A: Elizabeth Somer, M.A., R.D., author of The Essential Guide to Vitamins and Minerals (Harper Perennial, 1992) responds:
Low-carb diets restrict or eliminate many nutritious foods. As a result, you lose out on B vitamins and magnesium (from grains), calcium and vitamin D (from milk products), potassium (from potatoes and bananas) and beta carotene and vitamin C (from veggies). No pill can replace the thousands of health-enhancing phytochemicals found in intensely colored veggies and fruits.
Some low-carb supplements purport to aid weight loss by adding biotin. "[But] there is no evidence that this B vitamin helps shed pounds," says Jeffrey Blumberg, Ph.D., a professor at the Friedman School of Nutrition Science and Policy at Tufts University in Boston. "Besides, biotin is found in milk, liver, eggs and other foods allowed on low-carb diets." One low-carb supplement boasts that it offers potassium and calcium, yet supplies only 20 percent of the RDA for calcium and a mere 3 percent for potassium.
You may still want to supplement with a moderate-dose multivitamin and mineral supplement daily. One study found that even menus designed by dietitians using the USDA's Dietary Guidelines came up short when calories dropped below 2,200 a day.