As any dieter knows, losing weight is hard. Keeping it off can be even harder, and a small, new study by Australian researchers helps explain why: a symphony of hormonal changes sends the body relentless signals to slow metabolism and increase the urge to eat, for at least a year after weight loss.
The findings support obesity researchers’ long-held belief that dieters who regain weight aren’t just reverting back to old habits. Instead, they may be fighting their own biology.
Many previous studies have shown that when overweight people slim down, their bodies respond vigorously, by undergoing changes in hormones that affect hunger and satiety — “multiple compensatory mechanisms encouraging weight gain,” as the authors put it. For instance, when obese people lose body fat, levels of the hormone leptin, which is produced by fat cells, drop. That signals to the brain that the body’s energy stores are low, slowing metabolism and triggering hunger.
It’s an evolutionary survival tactic. “These mechanisms would be advantageous for a lean person in an environment where food was scarce,” write the authors, from the University of Melbourne and La Trobe University, in Australia, “but in an environment in which energy-dense food is abundant and physical activity is largely unnecessary, the high rate of relapse after weight loss is not surprising.”
What the new study adds to the picture is a longer view. By tracking participants for 16 months, the researchers found that these hormonal changes are persistent, possibly even permanent. The study involved 50 overweight or obese adults enrolled in an intense 10-week diet program. For the first eight weeks, the volunteers consumed just 500 to 550 calories a day. As a testament to how hard weight loss is, 11 participants dropped out during this early phase — they either quit or failed to lose the required 10% of body weight to continue.
Over the following two weeks, the 39 remaining participants, who had lost an average of 30 lbs. (14% of their initial body weight), were reintroduced to a normal diet and counseled on how to maintain their new weight. Over the next year, they were brought in every two months for follow-ups, contacted by phone between visits for dietary counseling and encouraged to exercise regularly. The researchers checked the participants’ hormone levels at baseline, at the end of the diet phase, and then again one year later; they also asked about the dieters’ subjective feelings of hunger. Before the year was up, five more participants had dropped out.
Despite the counseling, people gained back about 12 lbs. — nearly half of what they had lost — during the maintenance phase. And they reported feeling even hungrier at the end of the study than at the start. Their hormone profiles may help explain why.
Over the 10-week weight-loss phase, participants’ levels of leptin plummeted by 65%. A year later they were still a third lower than before weight loss, having climbed as people regained fat. Levels of several other appetite-regulating hormones, including ghrelin and peptide YY, which are produced in the pancreas and gut, also remained significantly changed a year later, in ways that would increase hunger and spur the urge to eat.
To many experts, the findings were not surprising. Weight loss is difficult — the study confirms it. But while compelling, the findings are limited by the study’s small size and the lack of a control group; more rigorous research that compares hormone levels in dieters to those in overweight people who maintain their body size would be helpful. Also, the current study doesn’t show whether hormonal changes caused the weight gain or coincided with it.
What is clear, however, is that obesity is not merely a problem of failed willpower or an unhealthy food environment. Solving it will require far better treatments than we’ve got now, perhaps including drugs that can balance dieters’ hormones and restore normal appetite. “A combination of medications will probably be required,” the authors write, although no such drug has been approved by the Food and Drug Administration.
Of course, this is not to suggest that long-term weight loss is impossible. Many people have done it. But dieters “who have lost weight need to remain vigilant and understand that once they have lost weight the battle is not over,” study author Joseph Proietto of the University of Melbourne told Bloomberg. “Indeed, the most difficult part of the weight loss program is the maintenance phase, which may be indefinite.”
The study’s essential finding is simple: prevention is key. “It’s better not to gain weight than to try to lose it,” Dr. George Bray of the Pennington Biomedical Research Center, in Baton Rouge, La., told the AP.
The new study was published Thursday in the New England Journal of Medicine.