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Deciding which vitamins to take daily can be a tricky process. Is it safe to overload on vitamins in order to avoid any deficiencies? The Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts University has devoted plenty of time and research to this question. The HNRCA is one of the nation's six Human Nutrition Research Centers and is backed by the U.S. Agriculture Dept. Recently, BusinessWeek's Sarah R. Shapiro talked to the center's Dr. Edward Saltzman. Edited excerpts from their conversation follow:Q: What daily vitamins would you advise the average person to take?A: For most people, I advise them to have a diet rich in vegetables and fruits. Beyond that, most people who want to take a vitamin can take a daily multivitamin. Women should consider additional calcium, especially if the diet is low in calcium or if they're postmenopausal.Q: Are there symptoms that most people ignore that could be caused by vitamin deficiencies?A: Symptoms of many vitamin deficiencies are often nonspecific, or are common symptoms that could be due to many different causes. Such symptoms include fatigue, poor concentration, changes in sensation, ulcers or sores on the lips or tongue, and rashes. There are virtually no symptoms that an untrained person could ascribe to a vitamin deficiency only. Some medical conditions may either predispose you to, or be associated with, certain vitamin deficiencies. So clinicians may pay more attention to signs and symptoms in these cases.Q: Is it safe to load up on vitamins daily?A: No, some vitamins can be taken in excess. Large doses of some vitamins can actually be harmful.Q: Are there any vitamins that most people are lacking?A: Most Americans aren't lacking any nutrients. If there's any nutrient that American women don't get enough of, typically, it's calcium. Some people who are older don't get enough B12.Q: Are there tests to see if we have specific vitamin deficiencies? A: Specific deficiencies of many nutrients can be diagnosed with blood or urine tests. For vitamin D and vitamin B12, blood tests are generally pretty accurate. There are also tests which give us a clue as to whether or not there are adequate amounts of a nutrient to allow normal function in the body -- these are called functional tests.An analogy is the yellow light on the gas gauge of your car. When you're about to run out of gas, the light goes on, even if the gauge doesn't read completely empty. However, for other nutrients it's sometimes difficult to interpret blood levels. For example, some blood nutrient concentrations may be influenced by disease or even obesity. So the blood concentration may be abnormal, but we don't really know if the concentration of nutrients in the tissues (which is where nutrients act) is normal or not.Q: Do you think there could be more tests in the future?A: Absolutely. Better tests are sure to be developed for nutrients we can already measure as well as for nutrients we currently can't measure. Importantly, we will also learn how to better interpret tests that are influenced by disease or other conditions.Q: Is it common for vitamins to react with drugs and other medicines?A: There are numerous drug-nutrient interactions. Examples are interactions between drugs for ulcerative colitis and folic acid, and tuberculosis drugs and vitamin B6.Some drugs result in impaired absorption or may alter metabolism (which either results in low levels or high levels of a nutrient in the body). For drugs such as warfarin that by design act by antagonizing the action of vitamin K, a consistent intake of food with vitamin K (not avoidance of these foods) is best. In addition, grapefruit or grapefruit juice may alter metabolism of some drugs. Information regarding these interactions is available on www.nutrition.gov or at the Tufts Nutrition Navigator site.Q: Are there any new studies that are more definitive on the topic than previous research has been?A: On testing for vitamin deficiencies, there's plenty of new research. Also, the new Dietary Reference Intakes (DRI) incorporate expected changes in absorption with aging, as well as changes through the lifespan for vitamins.For example, new evidence about deficiency of vitamin B12 in older persons has resulted in a specific B12 recommendation -- not just the amount but the form of B12 most likely to be absorbed to prevent deficiency.