Bioavailability: Definition, Calculation & Equation

Instructor: Victoria Leo

Victoria teaches college, authors books, has a therapy practice and masters degrees in anthropology and psychology.

Bioavailability measures how much of what you ingest or take intravenously (in the case of pharmaceuticals) enters your bloodstream to be available to your cells. Learn how bioavailability is calculated, and the equation that's commonly used.

What is Bioavailability?

The world of supplements, herbs and other nutritional products can be bewildering. We all can get confused, not only about what a product is supposed to do, but also how one version of a substance compares with another. If we know that a vitamin or herb has generally good effects, does Product A actually deliver the same amount of the substance to my body that Product B does? A related question, applied to pharmaceuticals, would be: does the generic drug really have the same effect on my disease or problem that the name-brand drug does?

All these questions have, at their base, the question of how much of a supposedly-helpful substance that you ingest will actually get to your bloodstream for distribution to all the cells of your body. This is called the substance's bioavailability - how much of what was in the pill or other formulation actually ends up being available to your body? The definition of bioavailability is somewhat different for pharmaceuticals (prescription drugs) than for herbals, supplements and other nutrition products.

In drugs, bioavailability is the fraction of the drug dose that actually reaches the bloodstream. If the drug is injected into a vein (intravenously), the bioavailability is 100%. Herbs, supplements and other nutrients, which are usually taken orally, can't be measured that way, so we measure the fraction of the product that is absorbed.

Why Bioavailability Varies

If you take a drug orally, all of it will not end up in your bloodstream. Why? Think about it. Absorption of any substance will be affected by whether or not you take the substance with food, and it varies with specific kinds of foods as well. That's why doctors are so insistent on the details in their directions - it really matters!

If the food needs to be taken on an empty stomach, it means that its bioavailability will be significantly decreased by competition for absorption (in the small intestine) by other foods, or even be completely inactivated by certain foods. Most people who lose thyroid function take a medication whose absorption will be decreased if you take it with food, for example. It's even more important to not take your calcium or multivitamin supplement - both wonderful for your health, of course - within 4 hours of that thyroid medication.

Our intestines respond differently at different times of the day, so you can reduce bioavailability of a drug by taking it at the wrong time. Other things that can reduce the bioavailability of an oral drug are: kidneys or liver that aren't functioning at full capacity; other substances that you are taking; how efficiently the absorption mechanism in your intestine works; and how healthy the bacterial colonies in your gut (your microflora) are. With nutritional supplements, whether it is successful or not is a lot more subjective. If I feel 'better,' I will keep taking something, but you can't measure 'feeling better' the same way that you can measure white cell count, right?

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