Bioavailability

Even if certain nutrients are present in plant-based foods, that does not automatically mean the body can absorb and utilize them to the same extent as from animal products. This argument is not about whether nutrients exist in a food, but about their bioavailability—that is, the proportion of a nutrient that actually reaches the body’s tissues and can be used.

Critics argue that plant foods contain so-called antinutrients, especially phytates (phytic acid), which can bind minerals such as iron, zinc, and calcium and may inhibit their absorption in the gut. Oxalates (e.g., in spinach) and tannins (e.g., in tea and coffee) are also cited, as they can influence the absorption of certain minerals.

In addition, a distinction is drawn between heme iron from animal foods and non-heme iron from plant sources, with heme iron often considered more bioavailable on average. Similar arguments are made for zinc and calcium.

Lower utilization is also discussed for certain vitamins and fatty acids. For example, beta-carotene must first be converted into retinol (vitamin A), and the plant-based omega-3 fatty acid ALA is only converted to a limited extent into the long-chain fatty acids EPA and DHA.

In the area of protein, it is argued that animal proteins have a higher biological value and achieve better scores in terms of PDCAAS or DIAAS, while plant proteins may contain limiting amino acids.

From these factors, critics conclude that even if nutrient intake is theoretically sufficient, a purely plant-based diet could lead to functional deficiencies because actual absorption and utilization in the body may be lower than in a diet that includes animal products.

At its core, this argument is more sophisticated than the simple claim that “nutrients are (or are not) present”: even if a nutrient is contained in a food, what matters is how much the body actually absorbs and uses. That is what bioavailability means. It depends not only on the food itself, but also on preparation methods, combinations with other foods, one’s current nutrient status, gut health, and even genetic factors.

That is why a careful classification is important: bioavailability is not an all-or-nothing problem, but a matter of probability, dose, and context. For many nutrients, a plant-based diet can have lower average absorption per serving, but in practice this can often be compensated very well through food choice, preparation, and pairing strategies. In other words, this argument mainly shows that a balanced vegan diet should be planned based on knowledge—not that it is inherently impossible. At first glance this may sound like it is very complicated to live vegan and healthy at the same time, but non-vegan diets also need to be well planned in order to be healthy. More on this can be found at the bottom under related topics.

Why plant-based bioavailability is sometimes lower

Plant foods contain certain accompanying substances that can inhibit the absorption of some minerals. The most commonly mentioned are:

• Phytates (phytic acid): found especially in whole grains, legumes, nuts, and seeds, and can bind iron, zinc, and sometimes calcium.
• Oxalates: e.g., in spinach or chard; they can reduce calcium absorption, especially from those foods.
• Tannins/polyphenols: e.g., in black/green tea and coffee; they can reduce iron absorption from a meal.
• High-fiber matrix: fiber is very valuable for health, but it can slow the release of some minerals from food. In practice, this is usually a moderate effect.

At the same time, there are strong absorption enhancers that can be used deliberately:

• Vitamin C significantly improves absorption of non-heme iron.
• Fermentation, sprouting, and soaking reduce phytates and can increase mineral availability.
• Iodized salt provides iodine very reliably regardless of plant-based inhibitors.

1) Iron: heme iron vs. non-heme iron

A classic point is that animal foods contain heme iron, which is absorbed more efficiently on average, while plant iron is non-heme iron and depends more strongly on inhibitors and enhancers. However, what matters is not the theoretical absorption percentage but whether adequacy works in real life. This can be managed well on a vegan diet because non-heme iron responds strongly to vitamin C.

Practical strategies:

• Combine iron-rich foods (e.g., lentils, beans, chickpeas, tofu, pumpkin seeds, oats, whole grains) with vitamin-C sources (e.g., bell peppers, citrus fruits, broccoli, berries).
• Avoid drinking tea/coffee directly with iron-rich meals (a time gap can help).
• Use fermented or sprouted products (e.g., sourdough bread, tempeh).

2) Zinc: phytates as a central factor

With zinc, the argument is similar: phytates can reduce absorption. Many plant zinc sources are also high in phytates, which makes the concern plausible. In practice, however, this is often manageable through amount, preparation, and food choice.

Practical strategies:

• Zinc sources: legumes, whole grains, nuts, seeds (e.g., pumpkin seeds, sesame), oats.
• Soaking, sprouting, and fermenting reduce phytates.
• For very high needs (e.g., in certain life phases), targeted planning or, if necessary, supplementation may be considered.

3) Calcium: not all vegetables are the same

With calcium, it is often broadly said that milk is better. The key educational point is: plant sources vary widely. Oxalate-rich plants such as spinach do contain calcium, but it is less available there. Other vegetables (e.g., kale, broccoli, bok choy) are often much better practical calcium sources.

Practical strategies:

• Calcium-rich sources: kale, broccoli, bok choy, sesame/tahini, almonds, calcium-rich mineral water.
• Fortified plant milks are especially convenient in practice.
• Oxalate-rich classics like spinach are healthy, but not the best main source for calcium.

4) Omega-3: ALA, EPA, and DHA

Here the issue is less about inhibitors and more about conversion: plant sources typically provide ALA (e.g., from flaxseed, chia seeds, walnuts, rapeseed/canola oil). The body can convert ALA into EPA and DHA, but this conversion is limited and varies between individuals. This yields a clear takeaway: if you want to shortcut the discussion, use the direct vegan source.

Practical strategies:

• Include ALA sources regularly: flaxseed, chia seeds, walnuts, rapeseed/canola oil.
• For direct EPA/DHA intake, algae oil can be used. This is the original source from which fish obtain their omega-3 fatty acids.

5) Vitamin A: beta-carotene as a precursor

Animal foods provide retinol. Plant foods often provide beta-carotene and other carotenoids, from which the body produces vitamin A. Conversion varies between individuals and depends, among other things, on fat in the meal. Educationally, this is important: it is not an argument against plants, but a note on how to combine them sensibly.

Practical strategies:

• Carotenoid sources: carrots, sweet potatoes, pumpkin, spinach, kale, red bell pepper.
• Combine with some fat (e.g., nuts, seeds, olive oil), because carotenoids are fat-soluble.

6) Protein quality: PDCAAS/DIAAS and limiting amino acids

Regarding protein, it is often argued that animal proteins score better in metrics such as PDCAAS or DIAAS. The educational core is: even if individual plant protein sources can have a limiting amino acid, this can be balanced very well in a varied diet across the day. In practice, soy (tofu/tempeh/soy yogurt), legumes, and combinations of grains and legumes form a robust foundation.

Practical strategies:

• Protein staples: tofu, tempeh, soy yogurt, lentils, beans, chickpeas, seitan, quinoa, oats.
• Variety across the day: e.g., legumes + whole grains + nuts/seeds.

7) Iodine and selenium: less about bioavailability, more about supply strategy

For iodine and selenium, in practice the issue is often less about inhibitors and more about whether sufficient amounts are consumed regularly. The most reliable vegan iodine strategy is iodized salt. Seaweed can contribute, but iodine content varies widely, so controlled use is sensible. Selenium depends strongly on soil content by region; Brazil nuts are often mentioned (in moderate amounts), along with whole grains, legumes, and nuts.

Summary: what follows in practice?

The bioavailability argument points to real mechanisms (phytates, oxalates, tannins, conversion rates), but it is not proof that vegan diets inevitably fail. It is primarily an indication that planning is useful: choose appropriate foods, use preparation methods, and in a few areas consider fortified products or supplements if needed.

Key takeaways:

• Phytates can be reduced through soaking, sprouting, and fermentation.
• Plant-based iron works especially well with vitamin C.
• Calcium: prefer kale/broccoli/bok choy and fortified plant milks rather than relying on spinach.
• For EPA/DHA, consider direct intake via algae oil if needed.