some populations, such as older adults and vegan women. Avoiding animal-sourced foods may reduce selenium status. Deficiency risks may be higher in vegans and vegetarians, depending on the biomarkers used to identify deficiency. Excessive intake can also be toxic.

Choline contributes to neural health, memory, and foetal brain development, offering protection in early and later life. Since the liver's production of choline is insufficient, it must be acquired from the diet. Rich dietary sources include organ meats and eggs. 

Many, if not most, vegetarians may not meet the recommended intake levels, which is particularly relevant for pregnant women. In older adults, decreased choline intake and low circulating choline levels are associated with an increased risk for cognitive decline and Alzheimer’s disease progression.

Food offers more than the sum of protein, vitamins, and minerals, leading to a 'matrix effect', which may at least be partially ascribed to the presence of overlooked biochemical compounds. 

Animal-sourced foods contain various bioactive molecules, such as taurine, creatine, carnosine, anserine, carnitine, conjugated linoleic acid, insulin-like growth factor-1, and bioactive peptides. 

Their health effects are multiple, including antioxidant, anti-inflammatory, and anti-aging outcomes. They contribute to gastrointestinal, cardiovascular, and cognitive health, as well as immune functions.