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Water-Soluble Vitamins and Nutrients continued...

The field of nutrition is ever-changing, and experts used to think that taking excess amounts of a water-soluble nutrient was harmless because the excesses would just be eliminated in urine. Today, we know that’s not the case, and that some water-soluble vitamins and nutrients are handled differently by the body than others.

Just because most water-soluble vitamins are not stored by the body, you can’t assume that it is safe or effective to take more than the safe upper limit. In addition, you need to account for the vitamins and nutrients you get from the food you eat, says Ruth Frechman, MS, RD, a spokesperson for the American Dietetic Association.

"Certain water-soluble vitamins in excess can cause problems, such as too much vitamin B6 can cause nerve problems, too much niacin can cause flushing, and excess vitamin C can cause kidney stones," Frechman observes. Excess folic acid may also mask a vitamin B12 deficiency, which is more common in people over age 50.

Fat-Soluble Vitamins

Vitamins A, D, E, and K are the fat-soluble vitamins. Unlike water-soluble vitamins, these vitamins dissolve in fat and are stored in body tissues. Because they are stored, over time they can accumulate to dangerous levels and can lead to a condition called hypervitaminosis, meaning excess amounts of a vitamin in the body, if more than the recommended amount is taken.

"Too much vitamin A, D, or K can lead to increased levels that are unhealthy and can cause health consequences," says Frechman. She adds that too much vitamin A can lead to birth defects, and too high levels of vitamin E may increase the risk of hemorrhaging. Excess vitamin K can lessen or reverse the effect of blood thinner medicines and prevent normal blood clotting. 

Vitamin D has been one of the more controversial vitamins. Even though it is a fat-soluble vitamin, it appears to be tolerated in the body at higher levels.

As of 2010, the Institute of Medicine recommends a daily allowance of 600 international units (IU) of vitamin D for everyone ages 1 to 70, with an upper limit of 4,000 IU for those ages 9 and older.

Some experts, like vitamin D researcher and Creighton University professor Robert Heaney, MD, think the upper limit levels are still not set high enough and that more vitamin D may be necessary to foster good health. "The new upper limit for vitamin D has been doubled to 4,000 IU per day, which will meet the needs of most healthy people, but the research shows the toxic level is much higher than the established ceiling," Heaney tells WebMD.

Frechman points out that vitamin D is also a hormone, which makes it unique in its properties. This dual function may explain why it functions differently than the other fat-soluble vitamins and renders it less harmful at higher intakes.