Bioavailability is a fundamental concept describing how well a substance, such as a medication, supplement, or nutrient, is absorbed and utilized by the body. It refers to the proportion of an administered dose that reaches the bloodstream and becomes available to produce its intended effects. Understanding this process is foundational for ensuring the efficacy and safety of substances consumed for health purposes.
Understanding Bioavailability
Bioavailability quantifies the fraction of an administered substance that enters the systemic circulation unchanged and becomes accessible to the body’s tissues. When a substance is taken, it journeys through the body from the point of administration (e.g., mouth) into the bloodstream. Not all of the administered dose may complete this journey intact.
For instance, an oral medication must navigate the digestive system, encountering digestive enzymes and varying pH levels. It then crosses the gut wall and may pass through the liver before reaching general circulation. Any portion metabolized or eliminated before reaching the bloodstream is considered not bioavailable.
Calculating Bioavailability
Bioavailability is commonly expressed as a percentage or a fraction. Absolute bioavailability compares the systemic availability of a substance given by a non-intravenous (non-IV) route to the same substance given intravenously. Intravenous administration is considered to have 100% bioavailability because the substance is delivered directly into the bloodstream, bypassing absorption barriers and first-pass metabolism in the liver.
The general formula for calculating absolute bioavailability (F) is:
F = (AUCoral DoseIV) / (AUCIV Doseoral)
AUC, or “Area Under the Curve,” represents the total exposure of the body to the substance over time, derived from plotting its concentration in plasma against time. A larger AUC indicates a greater amount of the substance in the bloodstream. Dose refers to the administered quantity of the substance.
For example, if a 100 mg oral dose of a drug results in an AUC of 60 mgh/L, and a 25 mg intravenous dose of the same drug results in an AUC of 100 mgh/L, the absolute bioavailability would be calculated as (60 25) / (100 100) = 0.15 or 15%. This means only 15% of the oral dose reached systemic circulation. Relative bioavailability compares the bioavailability of two different non-IV formulations of the same substance.
Factors Influencing Bioavailability
Several factors can influence a substance’s bioavailability:
- Physiological factors within the body, such as pH levels in the gastrointestinal tract, which affect solubility and stability, and stomach emptying rate, which influences movement into the intestines for absorption.
- The presence of digestive enzymes and the liver’s “first-pass metabolism,” which can break down a substance before it reaches systemic circulation.
- Chemical and physical properties of the substance itself, including its solubility in water or fat, particle size, and chemical stability.
- Dosage form, such as whether a substance is in a liquid, tablet, or extended-release capsule, which can significantly impact its absorption rate.
- Interactions with food or other medications, which can modify absorption. Some substances absorb better with food, while others may have reduced absorption or increased side effects.
Why Bioavailability is Important
Understanding bioavailability has practical implications across various fields. In drug development, it is a primary consideration for determining appropriate dosages, designing effective formulations, and choosing suitable administration routes. A drug with low bioavailability might require higher doses or more frequent administration to achieve its desired therapeutic effect, impacting both efficacy and safety.
In nutritional science, bioavailability helps determine how much of a nutrient from food or supplements is actually utilized by the body. For example, the bioavailability of certain minerals can be affected by other compounds present in food. Recognizing that a substance’s presence in a product does not automatically guarantee its effectiveness highlights bioavailability’s importance in ensuring optimal health outcomes and guiding consumer choices.