Bioavailability: Definition and Factors

Bioavailability is the fraction of an ingested nutrient that is absorbed across the gut and made available for physiological function or storage. Because absorption is rarely complete, the bioavailable amount of a nutrient is generally less than the amount present in food, and it varies with the nutrient's chemical form, the food matrix, and the rest of the diet.

Definition

Bioavailability is the proportion of an ingested nutrient that is absorbed and becomes available to support normal physiological functions or to be stored.

Scope

This entry defines bioavailability and distinguishes related terms, then surveys the principal factors that modify it: the chemical species of the nutrient, the food matrix, enhancing and inhibiting dietary components, and host and life-stage factors. It treats bioavailability as a quantitative concept in nutritional biochemistry, not as clinical advice.

Core questions

• What exactly does 'bioavailable' mean and how does it differ from total intake?
• How is bioavailability measured or estimated?
• Which dietary, chemical, and host factors raise or lower it?
• Why do nutrients of the same total amount differ in usable yield?

Key concepts

• Bioavailable fraction
• Absolute versus relative bioavailability
• Bioaccessibility (release from the food matrix)
• Chemical form and speciation of the nutrient
• Enhancing and inhibiting dietary components
• Host and life-stage factors
• Bioefficacy (conversion to active form)

Mechanisms

Bioavailability is set sequentially: a nutrient must first be released from the food matrix (bioaccessibility), then be in a chemical form the intestine can take up, then survive competition from other luminal components, and finally be transported across the enterocyte. Each step is modifiable. Chemical form is decisive — for example, iron bioavailability differs sharply between heme and non-heme forms and is depressed by ligands that bind it in the lumen (Hurrell & Egli, 2010). Dietary inhibitors such as phytate reduce the absorbable fraction of several minerals by forming poorly soluble complexes (Schlemmer et al., 2009), while host factors, including gastric acidity and changes with aging, alter uptake (Russell, 2001). Measurement approaches range from in vitro release assays to in vivo absorption studies (Srinivasan, 2001).

Clinical relevance

The concept explains why nutrient content in food does not equal the amount the body obtains, and why dietary form and combination affect nutrient adequacy. It is presented here for reference and education and is not a basis for individual dietary, supplementation, or treatment decisions.

Evidence & guidelines

Dietary reference values for some nutrients are framed in terms of absorbed fraction rather than total intake, explicitly building bioavailability assumptions into requirement estimates (Hurrell & Egli, 2010).

History

Bioavailability emerged as a formal concept in twentieth-century nutritional science as researchers recognized that ingested nutrient content overstates usable intake; practical frameworks for demonstrating availability were later set out in the nutrition literature (Srinivasan, 2001), and life-stage and dietary modifiers were characterized in subsequent reviews (Russell, 2001).

Key figures

• Richard Hurrell
• Robert M. Russell
• Ulrich Schlemmer

Seminal works

• srinivasan-2001
• hurrell-egli-2010

Frequently asked questions

What is the difference between absolute and relative bioavailability?

Absolute bioavailability is the absorbed fraction of a nutrient measured against a fully available reference, while relative bioavailability compares one source or form of a nutrient with another reference source.

Why is the chemical form of a nutrient so important?

Different chemical forms are recognized by different transport pathways and interact differently with luminal ligands, so the same element can be absorbed very efficiently in one form and poorly in another.