A diet that restricts your choices too much may leave you lacking in key nutrients.

RODALE NEWS, EMMAUS, PA—It stands to reason that going on a diet will impact your nutrient intake. After all, when you eat fewer calories, you eat less of everything. But according to a new study in the American Journal of Clinical Nutrition, that logic may not hold water. The authors followed women on four well-known fad diets and found that one diet in particular actually led to a boost in nutrient intake, while all the other dieters found themselves consuming fewer key nutrients, such as vitamin C, vitamin B12 and zinc.

THE DETAILS: The researchers randomly assigned 311 obese or overweight women, ages 25 to 50, to one of four popular diets: the low-carb Atkins diet; the Ornish diet, essentially a vegetarian diet that excludes nearly all fats except those from low-fat dairy; the LEARN diet, or Lifestyle, Exercise, Attitudes, Relationships, and Nutrition, which follows the USDA food pyramid and encourages low-fat eating; and the Zone diet, which stresses light consumption of lean protein and eating mostly complex carbohydrates from fruits and vegetables accompanied by healthy fats and oils.

Each participant was given a book about her diet and then attended a 1-hour training class each week of the 8-week study. However, they were left to their own devices as far as purchasing and preparing food according to each diets' requirements. Participants were called at random and asked what they ate in the previous 24-hour period, and that information was analyzed to determine nutrient intake.

All the participants consumed roughly the same number of calories per day, regardless of which diet they followed, but followers of the Zone diet were the only group to see significant improvements in overall nutrient intake. Atkins dieters experienced steep declines in thiamin, folic acid (which is especially important for pregnant women), vitamin C, magnesium, and, ironically, iron. The authors note that the primary source of iron in the American diet is not red meat but fortified breads and cereals, consumption of which is limited in the Atkins program. People on the Ornish diet saw significant drops in zinc and vitamin B12, which come primarily from animal foods, and LEARN participants saw worsened intakes for calcium, vitamin E, thiamin, and magnesium. None of those diets led to significant increases in nutrient intake (with the exception of vitamin K, which rose among Atkins dieters), but the Zone dieters, on the other hand, saw substantial increases in their nutrients with no decreases.

WHAT IT MEANS: Christopher D. Gardner, PhD, associate professor of medicine at the Stanford University School of Medicine's Prevention Research Center and lead author of the study, thinks the reason Zone dieters get a wider and better range of nutrients is that the diet itself is more nuanced and less of an all-or-nothing approach to certain food groups. "The Zone diet is lower in carbohydrates but not as extremely low-carb as Atkins," he says. "And the way women, we think, were approaching this was that they were eliminating the least nutritious carbs—added sugars and refined grains." When you eliminate entire food groups, he adds, and focus only on low-fat or low-carb, you wind up eliminating foods, such as olive oil, avocados, and a colorful array of vegetables that contain nutrients your body needs.