Last week's issue of Us Weekly (yes, I subscribe) reported that Mariah Carey dropped 20 pounds by following a diet prescribed by her trainer/nutritionist. One key, the magazine said: a ban on eating carbohydrates and protein in the same meal, on the theory that because these nutrients are absorbed differently in the gut, eating them separately is more efficient and promotes greater weight loss than eating a more balanced meal.
I thought that Mariah probably lost the pounds because her diet (which actually looked pretty good—lots of fruit, veggies, lean protein, and good fats) provided only about 1,000 to 1,500 calories a day, not because the carbs and protein were allowed to schuss through the digestive tract without the benefit of each other's company. But I called nutritionists to find out if there was anything to the idea of separating nutrients by meal (which is actually an old one; William Howard Hay popularized the concept in the U.S. in the early 20th century).
"Absolutely not—there is no scientific evidence to show this," says Suzanne Havala Hobbs, a registered dietitian and director of the doctoral program in health leadership at the University of North Carolina School of Public Health in Chapel Hill, N.C. I found one published study looking at this approach, appearing in the International Journal of Obesity in 2000. Researchers in Switzerland randomly assigned 54 obese adults to one of two diets, both of which had the same amount of calories and a similar carb-protein-fat ratio but which varied in how those nutrients were distributed throughout the day. One group ate balanced meals, while the other ate fat and carbs at different times. The result: no statistically significant difference in body fat lost or lean body mass between the groups. (Both groups lost weight, as you'd expect, since they were confined to a hospital and fed only 1,100 calories a day.)
Nutritionists I talked to said that humans have evolved over the years to accommodate and thrive on a variety of foods. "We don't give our bodies enough credit for adapting to varying dietary conditions, which we've had to do in order to survive as a species," says Havala Hobbs. In fact, whether you're trying to lose weight or not, it's probably better for you to make sure your meals and snacks have some combination of carbs, protein, and a smaller amount of fat, says Bonnie Taub-Dix, national spokesperson for the American Dietetic Association and director of BTD Nutrition Consultants in Woodmere, N.Y. "Yes, hello, I know the nutrients are absorbed at different rates," she says. "If you have peanut butter and crackers, the crackers are digested quickly to give you a boost of energy, and then because the peanut butter has protein and fat [which are absorbed more slowly], you won't have that crash but will have a more sustained energy during the day," she says. And if you're satisfied, you're less likely to snack on junk later in the day or overeat at the next meal.
There's some evidence that eating some nutrients together in a single meal can actually aid their absorption, especially of minerals, says Martha Stipanuk, a nutritional sciences professor at Cornell University. Iron from nonanimal sources like spinach or beans, for example, is better absorbed when it's accompanied by a source of vitamin C in the same meal. And vitamin A is better absorbed if it's digested with fat. But since there have been no large, "real world" studies showing that a diet where these nutrients are eaten together leads to better absorption than a diet where they're eaten separately, don't sweat when you eat which nutrients unless you have a deficiency.
"I don't know of any nutritionist who worries about this on a daily basis with [his or her] own diet," she says. That also goes for plant-based "incomplete" proteins like beans, nuts, and grains; it was once believed that you had to eat beans and grains together in order to get the full array of amino acids, but now the American Dietetic Association says it's only necessary to make sure your overall diet includes a variety of these proteins each day.
More important is to make sure you're getting a range of different foods, heavy on whole foods and light on the processed ones, says Stipanuk. (Here's a list of foods containing different nutrients.) And, says Taub-Dixon, if you're trying to lose weight, don't forget to watch your portion sizes. "There's no gimmick or miracle cure," she says. Not even for celebrities.