November 9, 2006 — 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.
The study by Thomas L. Halton, MD, of the Harvard School of Public Health in Boston, Massachusetts, and colleagues, appears in the November 9 issue of The New England Journal of Medicine.
"A low fat diet has been advocated to reduce the risk of coronary heart disease," Dr. 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."
Trimming the Fat Vs Curbing the Carbohydrates
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 consumption of carbohydrates, fats, and protein. A higher score reflected 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."
A Door Left Open
Commenting on the study to heartwire, 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 different intake of dietary fat. The door is open to examine lower carbohydrate diets to reduce cardiac risk."
Dr. Halton emphasized to heartwire 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.
"This is just one study and one study is never enough to make a claim as to whether something is safe or not," Dr. 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."
N Engl J Med. 2006;355:1991-2002.
According to the current authors, although a low-fat, high-carbohydrate diet is recommended for cardiovascular health, diets high in fat and protein and low in carbohydrate recently have been advocated and remain popular, but the long-term safety of such diets remains controversial.
The current trial is a prospective study of women in the Nurses Health Study who were followed up for 20 years and had their carbohydrate intake classified to examine the relationship between carbohydrate content in the diet and cardiovascular mortality.
* Participants were female registered nurses aged 30 to 55 years who completed a mailed questionnaire with a semiquantitative food frequency questionnaire every 2 years from 1976.
* Excluded were women with implausible intakes and 10 or more food items left blank.
* 82,802 women during 20 years of follow-up were analyzed.
* Dietary intake was assessed every 2 to 4 years by questionnaire.
* Nutrient values were computed by multiplying the frequency of consumption of each food by the nutrient content of the portion and then adding these products across all food items.
* Glycemic load was calculated by multiplying the carbohydrate content of each food by its glycemic index and then multiplying this value by frequency of consumption and the summed value for all foods.
* Women were divided into 11 strata each of fat, protein, and carbohydrate intake expressed as a percentage of energy.
* Women in the highest stratum received 10 points for each macronutrient, those in the next stratum received 9 points, etc, until the lowest stratum was reached.
* The points for each of the 3 macronutrients were summed to create the overall diet score, which ranged from 0 (the lowest fat and protein intake and the highest carbohydrate intake) to 30 (the highest protein and fat intake and the lowest carbohydrate intake).
* The final score was termed the "low-carbohydrate-diet score."
* Outcome was incident coronary heart disease, including deaths.
* Women were analyzed by deciles of low-carbohydrate-diet score.
* The cumulative average low-carbohydrate-diet score ranged from a median of 5.0 in the lowest decile to 26.0 in the 10th decile.
* Mean daily carbohydrate intake ranged from 234.4 g in the first to 116.7 g in the 10th decile.
* Higher scores were associated with smoking, higher body mass index, lower glycemic load, lower caloric intake, and higher saturated fat intake.
* Body mass index increased by 2.5 units from baseline to the end of follow-up.
* 1994 cases of coronary heart disease were documented for 1,584,042 person-years.
* After controlling for potential confounders, the relative risk for coronary heart disease for those in the 10th vs the first decile was 0.94 (P for trend .19), ie, not decreased.
* There was no evidence of body mass index, physical activity, smoking status, diabetes, hypertension, or hypercholesterolemia that influenced the effect of low carbohydrate intake on coronary heart disease risk.
* When energy from carbohydrate, vegetable protein, and fats were used to calculate multivariate risk, the relative risk for the 10th vs the first decile was 0.70 (P for trend, .002).
* Total carbohydrate intake was associated with a moderately increased risk for coronary disease.
* There was a significant direct association between dietary glycemic load and coronary heart disease (relative risk, 1.90; P for trend .003).
* The overall dietary glycemic index was positively associated with increased risk of heart disease (relative risk, 1.19; P for trend .003).
* There was a significant inverse relationship between vegetable fat consumption and the risk for coronary heart disease when comparing the first and the 10th deciles (relative risk, 0.75; P for trend .006).
Pearls for Practice
* 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.