A diet that is rich in carbohydrates is better than a low-fat diet for reducing the risk of heart disease and diabetes, according to a new study
Mark A. Pereira, a University of Minnesota epidemiologist, and his colleagues compared a diet rich in carbohydrates that have a low glycemic index with a conventional low-fat diet that included carbohydrates with a high glycemic index. Carbohydrates that have a low glycemic index tend to stabilize blood sugar.
Researchers assigned 39 young adults between the ages of 18 and 40, who were either overweight or obese, to either a low-glycemic diet or a low-fat one.
The low-glycemic diet consisted of 43% of calories from carbohydrates, 27% from protein and 30% from fat, Pereira reports. (For comparison, the popular Zone diet consists of 40% of calories from carbohydrates, 30% from protein and 30% from fat. Low-glycemic carbohydrates are recommended.)
People on the low-fat diet got 65% of their calories from carbohydrates, 17% from protein and 18% from fat.
Food was provided for all the participants, and both diets were restricted in calories to achieve a 10% weight loss. The low-glycemic menus featured foods such as steel-cut oatmeal, barley and whole-grain breads. The menus for the low-fat diet included carbohydrates that have a higher glycemic index, such as instant Oatmeal, white bread and white rice.
Pereira's study team measured blood pressure, insulin resistance (a predictor of diabetes), blood fats and other risk factors for diabetes and heart disease before and after the weight loss. "Generally, what we found across the board for blood fats and blood pressure and insulin resistance was the decrease in these measurements] was twice as great for the low-glycemic group compared to the low-fat group."
Both groups averaged the same weight loss -the 10% goal—but the dieters on the low-glycemic diet reported less hunger. In addition, the resting metabolic rate (the rate at which the body burns calories when at rest), which typically drops during weight loss, decreased less in the low-glycemic eaters.
"For the conventional low-fat dieters, the metabolic rate slowed down by 175 calories a day," Pereira reports. "For the low-glycemic group, there was a 95-calorie drop."
In the study, those people who were on the low-glycemic diets also achieved a greater improvement in blood pressure and blood fats than the low-fat group.
The data shows that carbohydrates with a low glycemic index are absorbed through the small intestine and converted to blood sugar at a much slower rate than high-glycemic (carbohydrates), resulting in a more stable blood sugar and much less insulin being produced," says Pereira. "So that would help prevent or control diabetes."
Not All Agree
Another expert says that it is difficult to credit the low-glycemic diet for the results. "I think it's a very interesting study," says Alice Lichtenstein, chair of the nutrition committee for the American Heart Association and professor of nutrition, science and policy at Tufts University. However, whether the effect was due to glycemic load or to the higher fat content (approximately 30%) of the low-glycemic diet can't really be distinguished, she says.
"Certainly, a number of studies have shown that low-fat diets are not optimal for promoting weight loss or decreasing cardiovascular risk," she says. The low-glycemic diet studies may be better because they use a more moderate level of fat, she continues.
Lichtenstein cautions that people trying to lose weight and reduce disease risk should not become too obsessed with total fat or glycemic load at the expense of calories. The most important aspect is watching "total calories and increasing activity," she says.
Glycemic Index Food Rank
The glycemic index ranks a food's effect on blood sugar levels. At the top of this 100-point scale is glucose. Consumption of glucose causes the sharpest rise in blood sugar. The higher a food's glycemic index, the greater its effect on blood sugar levels.
The chief determinant of the glycemic index of a particular food is the speed at which it is digested. Carbs that break down into glucose quickly raise blood sugar sharply. Slow-digesting carbs keep blood sugar levels on an even keel.
The body operates best when blood sugar stays at a constant level. Increasing the proportion of low-glycemic-index foods in your diet…
- Promotes weight loss. Low-glycemic foods remain in the digestive tract longer, and so provide a lasting feeling of satiety.
- Lowers the risk for diabetes. The risk of developing adult-onset (type 2) diabetes is determined largely by heredity. However, cells that are repeatedly subjected to surges of insulin eventually become less sensitive to the hormone's effects. Over time, the pancreas can't produce enough insulin to overcome the cells' insulin resistance and diabetes develops. By preventing blood sugar surges, low-glycemic index foods help forestall insulin resistance.
- Guards against heart disease. Since low-glycemic-index foods help prevent diabetes and obesity-both of which are risk factors for heart disease they also reduce heart disease risk. Low-glycemic-index eating also fights heart disease by raising levels of HDL ("good") cholesterol.
- Reduces the risk for hypoglycemia. Some people experience weakness, drowsiness and/or anxiety after eating because abnormally high insulin levels cause blood sugar to fall precipitously. Low-glycemic-index eating helps dampen these swings.