Boston, MA - A new study suggesting that low-carb diets do not increase the risk of CAD in women may help to allay fears that people who eat higher amounts of protein and fat while cutting back on carbs are not trading hopes of a slimmer waistline for increased coronary disease risk [1].

In fact, a new retrospective analysis of the Nurses' Health Study showed no differences in weight change over time between women who ate meals proportionally higher in carbs vs those higher in fats and proteins, nor were rates of cardiovascular events higher in the low-carb group over 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 eat a high proportion of fats from vegetable sources may actually lower their risk of disease.

The study, by Dr Thomas L Halton (Harvard School of Public Health, Boston, MA) et al, appears in the November 9, 2006 issue of the New England Journal of Medicine.

"A low-fat diet has been advocated to reduce the risk of coronary heart disease," Halton pointed out to heartwire. "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 et al 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 consumption of carbohydrates, fats, and protein. A higher score reflected higher consumption of protein and fats and lower amounts of carbs.

This study is just a piece of the overall picture, but it's very eye-opening.

When scores were examined by deciles, the relative risk of coronary heart disease over the 20 years of follow-up, comparing highest and lowest deciles of low-carb scores, was 0.94 (p=0.19). 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. Strikingly, 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 of coronary heart disease.

Diets low in carbohydrates were not associated with decreases in body weight over follow-up, but the authors point out that 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-carb dietary pattern, carbohydrate consumption was <30% of total energy—higher than that advocated by popular diets like Atkins and South Beach. But on the flip 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," Halton et al 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 for heartwire, Dr Eric Westman (Duke University Medical Center, Durham, NC) noted that the study primarily supports a link between higher-glycemic-load carbohydrates and increased cardiovascular risk but doesn't provide a lot of new information for stricter low-carb diets.

"I don't think the study included enough people who ate less than 30% carbohydrate diets to draw any conclusions," Westman stated. "What was most interesting to me was that cardiac risk did not vary with different intake of dietary fat. The door is open to examine lower-carbohydrate diets to reduce cardiac risk."

To heartwire, Halton emphasized that the study was not designed to measure weight as an outcome; that said, most low-carbohydrate studies have compared diets only over six months to one year, making the Nurses' Health Study observations an important contribution to understanding the implications of dietary choices.

"This is just one study, and one study is never enough to make a claim as to whether something is safe or not," Halton said. "You need to examine all of the literature, the short-term randomized trials, the longer-term cohort studies, as well as the metabolic studies on fat/carb consumption and blood lipid levels, etc. This study is just a piece of the overall picture, but it's very eye-opening."