Bioavailability: The Missing Piece in How We Think About Nutrition

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Bioavailability: The Missing Piece in How We Think About Nutrition

When we think about improving our diet, the conversation often centres around nutrient density. We’re told to eat more iron-rich foods, more calcium-rich foods, more foods packed with vitamins, minerals and phytonutrients.

This is the starting point. But it is only part of the picture.

Because we cannot talk about nutrient density without also talking about bioavailability.

A food may look impressive on paper, but what ultimately matters is how much of those nutrients your body can actually absorb and use.

What Is Bioavailability?

Bioavailability refers to how well a nutrient is absorbed from food and made available for use in the body.

Not everything we eat is fully absorbed. In fact, absorption can vary significantly depending on the food itself, how it is prepared, what it is eaten with, and the individual consuming it.

This applies to both whole foods and supplements.

It means that two foods with similar nutrient content can have very different nutritional value in practice.

Why Bioavailability Changes Everything

A common example is calcium. Spinach is often listed as a calcium-rich food. One cup of cooked spinach contains roughly 240 mg of calcium, which appears comparable to a standard glass of milk at around 300 mg.

However, spinach is also high in oxalates, compounds that bind to calcium and reduce its absorption. Estimates suggest that only around 5 percent of the calcium in spinach is absorbed. In contrast, calcium from dairy is generally more bioavailable, with absorption typically estimated at around 30 to 40 percent.

So while the numbers on paper look similar, the amount your body can actually use is quite different.

Iron provides another clear example. Haem iron, found in animal foods such as red meat, is typically absorbed at a rate of around 15 to 35 percent. Non-haem iron, found in plant foods, is absorbed at a lower and more variable rate, often estimated between 2 to 10 percent depending on the meal and the individual.

Non-haem iron is also more sensitive to compounds such as phytates and polyphenols, which can inhibit absorption under certain conditions. This helps explain why some people may consume iron-rich plant foods yet still experience low iron status.

A similar principle applies to supplements. Magnesium oxide is one of the most common forms found in supplements, yet it has relatively low bioavailability, with only a small proportion absorbed. As a result, much of it remains in the bowel, where it can draw water into the intestines and lead to symptoms like diarrhoea.

Magnesium glycinate, on the other hand, is bound to the amino acid glycine and is generally better absorbed and better tolerated, with fewer gastrointestinal side effects. The form is just as important, if not more, than the amount.

Food Synergy

Whole foods come packaged with a complex matrix of nutrients, enzymes, and cofactors that work together. This can enhance absorption in ways that isolated nutrients often cannot replicate.

In Ayurvedic traditions, turmeric, which contains the active compound curcumin, was rarely used in isolation. It was typically cooked with fats and combined with black pepper, which contains piperine, a compound known to enhance curcumin absorption. Similarly, soaking and fermenting grains and legumes was a common practice, helping to reduce phytates and improve the availability of minerals such as iron and zinc.

Supplements are often seen as a convenient solution, but they are not always the most effective approach. Many supplemental forms of nutrients are less well absorbed and lack the additional compounds naturally present in whole foods.

Carotenoids such as beta-carotene illustrate this well. Diets rich in carotenoid-containing foods, such as fruits and vegetables, have consistently been associated with lower disease risk in observational research. However, when beta-carotene was isolated and tested in supplement form in large clinical trials, it did not replicate the same benefits and in some cases was associated with adverse outcomes in high-risk populations. In foods like carrots, sweet potatoes, and leafy greens, beta-carotene exists within a broader food matrix that includes other carotenoids, fats, and plant compounds that influence its absorption and broader biological effects. This suggests that the benefits observed in whole foods are likely the result of multiple interacting compounds rather than a single isolated nutrient.

Explore our range of whole food meals here

That said, supplements do have their place, particularly in cases of deficiency or clinical need, but quality, form, and context are critical considerations. They should never be seen as a replacement for whole foods and are best taken under the guidance of a qualified professional.

For example, low iron is often assumed in cases of fatigue, leading some people to take therapeutic doses without confirming an actual deficiency. Excess iron can act as a pro-oxidant and may be harmful. Minerals work in balance within the body, so it is important to assess need carefully rather than supplementing indiscriminately.

A More Complete Approach to Nutrition

If the goal is to truly nourish the body, the focus needs to consider both nutrient density and bioavailability, alongside what works for the individual.

There is no single diet that suits everyone. For example, some people continue to produce the enzyme lactase beyond infancy, allowing them to digest lactose in dairy, while others do not.

This variability extends across many aspects of nutrition, including how nutrients are digested, absorbed, and tolerated.

Understanding the nutrient content of foods, how they are prepared, and how they are likely to be absorbed provides a more complete foundation.

From there, individual feedback becomes important. Signs and symptoms can offer useful clues as to what may need adjusting over time.

Health is an ever-evolving process.

References

• Institute of Medicine (2000). Dietary Reference Intakes: Applications in Dietary Assessment. Washington, DC: National Academies Press.
• Hurrell R & Egli I (2010). Iron bioavailability and dietary reference values. The American Journal of Clinical Nutrition, 91(5), pp.1461S–1467S.
• Weaver CM et al. (2016). Calcium in human health. Annual Review of Nutrition, 36, pp.127–148.
• Heaney RP & Weaver CM (1989). Oxalate: effect on calcium absorbability. The American Journal of Clinical Nutrition, 50(4), pp.830–832.
• Hallberg L & Hulthén L (2000). Prediction of dietary iron absorption. The American Journal of Clinical Nutrition, 71(5), pp.1147–1160.
• World Health Organization (2001). Iron Deficiency Anaemia: Assessment, Prevention and Control. Geneva: WHO.
• Schuchardt JP & Hahn A (2017). Intestinal absorption and factors influencing bioavailability of magnesium. Nutrients, 9(9), p.1020.
• Jacobs DR & Tapsell LC (2007). Food synergy: the key to a healthy diet. The American Journal of Clinical Nutrition, 86(5), pp.1256S–1262S.
• Liu RH (2003). Health benefits of fruit and vegetables are from additive and synergistic combinations. Nature, 423, pp.787–788.
• Shoba G et al. (1998). Influence of piperine on the pharmacokinetics of curcumin. Planta Medica, 64(4), pp.353–356.
• ATBC Study (1994). The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers. New England Journal of Medicine, 330(15), pp.1029–1035.
• CARET trial (1996). Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease. New England Journal of Medicine, 334(18), pp.1150–1155.
• Tolkien Z et al. (2015). Side effects of iron supplementation: a systematic review and meta-analysis. The American Journal of Clinical Nutrition, 102(4), pp.863–874.