Bioavailability: Definition and influencing Factor

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.

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