Losing weight is hard work, but many people who have lost weight may agree that keeping the weight off can be an even greater challenge. A lack of self-control or a few too many dietary indulgences are often cited as reasons for regaining weight. But a new study in the November issue of BMJ questions this conventional view, finding that the type of calories you consume may influence how likely you are to keep that weight off for the long term. 
The human body is designed to protect itself when it sheds weight, whether voluntarily or involuntarily, by causing an increased urge to eat and a slowdown in metabolism while more efficiently storing fat. Although it may be exciting to see pounds on the scale drop, the ability to keep losing weight or even maintain any weight loss becomes harder, because cravings to eat rise while the body more readily stores those calories as fat.
The purpose of the BMJ study was to see if different levels of carbohydrate in the diet could prevent these metabolic changes from occurring, so that any weight lost might stay off. The focus on carbohydrates was based on the “carbohydrate-insulin model” of obesity, which states that high insulin levels that result from eating a high glycemic load diet (i.e., highly processed carbohydrates like refined breads, crackers, cookies, and sugars) causes energy from the food to be stored more easily as fat, and may increase hunger and food cravings, lower energy expenditure, and promote weight gain.
Participants were first placed on a weight-reducing diet to lose about 12% of their starting weight (weight loss averaged 25 pounds) to kickstart metabolic changes. The next phase randomly assigned the 164 participants who achieved this weight loss to one of three test groups:
- high (60%) carbohydrate and low (20%) fat diet
- moderate (40%) carbohydrate and (40%) fat diet
- low (20%) carbohydrate and high (60%) fat diet
The protein amount was the same in all groups at 20%. Total calories were adjusted up or down to prevent any weight changes in each participant. All meals were provided to the participants during the weight loss phase and throughout the 20-week test phase. The types of foods in each diet group were designed to be as similar as possible, but varying in amounts: the high carbohydrate group ate more whole grains, fruits, legumes, and low fat dairy products. In contrast, the low carbohydrate group ate more fat but eliminated all grains and some fruits and legumes.
Participants followed the diets for 20 weeks and total energy expenditure was measured. During the 20 weeks, the participants in all groups maintained their weight and there was minimal difference in secondary measures including physical activity and resting energy expenditure (factors that could independently increase total energy expenditure).
- The low carbohydrate group showed an increased energy expenditure with a range of 209-278 calories/day compared with the high carbohydrate group. The moderate carbohydrate group showed a smaller increase in expenditure of about 100 calories compared with the high carbohydrate group. This trend was consistent throughout the 20-week period.
- The increased metabolic effect with the low carbohydrate diet was most significant in people who had high insulin secretion at the start of the study, with an increased energy expenditure of a range of 308-478 calories/day. (People with high insulin secretion tend to be shaped more like “apples” than “pears,” as excess body fat is stored predominantly around the mid-section.) This finding supports recent research to suggest that differences in biology may affect how people respond to weight loss diets over the long term.
- A hormone that works to increase appetite, ghrelin, decreased significantly on the low carbohydrate diet, which could help with weight loss maintenance. Another appetite-regulating hormone, leptin, also decreased. Leptin regulates energy balance and works to keep body weight stable. It typically counteracts ghrelin by sending signals to the brain to suppress appetite when the body has enough food. Previously, high leptin levels were thought to lower one’s appetite and cause the body to begin using stored fat for energy. However, some forms of obesity/overweight may lead to “leptin resistance” when the body has high levels of leptin. In this scenario, the brain does not receive an alert that leptin levels are already high, so it continues to send strong hunger signals while conserving body fat stores. In other words, high leptin levels may promote leptin resistance. Its significance in the BMJ study was that the lower carbohydrate diet appeared to improve leptin sensitivity by reducing high levels of leptin.
“This study raises the possibility that a focus on restricting carbohydrates, rather than calories, may work better for long-term weight control,” said Dr. David Ludwig, professor in the Department of Nutrition at the Harvard T.H. Chan School of Public Health, who led the study with Dr. Cara Ebbelingfrom Boston Children’s Hospital.
Dr. Walter Willett, professor of epidemiology and nutrition at the Harvard Chan School, who was not involved in the study, also noted that “these findings from a carefully conducted investigation can help explain why low fat/high carbohydrate diets are not successful for most people and have failed to maintain weight loss in formal randomized trials that have lasted for one year or longer.”
In a review featured in Science magazine the same week as the BMJ study, Dr. Ludwig discussed the controversy over specific fat-to-carbohydrate ratios in maintaining a healthy weight and lowering disease risk.  He, Dr. Willett, and other experts on the subject agreed that by focusing mainly on diet quality—replacing saturated or trans fats with unsaturated fats and replacing refined carbohydrates with whole grains and nonstarchy vegetables—most people can maintain good health within a broad range of fat-to-carbohydrate ratios. Read more at Dietary fat is good? Dietary fat is bad? Coming to consensus.