What Is Bioavailability Of Vitamins?

Whenever we eat or drink something, its nutrients are released. These nutrients then go into our bloodstream, through which they travel to their target destination. However, sometimes, our body isn’t able to use all nutrients the same way. This is because of their bioavailability. If you have a greater understanding of bioavailability, you will also be able to have a more nutritious lifestyle.

What Is Bioavailability?

Essentially, it describes the percentage of nutrients that are actually absorbed and used to support bodily functions. Bioavailability can change at different parts of the body, and due to a variety of different things, both internal and external factors. Some external factors include things such as the actual chemical in the nutrient ad the food matrix itself. Some internal factors include life stage, age and gender.

As a rule of thumb, it is said that the bioavailability of fats, proteins and carbohydrates (macronutrients) is about 90%. With vitamins and minerals, so-called ‘micronutrients’, however, those rates can vary tremendously. This is why it is important to learn more about the bioavailability of vitamins and minerals so that you can make sure your body is always healthy.

Five Things that Affect Bioavailability of Vitamins and Minerals

The table below highlights in greater detail some of the most common factors that affect the bioavailability of vitamins and minerals.

Factor What It Does
Enzymes There are a number of enzymes that stimulate our body to absorb vitamins, but others (like proteinase) inhibit it. Vitamin B1 deficiency, for instance, is common in countries where a lot of fish is consumed, as fish contains enzymes that destroy this vitamin.
Conversion With a number of vitamins, a conversion has to take place inside the body itself before it can actually be used. For instance, calcium needs vitamin D in order to be properly absorbed by the body. A deficiency of vitamin D will inevitably lead to a calcium deficiency, even if someone were to supplement with calcium.
Aging, toxicity and poor health The body’s nutrient pathways are very fragile and if certain people become ill or lead an unhealthy lifestyle, their ability to absorb vitamins and minerals may be impaired.
Low amounts of dietary fat Fat has become synonymous with evil, but we do actually need fat (the good kind) to survive. Since so many people shun it almost completely, they often start to build up deficiencies because their body cannot absorb nutrients properly anymore.
Advanced nutrient delivery systems This is the one element that can increase bioavailability. Some multivitamin tablets now have enteric coatings so that they can survive the passage into the stomach. This also increases their bioavilability.

The Food Matrix and Bioavailability

Food has to be liberated from the food matrix and it then has to be turned into a specific chemical that goes into the cells of the gut, or passes through them. This is called “bioaccessiblity”. Bioaccessibility is achieved through chewing and digestion in the mouth, after which is it is subjected to gastric juice before it goes into the small intestine. Food then gets broken down even further through contact with other enzymes.

Making food more digestible is also helped through pureeing and cooking it. However, as previously stated, food is not difficult to make bioavailable. The difficulty lies in the vitamins and minerals. Different ones direct differently to the various elements of the food matrix, and also depending on what else has been included in this matrix. A classic example is prescription medication that interacts with vitamins, for instance.

It is quite common for foods to be ‘fortified’ with certain vitamins and minerals. Vitamin D, for instance, is often added to milk and fruit juices. There are a number of vitamins and minerals, one of which is folic acid, that have much better bioavailability if they are synthetic. However, it is important that people see this as a supplement to a healthy diet, not a replacement.

What Enhances Bioavailability?

When nutrients are absorbed, they can interact with each other. In terms of bioavailability, there can be enhancers and there can be inhibitors. Enhancers, which make the effects of a nutrient stronger, can do a number of different things. For instance, they can cancel out the effect of an inhibitor, or they can increase absorption rates. Enhancers are found in a variety of things. A good example is a breakfast of fortified orange juice and fortified cereals. The vitamin C in the orange juice will enhance the bioavailability of the iron in the cereal.

What Inhibits Bioavailability?

As stated, however, there are also some significant inhibitors. They can do inhibit the absorption of vitamins and minerals in three different ways:

1. They can bind to the nutrient and create a form that the intestinal cells’ surface’s uptake systems don’t recognize and therefore discard.
2. They can make the nutrient insoluble, which means it can no longer be absorbed.
3. Some inhibitors need the same uptake system as a vitamin or mineral and are stronger, winning the race in other words.

A good example is phytic acid, which is found in nuts, seeds, wholegrain cereals and pulses, to name but a few. Phytic acid binds very strongly to zinc, iron and calcium, rendering them unavailable for absorption. This is why it is important to allow food to be fermented or germinated in order to reduce the phytic acid content.

Non-haem iron and calcium, interestingly, compete for the same uptake system. While this can be avoided by not taking them together and taking them after a meal, a lot of people take their supplements altogether, usually around meal time. This means that it is unlikely that they will be able to absorb calcium in particular, making it fully non-bioavailable.

Food constituents can also be strong inhibitors, but this can be used to your advantage as well. A good example of this are phytosterols, which are often added to enriched products. What these do is inhibit the absorption of cholesterol. As such, they are actually inhibitors that make you healthier overall.

Bioavailability of Vitamin D – An Example

Vitamin D is said to be one of the most bioavailable vitamins in our system. This is because our skin produces it through sun exposure. However, as much as the sun and our skin are therefore enhancers, there are also many inhibitors in place. The biggest inhibitor is our lifestyle: we no longer spend enough time in the sun. This can be due to our geographical location but it is most often because we spend too much time inside our homes and offices and, when we do go out in the sun, we lather on the sunscreen. An SPF 8 reduces the amount of vitamin D our skin can absorb by 95%.

There are many other factors that make vitamin D less bioavailable, however. One of the biggest ones, which is also a huge problem in our society today, is obesity. Research have been conducted on this subject to determine why people with obesity are at greater risk of having a vitamin D deficiency. While it was long accepted that it is because obese people are more likely to be housebound or to spend time indoors for personal reasons, research has now demonstrated that other factors are at play as well. When supplemented with a 50,000 IU dose of vitamin D2, the non-obese control group saw a marked increase in their vitamin D levels. In the obese group, however, the levels rose 54% less. It is now accepted that the fact that it is deposited in the fat compartments of the body reduces the bioavailability of vitamin D.

How Understanding Bioavailability Helps

It is important to have at least a baseline of understanding of bioavailability. This is particularly true for vitamin A, folate, zinc, iron, magnesium and calcium. By having a degree of awareness of the bioavailability, it also becomes a lot easier to consume a healthy, nutritious diet. Unfortunately, it isn’t that obvious, because recommendations in terms of what amounts to take and how often vary from one country to another, and even from one institution to another. Numerous charts exist that highlights the various nutrients, vitamins, minerals and other elements our bodies need in order to be healthy. These charts also often show the inhibitors and enhancers of bioavailability. A brief personal study, for which the online world can be used, is often sufficient to increase all levels of nutrients in the body.

Resources and References:

Bioavailability of Vitamins – Information on bioavailability of vitamins. (NIH.gov)
Human Bioavailability of Vitamins – Bioavailability of vitamins in people. (Nutrition Research Review)
Multivitamin and Multimineral Dietary Supplements: Definitions, Characterization, Bioavailability and Drug Interactions – Information on multivitamin supplements. (The American Journal of Clinical Nutrition)
Why Bioavailability Matters for Vitamins and Essential Nutrients – Importance of bioavailability of vitamins and nutrients. (The Recommended Daily)
Vitamin D Bioavailability: State of the Art – Review on current bioavailability of vitamin D supplements. (NIH.gov)
Evaluation of Vehicle Substances on Vitamin D Bioavailability: A Systematic Review – Review of the role of vehicle substances for vitamin D bioavailability. (NIH.gov)
Bioavailability of Vitamin D(2) and D(3) in Healthy Volunteers, a Randomized Placebo-Controlled Trial – Study on vitamin D2 and D3 bioavailability. (NIH.gov)
Decreased Bioavailability of Vitamin D in Obesity – Vitamin D bioavailability in obese people. (The American Journal of Clinical Nutrition)