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Bioavailability Formula – Definition & Calculation

The bioavailability formula calculates what fraction of an administered substance actually reaches the bloodstream. It is a key concept in pharmacology and nutritional science.

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Things worth knowing about "Bioavailability formula"

The bioavailability formula calculates what fraction of an administered substance actually reaches the bloodstream. It is a key concept in pharmacology and nutritional science.

What is the Bioavailability Formula?

The bioavailability formula is a mathematical tool used in pharmacology and nutritional science to determine what proportion of an administered substance – whether a drug, nutrient, or dietary supplement – actually reaches the systemic circulation and becomes biologically active. The so-called absolute bioavailability (F) is expressed as a percentage or as a decimal value between 0 and 1.

The Formula in Detail

The standard formula for calculating absolute bioavailability is:

F = (AUC_oral / Dose_oral) ÷ (AUC_iv / Dose_iv) × 100 %

Here, AUC stands for Area Under the Curve, which represents the total concentration of a substance in the blood over time. Intravenous (iv) administration serves as the reference standard, since 100 % of the substance enters the bloodstream directly when given intravenously.

Example Calculation

If a drug is taken orally and its AUC equals 80 % of the AUC following an intravenous dose (at equivalent doses), then the bioavailability F = 80 %.

Relative Bioavailability

In addition to absolute bioavailability, the concept of relative bioavailability is widely used. This compares two different formulations (e.g., tablet vs. capsule, or a generic vs. a brand-name product) without using an intravenous reference:

F_rel = (AUC_test / Dose_test) ÷ (AUC_reference / Dose_reference) × 100 %

Relative bioavailability is especially important for the approval of generic medications and pharmaceutical quality assurance.

Factors Affecting Bioavailability

The bioavailability of a substance is influenced by many factors:

Route of administration: Intravenous (100 %), oral, sublingual, transdermal, inhalation
First-pass effect: Metabolism of the substance in the liver after oral intake, before it reaches systemic circulation
Solubility: Lipophilic and hydrophilic properties of the substance
Formulation and release: Tablets, capsules, liquids
Gastric emptying rate and intestinal motility
Food interactions: For example, calcium ions can inhibit the absorption of certain antibiotics
Individual factors: Age, disease states, genetic variations (polymorphisms in metabolic enzymes)

Relevance in Nutritional Science

In nutritional science, bioavailability describes how efficiently the body absorbs and utilizes nutrients from food. For example, heme iron from animal sources has a bioavailability of approximately 15–35 %, which is significantly higher than non-heme iron from plant-based sources (2–20 %). The formula helps assess the true nutritional value of foods and dietary supplements.

Clinical Relevance

The bioavailability formula is essential for:

Dose determination of new drugs during clinical development
Comparing generic drugs to reference products (bioequivalence studies)
Dose adjustments in patients with impaired liver or kidney function
Optimizing dietary supplements and evaluating their effectiveness

References

Rowland, M. & Tozer, T. N. – Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. Lippincott Williams & Wilkins, 4th Edition (2010).
World Health Organization (WHO) – Multisource (generic) pharmaceutical products: guidelines on registration requirements to establish interchangeability. WHO Technical Report Series, No. 992 (2015).
Shargel, L. & Yu, A. B. C. – Applied Biopharmaceutics & Pharmacokinetics. McGraw-Hill Education, 7th Edition (2016).

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