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    Fat-Soluble Vitamins continued...

    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.

    Too Much Can Cause Harm

    Exceeding the government set tolerable upper limits can be a problem. "There is a reason for the tolerable upper limits that needs to be respected. Research has shown at which levels nutrients can cause potential problems, and these numbers take into account all sources of vitamins and minerals from food, fortified food and supplements," says Frechman.

    When the level goes beyond the safe upper limits, vitamins can act like drugs, says Roberta Anding, MS, RD, spokesperson for the American Dietetic Association, director of sports nutrition at Texas Children’s Hospital, and dietitian for the Houston Texans pro football team. Excessive calcium intake, more than 2,500 mg a day, can interfere with kidney function, cause kidney stones and constipation, and interfere with the absorption of iron and zinc. One 2,500 mg calcium supplement is like drinking eight glasses of milk, which goes beyond the reasonable food level, explains Anding.