The answer, according to a new study, is appetite. People who successfully lose weight get really hungry — more than anyone had ever expected that they might. The body prompts us to eat about 100 calories more than usual for every 2 pounds or so of weight lost, researchers found.
“That’s the very first time that number has been quantified. We never knew how big that number was before the study,” says researcher Kevin Hall, PhD, who studies how the body responds to weight loss at the National Institutes of Health in Bethesda, MD.
It’s this surge in appetite, even more than the drop in metabolism people have after weight loss, that drives weight regain, he says.
The effect of appetite is three times stronger than the slowing metabolism. The two together virtually assure that lost pounds will creep back on, Hall says.
Independent experts who reviewed the study, which will be published in the November issue of the journal Obesity and presented on November 2 at the ObesityWeek conference, say it will probably change how doctors treat patients who’ve lost weight.
“This is a landmark study,” says Ken Fujioka, MD, director of the nutrition and metabolic research center at the Scripps Clinic in Del Mar, CA. “It gives us very useful information that will actually help us develop new guidelines,” to prevent weight regain, he says.
“We get patients all the time that hit these plateaus, and we’re trying to figure out, what do we do?” Fujioka says. “It’s real clear to us that you really need to deal with the food intake side, the driven appetite, from this paper.”
Metabolism and Food Intake
By some estimates, 80% of people who successfully lose at least 10% of their body weight will gradually regain it to end up as large or even larger than they were before they went on a diet.
Obesity researchers have been working for decades to understand why it is so hard to maintain weight loss. The prevailing theory — proved dramatically in a study of contestants from “The Biggest Loser” reality TV show that Hall published earlier this year — is that the body’s ability to burn calories at rest, or its resting metabolism, slows down, making it easy to regain weight.
The other piece of the equation, food intake after weight loss, has been much harder to study.
That’s because people are notoriously bad at keeping track of how much they eat. One famous study found that people trying to lose weight only thought they were eating about half as much as they actually were. It’s also been hard to measure appetite experimentally with drugs. That’s because most weight loss medications work by decreasing appetite, which interferes with study results.
Hall’s team got at the question in a new way, by taking another look at data from a recent study of a new diabetes drug, Invokana. Invokana reduces blood sugar by causing the body to dump some sugar through the urine.
“Getting rid of those calories also leads to weight loss, but in a covert way,” says Scott Kahan, MD, director of the National Center for Weight and Wellness at George Washington University in Washington, D.C.
“People don’t notice major changes in weight from the medication, but it’s enough that we can study what the change in weight and appetite would be,” Kahan says.
The study gave 242 people with type 2 diabetes either a daily dose of Invokana or a placebo pill. Over the course of a year, both groups lost some weight. The 89 people in the placebo group lost about 2 pounds. The 153 people who were taking Invokana lost about 7 pounds.
The puzzling thing to researchers was why the group taking the drug hadn’t lost more weight. Lab tests showed they were losing about 360 calories a day through their urine. Over time, even though the drug was subtracting a substantial number of calories each day, their weights plateaued.
Hall used an equation developed in his lab to figure out why. It estimates the number of calories a person would need to be eating to have weight changes over time.
He found that even though people in the study didn’t know how many calories the drug was cutting each day, their bodies were fighting against the weight loss, prompting them to eat more to make up the deficit.
Here’s how that might look in real life. If a person who normally eats about 2,700 calories a day loses about 9 pounds, their body will prompt them to eat about 400 more calories than they were before — a total of 3,100 calories a day.
‘This Gives Us Direction’
The implications for weight loss in the real world are profound, Kahan says.
“What I see in my patients, they have worked their butt off to lose weight and then keep it off. They can’t understand why they have all this success in other areas of their life, and they have such difficulty in this area of their life,” Kahan says.
“This is one of the pieces of that puzzle. This helps to explain that it’s not all your fault. Your body fights against the long-term maintenance of that weight. That’s very important,” he adds.
The study has some limitations. For one, researchers were studying people with type 2 diabetes. The results might not accurately represent appetite changes in healthy people, Hall says. It’s also not clear whether the kinds of appetite changes calculated for people in the study would apply to different amounts of weight loss. It could be that small weight changes don’t prompt the same big jumps in appetite as more substantial weight loss.
If further research supports these findings, Fujioka and Kahan say it points to a new way that doctors can help their patients.
Almost all prescription weight loss medications work by turning down a person’s appetite. It may be that people who lose weight can keep it off with the help of one of these drugs.
“This gives us direction,” Fujioka says. “I may also need to give my patients who’ve recently lost weight an appetite suppressant so they’re not so driven to eat.”