Nutrient Bioavailability

Nutrient bioavailability means how much of a nutrient in food the body can actually absorb and use. More precisely, it is the proportion of an ingested nutrient that is absorbed and becomes available for normal physiological functions or storage. A food’s nutritional value therefore depends not only on its nutrient content, but also on the availability of those nutrients to the body.

Nutrient content versus usable nutrition

A laboratory measurement of nutrient content tells us how much a food contains, not necessarily how much a person will obtain from it. Two foods containing similar amounts of a nutrient can provide different nutritional benefits.

• Bioaccessibility refers to the release of a nutrient from the food during digestion so that it becomes available for absorption; it is a prerequisite for bioavailability.
• For example, haem iron, found in animal tissues, is generally more readily absorbed than non-haem iron, which is present in plant foods and also in animal foods.

What affects bioavailability

Bioavailability depends on the nutrient’s chemical form, the structure and composition of the food, the other foods eaten with it, and the individual’s nutritional and health status.

• Phytates in cereals and pulses can bind minerals such as iron and zinc, reducing their absorption.
• Vitamin C consumed in the same meal enhances non-haem iron absorption, while tea polyphenols can inhibit it.
• Dietary fat supports the absorption of fat-soluble vitamins A, D, E and K.
• Digestive and intestinal disorders can impair absorption, while the body’s nutrient status can influence uptake; iron absorption, for example, generally increases when iron stores are low.

Implications for evaluating and improving foods

A nutritionally improved food should be evaluated for the nutrients it delivers in usable form, not merely for a higher nutrient concentration.

• Soaking, germination and fermentation can reduce phytate levels and improve mineral availability, although effects depend on the food and processing conditions.
• Food preparation involves trade-offs: cooking can improve the release of some nutrients but can also cause losses of heat-sensitive or water-soluble vitamins.
• Fortification adds nutrients during food processing, whereas biofortification increases nutrient density in crops during their development or growth; neither higher content alone nor the method used establishes bioavailability.
• Assessment should consider the nutrient form, usual meal composition, preparation practices and target population, alongside evidence of absorption and improvement in nutritional status.