By Roberta H. Anding, MS, Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley and proofread by Angela Shoemaker, The Great Courses Daily

Over 80% of Americans take vitamins or supplements. Are you actually getting the full benefits from these vitamins? Professor Anding explains bioavailability and how it relates.

What Is Bioavailability?

One of the key concepts in vitamin nutrition is bioavailability. It is the amount of the nutrient that you can absorb and utilize. 

If you look up a specific food in a database or a nutrition book and find that the food has 60 milligrams of folic acid, that’s what is in the food. It’s not necessarily what you can get out of the food. 

One factor that influences bioavailability is the form of the vitamin. When we think of a vitamin, we think of a singular compound, but this is not always the case. For example, vitamin E actually has eight different forms or isomers, and they’re not all equally effective.

By definition, vitamins are organic substances. This is not the definition of “organic” that you’re most likely familiar with—a label designated for foods not made with synthetic pesticides. “Organic” in this context means that they have carbon as part of their structure. 

In the world of nutrition, 13 different vitamins have been isolated and classified. With one exception—vitamin D—our bodies can’t manufacture vitamins, and thus vitamins must be consumed in our diet or through supplementation.

Natural Versus Synthetic Vitamins

This leads to the difference between natural and synthetic vitamins. Natural vitamins come from our natural food sources, while synthetic vitamins are the kind you would buy in a bottle at a pharmacy or grocery store.

Most of us view natural as superior to synthetic. In the case of vitamin E, research has found that the food-based, natural version of vitamin E can be better for you than the synthetic version.

With folic acid, however, synthetic is better. It is more biologically available, so although there’s folic acid in food—think foliage, like green, leafy vegetables—synthetic folic acid like that found in fortified breakfast cereal is more biologically available.

Sometimes in the world of vitamins, a food or a food component could increase or decrease bioavailability. Folic acid, again, is better absorbed on an empty stomach than absorbed with food. In fact, synthetic folic acid on an empty stomach has a bioavailability of almost 100%. 

“When I take it with food, in a mixed meal—not just my breakfast cereal—the bioavailability actually drops to about 85%,” Professor Anding said. 

The bioavailability of iron, a mineral, is actually improved with vitamin C. You can liken this vitamin and mineral nutrition component to a house of cards. When you knock down one, you can influence your body’s ability to use, absorb, and utilize another. 

Too Much of a Good Thing

Keep in mind, though, that this bioavailability can also have some unintended consequences. When taken in excess, vitamins can have harmful side effects.

“I’ve had a patient come into my office and she just had chronic gastrointestinal upset—chronic diarrhea,” Professor Anding said. “She went to the gastroenterologist, and she had every test under the sun done.”

Professor Anding asked the woman if she took any supplements, vitamins, or minerals. The woman replied, “Well, yeah, I take vitamin C. Doesn’t everybody?” 

As it turned out, she was taking 10,000 milligrams of vitamin C, which was the cause of her diarrhea. Most of us don’t think about negative consequences associated with vitamins, but in the world of supplements, it is possible to have too much of a good thing.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.

This article was edited by Kate Findley, Writer for The Great Courses Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for The Great Courses Daily.