Evidence on the overall impact of low-carbohydrate diets on cardiovascular health is insufficient for making general recommendations to patients seeking weight loss advice, according to the findings of a metaanalysis.
Based on a review of five randomized controlled trials comparing low-fat and low-carb diets, Dr. Alain J. Nordmann of the Basel (Switzerland) Institute for Clinical Epidemiology and his colleagues concluded that while low-carb diets lead to greater short-term weight loss, their effect on lipid levels and other cardiovascular risk factors appeared to be mixed.
Across all studies, the 222 participants in the low-carb diet groups were permitted a carbohydrate intake of no more than 60 g/day but had no calorie restrictions. The 225 participants in the low-fat diet groups were allowed no more than 30% of daily calories from fats, and those with a body mass index of 25 kg/m
The 447 participants had a mean age range of 42–49 years. All five trials were unblinded.
At 6 months' follow-up, patients on low-carbohydrate diets had lost more weight than those on low-fat diets and were more likely to have completed the trial than were their low-fat diet counterparts (weighted mean difference −5.3 kg vs. − 1.4 kg). The investigators also noted a trend toward lower systolic and diastolic blood pressure among low-carb dieters.
In addition, compared with low-fat dieters, low-carb participants had better HDL and triglyceride levels. However, they also had less favorable total cholesterol and LDL-cholesterol values.
Differences in weight loss between the groups diminished in the three studies with 12-month follow-up (weighted mean difference −3.5 kg vs. −1.5 kg). Also, the attrition advantage for the low-carb group had become statistically insignificant, and the low-carb group's blood pressure advantage over the low-fat group was no longer detectable.
Moreover, the low-fat dieters' more favorable serum LDL and total-cholesterol profile remained favorable at the 12-month mark, as did their more favorable change in serum triglyceride, while the HDL-level advantage seen at 6 months for the low-carb group was no longer definitive at 12 months.
The authors concluded that “there is still insufficient evidence to make recommendations for or against” low-carb diets, “especially for durations longer than 6 months.” It's uncertain whether the positive effects of a low-carb diet on HDL cholesterol and triglyceride levels outweigh its less favorable effects on serum LDL cholesterol levels, they said.