Micronutrient Absorption and Bioavailability

Micronutrient absorption and bioavailability describe how vitamins and minerals are taken up by the gut and what fraction of an ingested amount becomes available to the body. Unlike the bulk macronutrients, micronutrient uptake is often tightly regulated and strongly modified by chemical form, the surrounding food matrix, and dietary enhancers and inhibitors, so the amount eaten and the amount absorbed can differ markedly.

Definition

Micronutrient absorption is the transfer of dietary vitamins and minerals across the intestinal epithelium, and micronutrient bioavailability is the fraction of an ingested micronutrient that is absorbed and made available for metabolic functions.

Scope

This topic covers the pathways by which vitamins and minerals cross the intestinal epithelium, the concept of bioavailability and its dietary determinants, and the enhancer and inhibitor interactions that shape micronutrient nutrition, using dietary iron as the principal worked example. It is reference physiology and nutrition and does not provide supplementation advice or dosing.

Core questions

• How are vitamins and minerals absorbed across the intestinal epithelium?
• What determines how much of an ingested micronutrient is actually absorbed?
• How do dietary enhancers and inhibitors modify micronutrient bioavailability?

Key concepts

• Bioavailability and absorption efficiency
• Chemical form (e.g., haem vs non-haem iron)
• Dietary enhancers (ascorbic acid) and inhibitors (phytate, polyphenols)
• Food matrix effects
• Regulated mineral uptake and homeostasis
• Fat-soluble versus water-soluble vitamin absorption

Mechanisms

Micronutrients are absorbed by a mix of carrier-mediated, receptor-mediated, and passive pathways across the small-intestinal mucosa, whose amplified surface provides the absorptive area for uptake (Helander & Fändriks, 2014; Wright et al., 2011). For many minerals, the absorbed fraction is governed by the nutrient's chemical form and by interactions in the meal: dietary iron is the canonical case, where haem iron is well absorbed, non-haem iron absorption is enhanced by ascorbic acid and inhibited by phytate and certain polyphenols, and whole-body iron status further modulates uptake (Hurrell & Egli, 2010). Because of these interactions, bioavailability — not intake alone — determines the physiological supply of a micronutrient.

Clinical relevance

These principles inform how micronutrient deficiencies and the design of food fortification and dietary reference values are reasoned about in the health sciences. The entry is reference and educational material describing absorption and bioavailability; it is not a basis for individual supplementation, dosing, or treatment decisions.

Debates

How should dietary reference values account for variable bioavailability?

Because the absorbed fraction of minerals such as iron depends heavily on diet composition and body status, reference values must assume a representative bioavailability, and the choice of that assumption shapes the recommended intakes derived from it.

Seminal works

• hurrell-2010

Frequently asked questions

Why isn't the amount of a mineral eaten the same as the amount absorbed?

Mineral absorption is regulated and is strongly affected by chemical form and by other meal components; enhancers can raise and inhibitors can lower the absorbed fraction, so bioavailability often differs substantially from intake.

What makes haem iron more available than non-haem iron?

Haem iron is absorbed through a relatively efficient pathway that is less affected by dietary inhibitors, whereas non-haem iron absorption varies widely and is enhanced by ascorbic acid and reduced by phytate and some polyphenols.