Convinced of their benefits, a growing number of Americans are turning to vitamins and minerals to help keep them healthy. The latest federal National Health and Nutrition Examination Survey found that 40 percent of Americans had taken a vitamin or mineral supplement during the past month.
But taking vitamins can be as complicated as playing with a chemistry set. Vitamins and minerals can interact with each other and with food. In some cases, these interactions boost absorption. In others, they block it.
It may take a doctorate in nutrition to know all the permutations. Even then, new research is rewriting the rules every year. Manufacturers say interactions are very unlikely to occur in multivitamins and are mostly confined to megadoses of the fat-soluble vitamins, A, D, E and K.
Multivitamins that have ingredients that significantly interact "won't pass FDA (Food and Drug Administration approval)," said Vishwa Singh, director of human nutrition research for Hoffmann-La Roche, one of the leading vitamin manufacturers. "You have to prove at the time of sale that 100 percent of the ingredients are still there. Those things are taken care of."
For consumers who take vitamins and minerals, here are some guidelines to make sure you're getting, and absorbing, what you need.
Try to take vitamins at about the same times every day. That goes for a multivitamin tablet as well as individual vitamins and minerals. Making them part of a daily routine means that they don't simply sit on your shelf unused. "If you don't remember to take a vitamin, the amount you absorb is zero," said Johanna Dwyer, a nutrition researcher at the Tufts University School of Nutrition and Medicine.
Don't take a handful of pills at one time. Since vitamins and minerals often interact, which can significantly decrease absorption, experts advise staggering supplements throughout the day. Set up a regular schedule for each one and take some at breakfast, some at lunch, some at dinner.
Try not to take calcium at the same meal with multivitamins or with supplements containing either iron or zinc, since calcium blocks their absorption. While it may not make a big difference once in a while, "if you are taking a lot of supplementary calcium, it may be interfering with iron absorption, and that's not a very good trade-off," said Irwin Rosenberg of Tufts University.
The same is true for interactions between calcium and multivitamins. "So it might be good not to take calcium within one to two hours of taking a multivitamin," said John Hathcock of the Council for Responsible Nutrition.
Avoid the exotic. A growing number of vitamin products now often include herbs such as echinacea and other botanicals that promise to deliver a wide range of health benefits but have little scientific evidence to back their claims. "There are so many different brands and combinations that you want to avoid anything strange, particularly natural products that have herbs and unregulated substances with multivitamins," said Benjamin Cabellero, director of the Center for Human Nutrition at Johns Hopkins University's School of Public Health and Medicine in Baltimore.
Take adequate amounts, but not too much. Forget the megadoses. There is no scientific proof that they help, and there is growing scientific evidence that they may cause harm.
Simple is best. A whole variety of supplements is now available in timed-release or sustained-release forms. They often cost more than standard vitamins. But an expert panel recently convened by the National Academy of Sciences found no benefit to their use.
Nor does it make a difference "whether you take these vitamins at the beginning or at the end of the meal," said Robert Russell, also of Tufts. "The idea of these tablets is that if you flatten out the absorption curve you will get a more-sustained effect," Mr. Rosenberg said. "But there's no evidence that a sustained effect is better for vitamin nutrition."
Consider your stomach. Some vitamins and minerals are absorbed best on an empty stomach, but they can also cause nausea or stomach irritation when they are taken without food -- the reason that many experts advise taking most supplements with food. Iron and calcium are both better tolerated with food, although their absorption is slightly decreased.
Fat soluble vitamins -- A, D, E and K -- are better absorbed with food, especially meals containing some fat. "Since it's hard to get a totally fat-free meal, it doesn't matter whether you take them with breakfast, lunch or dinner," said Jeffrey Blumberg, at Tufts University.
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