Once you eat a meal or swallow a vitamin tablet, it begins a complex journey through your digestive system. Bioavailability is the portion of a given active ingredient that is capable of being absorbed through your intestinal membranes into your bloodstream and made available for use by your organs. According to Bates and Heseker, the amount of each vitamin which is made bioavailable varies from as low as 20% to as high as 98%.
What accounts for this significant variance? The bioavailability of vitamins is affected by the following five factors:
1. Certain enzymes stimulate vitamin absorption while others inhibit it
When you eat, the absorption of vitamins is usually enhanced by increased secretion of enzymes and bile acid. However, some foods contain compounds which inhibit the utilization of vitamins by way of reducing solubility or the amount released. For instance, proteinase (that is, an enzyme which breaks down protein) inhibitors in some raw fruits and vegetables can interfere with vitamin absorption. Enzymes that destroy or inactivate thiamine (vitamin B1), are present in fish, shellfish, and other foods. This has led to vitamin B1 deficiency in countries, such as Japan, where consumption of these foods is common.
2. Some vitamins must undergo a conversion to become bioavailable
Certain vitamins are not present in food in a bioavailable form. For example, riboflavin (vitamin B2) is present in milk and eggs but is bound to a carrier protein. The combined compound is known as riboflavin phosphate. In order for the organism to extract the necessary riboflavin from it, this compound must be converted in the intestines with the help of phosphatease enzymes. This conversion process however may be hindered by medications, such as certain anti-malarial drugs and tricyclic antidepressants, which have a chemical form similar to riboflavin. If you are taking such medications the bioavailability of riboflavin extracted from food is very low. In such cases taking a vitamin supplement where the nutrient is present in its pure form may be the only way to prevent riboflavin deficiency.
3. Poor health, toxicity and aging may reduce vitamin absorption
The body has many complex nutrient absorption pathways, and many of these are fragile and easily affected by illness or lifestyle, leading to a cascade of other symptoms. For example, chronic excess alcohol consumption can inhibit the absorption of many vitamins. Excess alcohol inhibits an enzyme in the liver, which in turn inhibits the absorption of thiamin (vitamin B1) by the organism. In addition, elderly people have greater requirements for many vitamins, because the ability of their organism to convert and absorb vitamins may be impaired.
4. Lack of dietary fat may reduce the absorption of fat-soluble vitamins
Vitamins can be water soluble or fat soluble. Vitamin D is an example of a fat-soluble nutrient. It is stored in fatty tissue and is released as the tissue is broken down by the body. In addition, fat must be present in your intestines for vitamin D to be properly absorbed. If your diet does not include healthy fatty acids your ability to absorb vitamin D may be reduced.
5. Advanced nutrient delivery systems can enhance bioavailability
The way vitamins and other nutrients are delivered to the intestines can have a profound effect in how much of them ends up in your bloodstream. When you take a vitamin tablet, it passes through the stomach where it is attacked by stomach acid. Unfortunately some important nutrients such as Resveratrol, L-Glutathione and SAMe are impaired during that process. To prevent this, tablets can be covered with enteric coating which enables them to pass through the corrosive environment in the stomach without any damage. Because of that, enteric-coated supplements tend to have greater overall bioavailability.
As you can see, many factors may affect how much of a given nutrient is actually absorbed into your bloodstream. This in turn determines how much of the vitamin your organism can use for essential physiological processes. As an educated consumer, you need to be aware of these factors and take them into account when choosing a vitamin supplement. To get all the nutrients your body needs with greater bioavailability, you can take an enteric-coated multivitamin.