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

While we are aware that we should not take more than the recommended dose of a prescription medication, most of us wouldn’t think twice about “overdosing” on vitamins. Some people even believe this gives an extra “boost” to your health. According to Professor Anding, though, vitamins are not harmless when consumed in excess.

Fat-Soluble Vitamins

Vitamins fall into two major categories: fat-soluble and water-soluble. These categories have to do with how the vitamins are stored in your body. Fat-soluble vitamins dissolve and remain in the body’s fat stores or adipose tissue. 

For example, when you take in vitamin D, if you’ve got excess body fat, that vitamin D is sequestered or held in your body fat, making it less biologically available to other tissues within the body. Because they are fat soluble, you can store them, and there’s a less urgent need to have them on a daily basis. 

As a group of vitamins, fat-soluble vitamins include vitamins A, D, E, and K. They should not be consumed in excess without medical supervision or advice. 

Toxic reactions can occur at much lower multiples of the recommended dietary allowances than with water-soluble vitamins, most notably with vitamin A. Some experts suggest that one serving of polar bear liver can cause death from vitamin A toxicity. 

A bottle may give the vitamin A content as 100% of the daily value, and in parentheses, it’ll say, “40% as beta-carotene.” This means that they’ve taken out the more active—in this case, more toxic—version of vitamin A and replaced it with a precursor. Beta-carotene in your body can be converted to vitamin A.

Water-Soluble Vitamins

Unlike fat-soluble vitamins, water-soluble vitamins are ones that you probably need to consume on a daily basis. Water-soluble vitamins are dispersed in body fluids, and they’re not going to be stored to any great extent. The exception is vitamin B₁₂. 

You can store vitamin B₁₂for up to one to two years in your liver. They exert their greatest influence for about eight to 14 hours after ingestion, so if this is something that needs to be consumed on a regular basis, their potency decreases over time. 

In general, these water-soluble vitamins act as coenzymes. These are small molecules that combine with a larger compound to form an active enzyme.

Think about it as the grease of the metabolic machinery. It helps you to use your metabolic pathways more effectively. In essence, it accelerates chemical reactions that need acceleration within the body. 

This category of water-soluble vitamins includes vitamin C, B₁ (thiamine), B₂ (riboflavin), B₆ (pyridoxine), niacin (sometimes known as nicotinic acid), pantothenic acid, biotin, folic acid, and vitamin B₁₂. If your diet contains less than 50% of the recommended value for a water-soluble vitamin, deficiencies can begin to show up within approximately four weeks. 

Dangers of Excess

Although the excess of a water-soluble vitamin is voided by urination, these vitamins can still be harmful when taken in quantities high above the recommended daily allowance. For example, Vitamin B₆ can have a toxic effect. 

“I had a gymnast who came into my practice, and I work in adolescent and sports medicine, so she came into the sports medicine side,” Professor Anding said. “What ended up happening was she was doing something on the uneven bars, and she lost her grip … She actually fractured six vertebrae in her back, and that ended her gymnastics career, and she had been an Olympics hopeful.”

As it turns out, her coach had recommended that she take B vitamins to combat fatigue. She took enough B₆, or pyridoxine, that she developed a neuropathy—a numbness and tingling in her hands—that caused her to lose her grip. 

Therefore, when we are taking vitamins to supplement our diet, we must use moderation and check with a physician or nutritionist if we are making drastic changes. As with prescription medications, both fat-soluble and water-soluble vitamins are not benign and can present harmful side effects when taken in excess.

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.