Leptin: Could This Hormone Be The Reason It's So Difficult To Keep Weight Off?

New research has found that significant weight loss can wreak havoc on your hormones, making it much more difficult to keep those pounds off.

Why is it so tough to keep weight off?

It's a question many Americans struggle with. A 2015 study found an average obese person who lost more than 5 percent of their body weight within one year had only about a one in five chance of keeping the weight off for the next five years. While some might blame lack of willpower or motivation, there's increasing evidence that biological changes make it intensely challenging for once-obese people to sustain weight loss.

Much of this evidence revolves around two important hormones—leptin and ghrelin—and how drastic weight loss can affect their levels. These hormones have a tremendous impact on how the body processes hunger and satiety.

Here's why these hunger hormones could be the invisible reason why it's so hard to keep weight off.

The Hunger Hormones

Business People Eating Pizza at a Meeting

Leptin and ghrelin are both hormones.

Produced by glands throughout the human body, hormones are a class of signaling molecules that enter the circulatory system and target different organs. They essentially help different areas of the body talk to each other and get on the same page, and they play a role in everything from metabolism to sensory perception to reproduction. Collectively, leptin and ghrelin are often referred to as "the hunger hormones."

Ghrelin is an appetite increaser that's released primarily in the stomach and is believed to signal hunger to the brain. When your stomach is empty, ghrelin is secreted and you experience hunger. Ghrelin has fast-acting capabilities, and it can rise in correlation with your normal meal routine. If you normally eat lunch at 11 a.m., but have to wait until 1 p.m. on a given day, the intense feelings of hunger you experience are tied to ghrelin. Once you eat, ghrelin levels decline. As you go from feeling hungry to eating until you're full, the rise and fall of ghrelin levels throughout the day is a healthy human function.

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Whereas ghrelin plays more of a role in short-term, everyday hunger and satiety, leptin helps regulate long-term weight and energy expenditure. Released in fat cells throughout the body, leptin sends signals to the brain that reduce hunger. The more leptin you have inside your body, the less hungry you should be. Since leptin lives in fat cells, it is more abundant in those with higher levels of body fat. This makes sense, right? A person with a higher level of body fat shouldn't feel as much of a biological need to eat as someone who is underweight, so their leptin levels are higher.

These roles of leptin and ghrelin might sound fairly straightforward, but obesity has some bizarre effects on how they behave.

Obesity Makes Hormones Go Haywire

Fatigue on an Exercise Bike

Although leptin and ghrelin work efficiently and fairly predictably in most healthy people, obesity can wreak havoc on them.

Leptin levels rise as body fat levels go up, but there's a point where the body produces so much leptin that it loses its ability to send signals to the brain. While the exact mechanism behind this process is unknown, "leptin resistance" is a major biological factor in the development of obesity. The excess buildup of leptin prevents its signals from reaching the brain, which tricks the body into thinking it needs to eat more and reduces energy expenditures. Leptin resistance is perhaps the biggest biological reason why people who are overweight or slightly obese often see their weight skyrocket unchecked. Their natural hormones have lost the ability to do their job effectively.

But what happens when those same obese people manage to lose significant amounts of body fat?

A recent study in the journal Obesity provides a clearer view of the biological changes obese people experience when they lose weight. For the study, researchers tracked 14 contestants from season eight of NBC's The Biggest Loser over a six-year period. Over those six years (season eight aired in 2009), all but one regained a significant amount of weight, and four contestants became heavier than they were when they started the show.

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The researchers found significant biological changes that made it extremely difficult for the contestants to keep weight off over an extended period of time. Many of them saw their metabolisms slow down dramatically, which meant they burned fewer calories at rest. But also, their leptin levels plummeted. By the end of the show, contestants had almost no leptin in their systems. Such a drastic lack of leptin makes people ravenously hungry. They feel a strong biological need to consume more calories and reduce energy expenditure (which could be a big reason for the slower metabolisms). Leptin also has an affect on the brain's reward system—when levels are low, consuming food feels even more rewarding.

Contestants described intense hunger attacks which made binge eating all but unavoidable. "What people don't understand is that a treat is like a drug," Erinn Egbert, a season eight contestant, told the New York Times. "Two treats can turn into a binge over a three-day period. That is what I struggle with." Contestants began the show with average leptin levels of 41.1 nanograms per millimeter. By the end of the contest, those levels had nosedived to an average of 2.6 ng/mL. As the contestants began to regain the weight they had lost, their leptin levels rose—but only to about half of their baseline levels

The Biggest Loser study didn't monitor ghrelin levels, but a 2011 study in The New England Journal of Medicine did. Researchers studied 50 overweight participants who lost weight on a 10-week program with a low-calorie diet. Their weight loss was accompanied by significant reductions in leptin and significant increases in ghrelin and appetite.

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"One year after initial weight reduction, levels of the circulating mediators of appetite that encourage weight regain after diet-induced weight loss do not revert to the levels recorded before weight loss. Long-term strategies to counteract this change may be needed to prevent obesity relapse," the authors concluded.

In both of these studies, one thing was abundantly clear: when an obese person loses a significant amount of weight, his or her body fights to put the weight back on by making hormonal changes.

Shedding Pounds For Good

Woman Weighing Herself

There is still much to be learned about how the human body responds to obesity and weight loss, but it's clear that sustaining weight loss is much more difficult than most people anticipate. Major changes in metabolism and hormones indicate that the body, for whatever reasons, has a strong, natural desire to return to its starting weight—even if that starting weight was unhealthy.

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The best thing someone can do to deal with this issue is to avoid becoming obese in the first place. It's fairly common for people to watch their weight gradually climb over an extended period of time and think to themselves that they'll get it under control eventually. They might think they're too busy to make dietary or lifestyle changes, but vow that they will do it in the future. What they don't realize is that for every extra pound they put on, they make it exponentially more difficult for their future selves to achieve and maintain a healthy weight.

For those who are already obese, slow, sustained weight loss may give them a better shot at keeping the weight off for good. Pharmaceutical companies are also developing medications that might help those who've achieved drastic weight loss battle the biological changes that come along with it.

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