The Scoop on Carbs and Fats
A new study tries to make sense of diet and the risk of heart disease
Since the day in 1972 that cardiologist Robert Atkins first told dieters to swap carbohydrates for steak, cheese, and bacon, low-carb, high-fat diets have inspired endless controversy. The latest salvo comes from researchers at the Harvard School of Public Health, who reported last week in the New England Journal of Medicine that low-carb diets don't increase the risk of heart disease. But other scientists say this latest exam hardly gives low-carb diets a clean bill of health.
The researchers analyzed data from 20 years of food diaries created by the 82,802 participants in the renowned Nurses' Health Study and found that the risk of heart disease was the same for women who ate either low-fat food or low-carb meals similar to the maintenance phase of the Atkins program, which allows dieters to eat some carbohydrates. (The women weren't necessarily trying to lose weight.) Neither diet was particularly healthful, the researchers noted, with the women on the low-fat diets eating too much sugar and other refined carbohydrates, while the women on the low-carb diet ate too much animal fat.
By contrast, women in the study group who ate a low-carb diet that shunned animal fats and embraced vegetable sources of fat and protein, like olive oil, nuts, and beans, saw their risk of heart disease drop by 30 percent. "You can have a healthy low-carb diet as long as you include healthy sources of fats and proteins," says Frank Hu, an associate professor of nutrition and epidemiology at the Harvard School of Public Health and senior author of the report. "The study doesn't mean that you should load your plate with a lot of bacon."
Although the Harvard study is the first to examine the long-term health effects of low-carb diets, other scientists said the analysis raises more questions than it answers. For instance, the women who ate a low-carb, high-fat diet ended up gainingmore weight overall than their peers who stuck with low-fat foods, but the study authors adjusted the data to erase the effects of body fatness on health. "It is very well known that increased body fat is associated with an increased rate of heart disease," says Susan Roberts, a professor of nutrition at Tufts University. For study participants, "eating a low-carb diet and being overweight go together." Rather than absolve low-carb diets, Roberts says, the results are further confirmation that vegetable protein and vegetable fat-the kind found in olive and canola oil, nuts, and avocados-is heart healthful. "I don't think this study should be used to justify" that there's no heart disease effect of low-carb diets.
Glycemic load. The study also wades into another contentious diet topic, the glycemic index. The index was developed as a way to calculate a carbohydrate-rich food's effect on blood glucose levels. Bagels, for example, have a high glycemic index, which means they cause a rapid spike in blood glucose. Whole-wheat bread, barley, and other slow-to-digest carbohydrates have a low glycemic index. But in recent years, glycemic index has been used as the rationale for low-carb diets, despite the fact that eating high-fiber carbs like oatmeal and whole grains demonstrably reduces the risk of diabetes and heart disease. Low glycemic index diets also say baked potatoes are bad for you, a notion that makes nutritionists shudder.
The Harvard group found that women who ate higher glycemic index foods were twice as likely to have heart disease. This reprises research published in 2000 that found that overweight women with a high glycemic load diet were twice as likely to develop heart disease as were overweight women who favored low glycemic load foods. "You can eat a low-fat diet as long as most of your carbohydrates come from whole grains,"Hu says. "Unfortunately, the low-fat diets practiced by most people are not like that-there are a lot of refined carbohydrates and sugar beverages." Indeed, Hu speculates that one reason his research didn't find more heart disease among the high-fat eaters is that they were eating fewer refined carbohydrates.
Robert Eckel, immediate past president of the American Heart Association, says that the jury is still out on the glycemic load's effect on weight loss and heart disease. A study earlier this year based on data from the federal Women's Health Initiative, which tracked nearly 50,000 women over about eight years, found no increased risk of heart disease or diabetes from eating grains, starches, and sugars. Glycemic load, which is calculated by multiplying a food's glycemic index by the amount consumed, is a tricky measure, Eckel says. That's not only because it changes when foods are eaten together (eating a high-index bagel with low-index chili slows the bagel's conversion to glucose) and depending on how foods are cooked (al dente pasta has a lower glycemic index than overcooked pasta) but also because it's not a proxy for carbohydrate intake. "To come away from this saying that higher glycemic load causes more heart disease is unacceptable."
Although the Harvard study is "interesting and may be important," Eckel says people should stick with the federal government's 2005 recommendations for healthful eating: Keep fat intake at 20 to 35 percent of calories a day in the average 2,000-calorie diet, with most fats coming from fish, nuts, and vegetable oils. Saturated fats should account for less than 10 percent of calories daily, equivalent to about 200 calories, or two pats of butter. Trans fats are out altogether, and avoid refined sugar and caloric sweeteners in food and drinks. The U.S. Department of Agriculture also recommends eating three or more whole-grain foods a day, as well as 2 cups of fruit and 2
The bottom line, Hu says, is that there is no single optimal diet for everyone. "You can have a healthy low-carb diet or a healthy Mediterranean diet [with complex carbohydrates, vegetable oil, and fish]."But the emphasis is on the healthy. l
This story appears in the November 20, 2006 print edition of U.S. News & World Report.