Bioavailability Definition

When a substance such as a medicine or supplement enters your system, the portion of the total substance introduces which can effectively create a response determines that substance’s bioavailability. The bioavailability of a substance can fluctuate, depending on the route of administration. Intravenous administration, or a direct line into the bloodstream, is typically considered 100% bioavailability, as all of the substance will reach target cells. In oral administration routes, AKA when you take a pill, the amount of medicine or supplement you receive depends on many factors, including your diet and your personal metabolism.

Bioavailability has become a new and upcoming science in recent decades. Many researchers have been concerned with marketing for food and supplements. Many producers make claims that their foods or nutritional supplements carry certain nutrients. However, the science behind how these nutrients are absorbed into our system is very different. For example, milk claims to have huge amounts of calcium. Calcium is known to be a constituent of bone. Therefore, milk producers have claimed the enormous benefits of milk. However, the bioavailability of calcium in milk has never been shown. In fact, researchers are finding that milk and dairy products tend to pull calcium from the bones, to correct for the acidity they caused in the bloodstream. Countries that drink larger amounts of milk are shown to have higher incidences of hip fractures and poor bone health.

Clearly, the bioavailability of the calcium in milk is very low. On the other side of the spectrum, spinach also has a lot of calcium. Scientists have found that when you eat spinach, calcium is not depleted from your bones, and is able to be extracted from the spinach. In part, this is due to the high amount of fiber spinach has, which changes the way it moves through the intestines. This allows more calcium, and other nutrients, to be extracted. The bioavailability of nutrients in plants is typically higher than that of nutrients in animal products. In part, this is because the human body has evolved to be a frugivorous, not necessarily an omnivore or carnivore.

Factors Influencing Bioavailability

Route of Administration

Every medicine and nutrient must be taken into the body in some way. One of the largest hurdles to pass when creating a medicine is to understand how the medicine will reach the cells it needs to target. While it was mentioned before that the intravenous route is often considered 100% bioavailable, this is not always the case. A medicine which has hydrophilic (water-loving) tendencies will find it hard to make it through the cell membrane, which is very hydrophobic. To increase their bioavailability, they must often be coupled with another substance which is hydrophobic, so they can slip into cells.

Oral supplements must also conform to this rule. Further, they must make it through the digestive system and into the bloodstream. To do this, they often need to be designed to endure acid pH balances and high temperatures. Once they make it to the intestines, they can be absorbed into the bloodstream. But, like all nutrients and the food we eat, not all of it will make it into the bloodstream before we defecate and remove the substance from our system. These limitations severely lower the bioavailability of most orally administered drugs.


Every person’s biochemistry is slightly different, based on their DNA and how it has interacted with the environment over the course of their life. Therefore, their body will react differently with every substance. This will also affect a drug’s ability to enter the body, absorb through the tissues, and a drug’s overall ability to affect target cells. Thus, the bioavailability of any substance is also affected by individual and unique metabolisms.

Further, beyond your individual metabolism, all bodies go through different phases. When you are full of food, your body is actively working on digesting it. Your membranes become more active, your stomach and intestines actively work to move food around, and your cells are ready to receive materials. In this state, the bioavailability of supplements and medicines increases. In a fasted state, your body is not ready to move materials quickly from the intestines to blood stream, which may significantly lower the bioavailability of many substances.

Type of Substance

As discussed in the definition of bioavailability, the type, size, shape, and chemical properties of any given substance are of utmost importance. These properties determine if the molecule will even be able to make it into the body, and will determine how it interacts with the cell. Some substances are less bioavailable than others. This has become markedly clear in the use of supplements. While nutritional supplements do have some bioavailability, it is often found that the same nutrients found in natural foods have a much higher bioavailability. This is often because the supplements do not have any of the fiber or sugars, which are needed to help move the nutrients into the body.


1. Which of the following substances would have the highest bioavailability?
A. Pain-reliever delivered orally
B. Pain-reliever delivered intravenously
C. Pain-reliever as a topical gel

Answer to Question #1
B is correct. While the other two routes have to make it through multiple layers of tissue to enter the bloodstream, the intravenous route uses a needle to introduce it directly into the blood. In this way, in a matter of minutes, the pain-reliever will be accessible to all cells of the body. With the other routes of administration, more dose than is needed must be administered because not all of it will be able to get through the tissues into the blood.

2. Two people are given the same dose of medicine, in a pill form. One person took the pill with food, the other person took the pill on an empty stomach. For which person will the medicine have a higher bioavailability?
A. With food
B. Without food
C. They will be the same

Answer to Question #2
A is correct. The food in this patient’s system will stimulate their digestive system to get to work. This means that it will produce more acid and become generally more active. This will pull more of the medicine from the pill as it passes through the system. This is often why pills are labeled to “take with food”.

3. Birds are often seen eating rocks. These rocks are used to grind up food into more digestible bits. Do the rocks themselves have a bioavailability to birds?
A. Yes
B. No
C. Maybe

Answer to Question #3
B is correct. No, the rocks themselves are not bioavailable to the birds. The rocks are stored in a muscular organ called the gizzard, which helps grind food. But the cells of the birds do not use the rocks, and the rocks don’t dissolve into the bloodstream. In this way, they are completely unavailable.


  • Bruice, P. Y. (2011). Organic Chemistry (6th ed.). Boston: Prentice Hall.
  • Campbell, T. C., & Campbell, T. M. (2006). The China Study. Dallas: Benbella Books.
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.
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