Low-carbohydrate-diet score and risk of type 2 diabetes in women

Nutrition Research Newsletter, March, 2008

Obesity in the United States has become a major public health concern. At any given time, ~45% of women and ~30% of men are attempting to lose weight. A low-fat, high-carbohydrate, hypocaloric diet has been advocated by a variety of research and medical societies for weight management. Despite these guidelines, low-carbohydrate diets remain a popular option for those attempting to lose weight. A number of best-selling books promote this strategy for weight loss. The long-term effects of low-carbohydrate diets are yet to be determined. Low-carbohydrate diets result in an increase in total and saturated fat intakes and a decrease in consumption of whole grains, cereal fiber, fruit, and vegetables. These changes in diet have the potential to increase the risk of type 2 diabetes. The American Diabetes Association advocates a low-fat diet to prevent type 2 diabetes.

The researchers of the present study created the low-carbohydrate-diet score by dividing women from the Nurses' Health Study into deciles of fat, protein, and carbohydrate consumption as a percentage of energy consumed. The highest score, 30, represents the highest intake of fat and protein and the lowest intake of carbohydrate, whereas the lowest score, 0, represents the lowest intake of fat and protein and the highest intake of carbohydrate. The low-carbohydrate-diet score, therefore, represents how closely a participant followed a low-carbohydrate diet. In a previous investigation, they found that this score was not associated with an increased risk of coronary heart disease in women from the Nurses' Health Study. In the present study, they examined prospectively the association between a low-carbohydrate-diet score and the risk of type 2 diabetes in participants from the Nurses' Health Study.

The researchers of the current study prospectively examined the association between low-carbohydrate-diet score (based on percentage of energy as carbohydrate, fat, and protein) and risk of diabetes among 85,059 women in the Nurses' Health Study. During 20 y of follow-up, they documented 4670 cases of type 2 diabetes. The multivariate relative risk (RR) of diabetes, after adjustment for body mass index and other covariates, in a comparison of the highest decile of low-carbohydrate-diet score with the lowest was 0.90. The multivariate RR for the comparison of extreme deciles of low-carbohydrate-diet score based on total carbohydrate, animal protein, and animal fat was 0.99, whereas the RR for a low-carbohydrate-diet score based on total carbohydrate, vegetable protein, and vegetable fat was 0.82. A higher dietary glycemic load was strongly associated with an increased risk of diabetes in a comparison of extreme deciles. A higher carbohydrate consumption was also associated with an increased risk of diabetes in a comparison of extreme deciles.

In this large prospective cohort of women, the researchers found that after adjustment for confounding variables, especially BMI, a higher low-carbohydrate-diet score was not associated with risk of type 2 diabetes. This dietary score was associated with a modest decreased risk of type 2 diabetes when vegetable sources rather than animal sources of fat and protein were chosen.

A carbohydrate-restricted diet tends to have a lower glycemic index and lower glycemic load than does a high-carbohydrate diet. The glycemic index of a carbohydrate is a measure of how much that food raises blood glucose compared with a standard carbohydrate (usually glucose or white bread). The glycemic load takes into account the amount of carbohydrate in addition to its glycemic index. Compared with higher glycemic diets, low glycemic diets have been shown in epidemiologic studies to decrease glucose and insulin responses and glycated hemoglobin and to increase insulin sensitivity. In the present investigation researchers found a statistically significant positive association for both dietary glycemic load and total carbohydrate and risk of type 2 diabetes. The positive association between dietary glycemic load and type 2 diabetes was much stronger than that for total carbohydrate because glycemic load captures both the quality and quantity of carbohydrate.

In conclusion, a diet lower in carbohydrate and higher in protein and fat did not increase the risk of type 2 diabetes in this cohort of women. In fact, when vegetable sources of fat and protein were chosen, these diets were associated with a modest reduction in the risk of type 2 diabetes. These data support a potential benefit in reducing the glycemic load of the diet and for substituting low-glycemic fruit, vegetables, whole grains, and healthful sources of fat and protein for high-glycemic refined carbohydrates.

T Halton, S Liu, J Manson, et al. Low-carbohydrate-diet score and risk of type 2 diabetes in women. AJCN; 87:339-346 (February 2008) [Correspondence: FB Hu, Department of Nutrition, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02215. E-mail: frank.hu@channing.harvard.edu]