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At a recent American Heart Association (AHA) meeting, those debating whether low-fat or low-carbohydrate diets are better for weight loss agreed on one thing: It's the calories.
At the Obesity, Lifestyle, and Cardiovascular Disease Symposium, held January 18–20 in Washington, D.C., Alice H. Lichtenstein, DSc, of Tufts University in Boston, started with a literature review of studies comparing low-carb with low-fat diets.1,2
Potential benefits of a low-carb diet include its atherogenic profile and improved measures of glucose homeostasis and inflammation, Lichtenstein summarizes. Potential risks are increased urinary ketones, constipation caused by low fiber intake, and potentially compromised micronutrient intakes. The difference in weight loss is not significant, she says.
But Lichtenstein concludes that the lack of long-term data makes it difficult to properly assess the risks and benefits of any type of restrictive diet.
"Measures were assessed during the active weight-loss phase, and may be different during weight maintenance," Lichtenstein says. "We really need long-term data."
"Extreme" diets work in the short term because they eliminate large categories of foods, she says. "After a while it gets old, and they get tired," Lichtenstein says. "In the long term, the diets don't seem to be working. The bottom line is calories. We have to get a lot clearer on our messages."
LOW-CARB: TERRA INCOGNITA
Internist Eric C. Westman, MD, MHS, of Duke University Medical Center in Durham, N.C., calls low-carb diets the "terra incognita" of diet studies.
"What we've known from mixed-diet studies really doesn't apply," Westman says.
Carbohydrates, fat, and protein are all part of the excess weight problem because of calories, he says. Westman cites a 2005 study for helping define what a low-carb diet really is—high-fat, not high-protein.3 He proposes one possible mechanism for why these diets might aid weight loss: When dietary carbohydrates are restricted, appetite is suppressed, and the body draws on stored fuel. Another possibility, which Westman says is unproven, is that the body's inefficient processing of protein and fat leads to excess energy loss.
He notes the apparent paradox that an increase in fat consumption in low-carb diets has been shown to decrease triglycerides as well as total and LDL cholesterol in many individuals. The hypothesis is that replacing carbs with fat reduces glucose, causing a reduction in insulin, which in turn reduces cholesterol.
Before insulin therapy was discovered in the 1920s, a low-carb diet was the treatment of choice for people with diabetes, Westman says. One recent study shows that after 16 weeks on a low-carb diet, subjects showed a reduction in glycated hemoglobin (A1C) from 7.4 to 6.3.4 "Many people came off their [diabetes] medication in that time," he says.
He reminds clinicians to adjust diabetes medications as necessary for patients who are cutting calories and losing weight.
Westman says what's been learned about low-carb diets is that decreasing carbohydrates decreases calories. As for weight loss, Westman concedes that low-carb may not be better than low-fat: "I don't understand the argument that low-carb doesn't work better, when all I'm trying to say is that it works equivalently."
He recommends tailoring diets to personal taste and metabolic outcomes.
WEIGHT MAINTENANCE
The Women's Health Initiative (WHI) Dietary Modification Trial,5 while not a weight-loss study, presented an opportunity to ask how lowering fat (and therefore increasing carbohydrate intake) affected weight over 7.5 years.
The trial included almost 49,000 ethnically diverse postmenopausal women
aged 50–79 years whose diets upon entering the study comprised 
32%
fat. The women were randomized to either a low-fat dietary intervention or a
self-selected dietary control group.
The intervention was designed to change diet patterns, but did not encourage weight loss or caloric reduction. It tracked dietary changes based on a nutritional counseling program that advised participants to reduce fat intake (setting no limits on type of fat) by replacing it with fruits, vegetables, and grains. The program included 18 meetings during the first year and quarterly sessions in the years following.
Compared with the control group, those in the diet intervention reported a significantly lower percentage of energy from total, saturated, and unsaturated fat intake during followup, and a significantly higher percentage of energy from carbohydrates. The intervention group also increased intake of fiber, vegetables and fruits, total grains, and whole grains.
At year 1, the intervention group's mean weight was 2.2 kg (4.6 lb) less than the control group's. The weight difference diminished as the years went by, but women in the intervention group maintained a modest weight loss, compared with the control subjects.
"A low-fat eating pattern doesn't result in weight gain,"
concludes investigator Barbara V. Howard, PhD, of the MedStar Research
Institute in Hyattsville, Md. "If calories are restricted and activity
is increased, it could be a feasible approach to weight loss."
References
2. Stern L, Iqbal N, Seshadri P, et al.: The effects of
low-carbohydrate versus conventional weight loss diets in severely obese
adults: One-year follow-up of a randomized trial. Ann Intern
Med 140:778–785, 2004.
3. Boden G, Sargrad K, Homko C, et al.: Effect of a low-carbohydrate
diet on appetite, blood glucose levels, and insulin resistance in obese
patients with type 2 diabetes. Ann Intern Med 142: 403–411, 2005.
4. Yancy WS, Foy M, Chalecki AM, et al.: A low-carbohydrate, ketogenic diet to treat type 2 diabetes. Nutr Metab 2:34, 2005. Available online at www.nutritionandmetabolism.com/content/2/1/34. Accessed January 26, 2006.
5. Howard BV, Manson JE, Stefanick ML, et al.: Low-fat dietary pattern
and weight change over 7 years: The Women's Health Initiative Dietary
Modification Trial. JAMA 295:39–49, 2006.
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