This theory could explain why you can't seem to ever turn off your cravings.

By By Christina Stiehl

When you treat yourself to your favorite snack or dessert, it's sometimes hard to stop eating once you get started. Luckily, your body has a built-in system to stop yourself from overindulging. Even if your mouth wants just one more bite, your body fires off signals to your brain letting you know you're satisfied. But what if those signals get lost in translation, never delivering the message that you're full?

It's an unfortunate reality for some people, making it almost impossible to stop hunger and mindless eating. And that, of course, leads to weight gain. Although researchers are still examining the complexities tied to obesity, some are pinning the blame on a hormone called leptin, a protein made by fat cells in the body. Leptin was just discovered in 1994, and doctors believe it could hold the key to unlocking the physiology of obesity and weight gain.

What Is Leptin?

Leptin is known as the appetite or hunger control hormone. After you eat, your fat cells secrete leptin into the bloodstream, where it travels to the brain, signaling that you're full. Someone whose leptin is functioning properly can eat to the point of satisfaction and not crave any more food. But when the brain struggles to detect the hormone, it doesn't trigger that necessary response. This is called leptin resistance.

If you have leptin resistance, even as your body goes into overdrive, producing more leptin and having it float around in your bloodstream trying to tell your brain that you've had enough, your brain isn't picking up on it. That leaves you with a hankering for more food. It's also a cycle: The more you eat, the more engorged your fat cells, and the greater the risk of worsening your leptin resistance, says Wendy Scinta, M.D., M.S., assistant clinical professor of family medicine at SUNY Upstate. And the more you gain weight, the less sensitive your body becomes to leptin.

What Causes Leptin Resistance?

Since leptin resistance is still being researched, scientists just don't know for sure what causes it, says Steven R. Hendrick, M.D., F.A.C.S., board-certified general and bariatric surgeon for the Harper Bariatric Medicine Institute at the Detroit Medical Center. However, there's evidence to suggest it's related to being overweight. "We only know that leptin resistance is associated with certain other medical conditions, such as obesity, [and its related diseases] type 2 diabetes, thyroid issues, and elevated triglycerides in the bloodstream," he explains.

A sedentary lifestyle and a diet rich in processed foods and simple carbohydrates could also lead to leptin resistance, in the same ways those factors lead to weight gain and other obesity-related illnesses. (Psst, here's how to eat carbs and still lose weight.)

How to Tell If You Have Leptin Resistance

Unfortunately, there's no set blood test or definitive way to determine if you have leptin resistance. But there are physical symptoms doctors consider, such as being overweight and the presence of visceral belly fat. A waist measurement larger than 35 inches for women or 40 inches for men could signal a problem.

"If you are always hungry, regardless of how much you eat, and often eat past what you feel should be your fullness point, there's a good chance you have leptin resistance," says Scinta. "People with leptin resistance will tell me, 'I am never full.'"

Scinta points to other related blood tests that could signal leptin resistance, such as fasting insulin levels. With insulin resistance inherently comes leptin resistance, she explains, because both hormones play a role in regulating hunger and therefore body fat. So high fasting insulin levels could indicate leptin resistance, as well. Elevated triglycerides, meaning fasting triglycerides above 200, could also signal a leptin resistance.

How to Treat Leptin Resistance

Although there's no medication to specifically target leptin resistance, you can relieve symptoms through lifestyle changes, says Hendrick. Since leptin resistance is a complicated issue intertwined with many other obesity-related illnesses, leptin sensitivity can be increased with proper diet and physical activity. Cutting out highly processed, inflammation-triggering foods is one good place to start, according to Scinta. Limiting simple carbohydrates and sugars can especially help since it will bring down glucose and insulin levels, which Hendrick says can improve your brain's leptin sensitivity.

Exercise will also treat insulin resistance. Working out by itself increases leptin sensitivity, especially longer aerobic exercise (think: your one-hour spin class). Leptin production was also associated with exercise that specifically reduced body fat mass, such as a combination of strength training and high-intensity interval training. And the more you build lean muscle mass, the more calories you burn at rest, and the healthier your resting glucose and insulin levels will be, says Hendrick.

Ideally, a combination of exercise and a diet rich in fresh fruits, vegetables, lean protein and some whole grains will not only promote insulin sensitivity on their own but also help you lose weight, which again, will help with insulin resistance-it's a cyclical process.

The Leptin Debate

Although the notion of leptin resistance is gaining more popularity, its role in managing obesity is still disputed. Since there's no official way to detect leptin resistance, some doctors remain skeptical of just how much of an impact leptin resistance has on being overweight. "We're all very different in how areas of the brain drive us to eat even when we have enough energy stored," says Eduardo Grunvald, M.D., program director at UC San Diego's Weight Management Program. "[Leptin resistance] doesn't have much value for people in the real world trying to figure out how to lose the end of the day, our brains are driving us to consume calories, and part of that is the cues we are getting from the environment around us."

The landmark 1990s study that identified leptin initially was done in mice. Although studies on rodents can sometimes help predict similar patterns in humans, Grunvald says that how leptin works in mice doesn't necessarily translate to how it works in humans.

Doctors are still researching how leptin works with other weight-regulating hormones, such as ghrelin, the hunger hormone; insulin, which builds muscle and regulates blood sugar; and adiponectin, which boosts your metabolism.

"We truly don't know all the relationships between these gut hormones and obesity," Hendrick says. But let's be honest, exercising and eating healthy are a good idea no matter what.

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