Dieters on low-fat and low-carb diets both lost the same amount of weight after two years, according to a just-published article in Annals of Internal Medicine. Both groups received intensive behavioral treatment, which may be the key to success for many. Low-carb eating was clearly superior in terms of increased HDL cholesterol, which may help prevent heart disease and stroke.
The study was funded by the National Institutes of Health and was carried out in Denver, St. Louis, and Philadelphia.
How Was It Done?
Healthy adults aged 18-65 were randomly assigned to either a low-fat or low-carbohydrate diet. Average age was 45. Average body mass index was 36 (over 25 is overweight; over 30 is obese). Of the 307 participants, two thirds were women. People over 136 kg (299 lb) were excluded from the study—I guess because weight-loss through dieting is rarely successful at higher weights. Diabetics were excluded.
The low-carb diet: Essentially the Atkins diet with a prolonged induction phase (12 weeks instead of two). Started with maximum of 20 g carbs daily, as low-carb vegetables. Increase carbs by 5 g per week thereafter as long as weight loss progressed as planned. Fat and protein consumption were unlimited. The primary behavioral goal was to limit carb consumption.
The low-fat diet: Calories were limited to 1200-1500 /day (women) or 1500-1800 (men). [Those levels in general are too low, in my opinion.] Diet was to consist of about 55% of calories from carbs, 30% from fat, 15% from protein. The primary behavioral goal was to limit overall energy (calorie) intake.
Both groups received frequent, intensive in-person group therapy—lead by dietitians and psychologists—periodically over two years, covering such topics as self-monitoring, weight-loss tips, management of weight regain and noncompliance with assigned diet. Regular walking was recommended.
Body composition was measured periodically with dual X-ray absorptiometry.
What Did They Find?
Both groups lost about 11% of initial body weight, but tended to regain so that after two years, both groups average losses were only 7% of initial weight. Weight loss looked a little better at three months in the low-carb group, but it wasn’t statistically significant.
The groups had no differences in bone density or body composition.
No serious cardiovascular illnesses were reported by participants. During the first six months, the low-carb group reported more bad breath, hair loss, dry mouth, and constipation. After six months, constipation in the low-carb group was the only symptom difference between the groups.
During the first six months, the low-fat group had greater decreases in LDL cholesterol (with potentially less risk of heart disease), but the difference did not persist for one or two years.
Increases in HDL cholesterol (potentially heart-healthy) persisted throughout the study for the low-carb group. The increase was 20% above baseline.
About a third of participants in both groups dropped out of the study before the two years were up. [Not unusual.]
Contrary to several previous studies that suggested low-carb diets are more successful than low-fat, the study at hand indicates they are equivalent as long as dieters get intensive long-term group behavioral intervention.
Low-carb critics warn that the diet will cause osteoporosis, a dangerous thinning of the bones that predisposes to fractures. This study disproves that.
Contrary to widespread criticism that low-carb eating—with lots of fat and cholestrol— is bad for your heart, this study notes a sustained elevation in HDL cholesterol (“good cholesterol”) on the low-carb diet over two years. This also suggests the low-carbers followed the diet fairly well. The investigators also note that low-carb eating tends to produce light, fluffy LDL cholesterol, which is felt to be less injurious to arteries compared to small, dense LDL cholesterol.
A major strength of the study is that it lasted two years, which is rare for weight-loss diet research.
A major weakness is that the investigators apparently didn’t do anything to document the participants’ degree of compliance with the assigned diet. It’s well known that many people in this setting can follow a diet pretty well for two to four months. After that, adherence typically drops off as people go back to their old habits. The group therapy sessions probably improved compliance, but we don’t know since it wasn’t documented.
How often do we hear “Diets don’t work.” Well, that’s just wrong.
Overall, it’s an impressive study, and done well.
Individuals wishing to lose weight on their own can’t replicate these study conditions because of the in-person behavioral intervention component. There are lots of self-help calorie-restricted balanced diets (e.g., Sonoma Diet, The Zone, Advanced Mediterranean Diet) and low-carb diets (e.g., Atkins Diet, Banting’s Letter on Corpulence, Low-Carb Mediterranean or Ketogenic Mediterranean Diets). On-line support groups—e.g. Low Carb Friends and SparkPeople and 3 Fat Chicks on a Diet—could supply some necessary behavioral intervention strategies and support.
Choosing a weight-loss program is not as easy as many think. [Well, I'll admit that choosing the wrong one is easy.] I review the pertinent issues in my “Prepare for Weight Loss” page.
Reference: Foster, Gary, et al. Weight and metabolic outcomes after 2 years on a low-carbohydrate versus low-fat diet: a randomized trial. Annals of Internal Medicine, 153 (2010): 147-157 PMID: 20679559