A study suggesting that low-carbohydrate diets do not increase the risk for coronary artery disease in women may help to allay fears that people who eat higher amounts of protein and fat, while cutting back on carbohydrates, are not trading hopes of a slimmer waistline for increased coronary disease risk.
In fact, a retrospective analysis of the Nurses Health Study showed no differences in weight change over time between women who ate meals proportionally higher in carbohydrates vs those higher in fats and proteins but nor were rates of cardiovascular events higher in the low-carb group during 20 years of follow-up. A closer look at the types of foods being eaten, however, suggested that diets with a higher glycemic load were strongly associated with increased risk of coronary heart disease, while women who ate a high proportion of fats from vegetable sources may actually lower their risk for disease.
“A low-fat diet has been advocated to reduce the risk of coronary heart disease,” Dr. Halton pointed out. “This study shows that a low-fat diet was not more protective than a low carbohydrate diet over the long term. In fact, when vegetable sources of fat and protein were chosen, the lower carbohydrate diet was associated with a reduced risk of coronary heart disease in this cohort.”
Halton and colleagues looked at food frequency questionnaires completed by almost 83,000 women who participated in the Nurses Health Study, using responses to calculate a low-carbohydrate-diet score based on the consumption of carbohydrates, fats, and protein. A higher score reflected a higher consumption of protein and fats and lower amounts of carbohydrates.
When scores were examined by deciles, the relative risk for coronary heart disease during the 20 years of follow-up, comparing highest and lowest deciles of low-carbohydrate scores, was 0.94 (P = .19). The relative risk was unchanged in analyses that compared highest and lowest scores when a high percentage of fats and proteins came from animal sources. However, the relative risk was significantly lower among women who ate low carbohydrate diets with a higher proportion of fats from vegetable rather than animal sources. Conversely, diets with higher glycemic load — reflecting a higher proportion of foods that rapidly increase blood glucose — were associated with an increased risk for coronary heart disease.
Diets low in carbohydrates were not associated with decreases in body weight during follow-up, but the authors point out this is not unexpected, since participants in the Nurses Health Study were not following particular diets for the purposes of weight loss. In even in the highest decile score, representing the most low-carbohydrate dietary pattern, carbohydrate consumption was less than 30% of total energy — higher than that advocated by popular diets like Atkins and South Beach. But on the other side, this observation “does indicate that the effects of the low-carbohydrate-diet score on outcomes in this analysis were not mediated by weight loss,” the authors note.
“Diets lower in carbohydrate and higher in protein and fat were not associated with an increased risk of coronary heart disease in this cohort of women,” Dr. Halton and colleagues conclude. “When vegetable sources of fat and protein were chosen, these diets were related to a lower risk of coronary heart disease.”
Commenting on the study, Eric Westman, MD, of the Duke University Medical Center in Durham, North Carolina, noted that the study primarily supports a link between higher glycemic load carbohydrates and increased cardiovascular risk, but does not provide a lot of new information for stricter low-carbohydrate diets.
“I don’t think the study included enough people who ate less than 30% carbohydrate diets to draw any conclusions,” Dr. Westman stated. “What was most interesting to me was that cardiac risk did not vary with the different intake of dietary fat. The door is open to examining lower carbohydrate diets to reduce cardiac risk.”
Dr. Halton emphasized that the study was not designed to measure weight as an outcome, and with that said, most low-carbohydrate studies have only compared diets during 6 months to 1 year, making the Nurses Health Study observations an important contribution to understanding the implications of dietary choices.
In conclusion, the findings suggest that diets lower in carbohydrate and higher in protein and fat are not associated with increased risk of coronary heart disease in women. When vegetable sources of fat and protein are chosen, these diets may moderately reduce the risk of coronary heart disease. This study is just a piece of the overall picture, but it’s very eye-opening.”
· Low carbohydrate diet score is associated with a moderately lower risk and high carbohydrate diet is not associated with lower risk for coronary heart disease incidence.
· A high dietary glycemic load and index is linked with increased risk for coronary heart disease.