PHILADELPHIA — Dieters assigned a low-carbohydrate diet over a 12-month period fared better than those on a low-fat diet as measured by weight loss and reduction in cardiovascular risk factors, according to research recently published.
The findings, “Effects of Low-Carbohydrate and Low-Fat Diets,” representing the latest salvo in the decades old debate over the efficacy of macronutrient diets, were published in the Sept. 2 edition of Annals of Internal Medicine.
The study was published by a team led by Lydia Bazzano, an associate professor of epidemiology at Tulane University and Tian Hu, a doctoral fellow at Tulane University School of Public Health. The primary funding source for the study was the National Institute of Health.
Participants in the study were overweight or obese men and women who did not have clinical cardiovascular disease, type II diabetes, or kidney disease, use of prescription weight loss medications, surgery, and those with weight loss greater than 6.8 kg within six months of study entry. A total of 148 participants were included. Roughly half of the subjects were black, a group the researchers said were underrepresented in earlier research.
The participants were divided into two groups, half of whom were told to consume less than 40 grams of carbohydrates per day (the low-carbohydrate group) and the other half were told to consume less than 30% of their daily energy intake from fat.
Participants received dietitian guidance — weekly and individualized for the first four weeks; every other week in small group counseling sessions the next five months; and monthly in small groups for the final six months.
Data were collected using two 24-hour dietary recall methods. One recall reflected consumption on a weekday, the other on a weekend. The recalls were done at the start, and at 3, 6 and 12 months to track dietary intake.
At baseline, reported dietary composition was similar between those in the low-carbohydrate and low-fat groups. Also, physical activity levels remained similar throughout the study.
The participants in the low-carbohydrate group showed significantly greater body weight reduction than those in the low-fat group (mean difference in change at 12 months: -7.7 lbs) and showed significantly greater proportional reductions in fat mass (mean difference in change at 12 months: -1.5%). Changes in waist circumference favored the low-carbohydrate diet early in the study but did not differ significantly by the end. Also comparable between the two groups were 12-month serum levels of L.D.L. cholesterol — little changed. By contrast, H.D.L. cholesterol increased significantly more in the low-carbohydrate group than in the low-fat group.
“Our study found that a low-carbohydrate diet induced greater weight loss and reductions in cardiovascular risk factors at 12 months than a low-fat diet,” the researchers said.
“Findings from this trial may offer new evidence for the recommendation of a low-carbohydrate diet to obese persons as an additional nonpharmacologic approach for weight loss and reduction of cardiovascular disease risk factors,” the authors said.
The authors criticized earlier studies for failing to examine a “typical low-carbohydrate diet,” which they defined as restricting carbohydrates to less than 20% of daily energy intake. The authors estimated participants in their study “achieved an average of 30% of daily energy from carbohydrate.” They did not describe this difference as a shortcoming of their research. Over the course of the 12 months, the low-carbohydrate dieters consumed between 93 and 127 grams of carbohydrates per day (versus the objective of 40 grams). The low-fat dieters consumed between 27.5% and 29.8% of their calories from fat (versus a target of 30%).
Noting limitations to their study, the researchers acknowledged that self-reporting dietary intake may be subject to recall and memory issues. Additionally, the dietitians were not blinded to the study hypothesis, but were given specific and detailed scripts to avoid potential differences in counseling. In the paper the authors did not disclose their hypothesis, though the certain of the researchers have in the past advocated low-carbohydrate diets.
The researcher said their conclusions were limited by the lack of c.v.d. clinical end points, but risk factors were still assessed. Finally, the researchers said that their findings are difficult to generalize.
“Although our findings show what can be achieved, they may not be generalizable to more common situations where intensive and repeated dietary counseling is not available,” the researchers said.
In widespread media coverage of the article, most outlets, but not all, characterized the study as a victory for low-carbohydrate dieting. One skeptic was Elle Penner, a registered dietitian and food and nutrition editor.
In a Huffington Post blog, Ms. Penner identified a number of reasons the findings may not be reproducible to the public at large, including the participants access to regular counseling. She also noted low-carbohydrate dieters were eating three times as much carbohydrate by the end as they were instructed to consume.
“As it turns out, the study doesn’t actually prove that low-carb diets lead to greater weight loss compared to low-fat diets,” Ms. Penner said. “Instead, it shows consuming fewer carbohydrates may increase protein consumption, and something about that combination seems to enhance weight loss — however, the cause is not yet clear.“What is clear is that adhering to a diet of 40 grams of carbohydrate per day for any amount of time is really difficult, even with low-carb meal supplements and nutrition counseling.”