New research has concluded that regular exercise can decrease the levels of leptin, "the fat hormone," reducing the risk of obesity and obesity-related diseases (American Heart Association, Mar. 3, 2000). Leptin is a protein hormone that circulates in the bloodstream and is produced by the body's fat cells. It helps regulate body weight by stimulating the brain to give a feeling of fullness and reduce appetite. Leptin also tells the brain how much fat is stored in the body and seems to increase as physical activity decreases.

Leptin and Movement

A recent study looked at the levels of leptin and physical activity in 268 men aged 47 to 83 who did not have disease, cancer or diabetes. The participants were questioned on what types of food they ate, how much they exercised, if they smoked or consumed alcohol, and if so, how much, to determine their dietary and lifestyle habits.

The researchers found that the men with the highest leptin levels did not exercise regularly, weighed more and ate foods higher in saturated fat than men with low leptin levels. The men with higher leptin levels usually produced more insulin, a risk factor that could lead to diabetes, but more research needs to be done to confirm this finding.

Correlation between leanness and lower leptin levels still needs to be determined. Some researchers also speculate that fat people stay fat because their brain cells may be unresponsive to leptin just like people with diabetes are unresponsive to insulin.

The best way to manage your leptin levels is to engage in any kind of physical activity. An increase of physical activity by 20 MET, about 25 minutes of jogging per day, was found to decrease leptin levels by 10 percent. In addition to decreasing leptin levels, regular exercise will help reduce your risk of heart disease, diabetes and other chronic diseases, as well as lose body fat, and help boost self-confidence.

Leptin and Fat-Burning

The study, published in the Proceedings of the National Academy of Sciences found that increasing the levels of leptin in rats led to significant weight loss by changing the structure and function of the fat cells- from fat-storing to fat-burning. This occurred by increasing the energy powerhouse called mitochondria within the cells.

In the study, researchers looked at the effects of injecting one of two groups of rats with the leptin gene. Compared with normal rats, animals that got the leptin injection experienced much greater weight loss and lost an average of about 26% of their total body weight in 14 days. This is due to a decreased appetite and increased physical activity that occured naturaly after leptin treatment. On the other hand, the normal rats fed a restricted diet were constantly searching for food and had lower levels of physical activity.

When the researchers examined the rats' cells under the microscope, they found that rather than storing fat, the cells had inceased mitochondria. So, not only leptin-fed rats wanted to eat less and move more, they also burned more fat.

Leptin and Diet

The problem with obesity is just as complex as the human body itself. So, the leptin dilemma gets more complicated. The well-known saying "you are what you eat" received some confirmation in the study published in the December 2003 issue of Obesity Research.

Adult rats fattened on a high-calorie diet returned to their original weights when scientists injected them with leptin, making more of the hormone available for rats' brains.One group of rats had a standard diet containing 11% fat and another group consumed a high-calorie diet with 45% fat. After 80 days, the average weight of the rats on the high-fat diet increased by 20%. At that point, one group received an injection of a gene that produces leptin directly in their brains and another was given a placebo.

Fifty-six days after the injections were given, the weight of the obese rats given the leptin gene had dropped to only 3% above their beginning weight. Those given the placebo remained 20% heavier than their beginning weight. Even though the rats were offered as much of the high-fat food as they wanted, after getting the gene therapy, they opted to eat 10%-15% less and increased their energy expenditure on their own.

Blood levels of leptin in the obese rats that got the leptin injection naturally dropped as they lost excess fat, but they also produced more leptin in the brain, where the hormone has the biggest impact on influencing behavior. Researchers speculate that overweight people eat too much fat, which leads to excessive leptin production and dampens the fat-fighting effect of the hormone in the brain. For reasons that are not clear yet, when leptin levels are increased in the blood, leptin is ineffective in performing its normal function in the brain. Scientists call this leptin resistance which increases with high-calorie diet.

Leptin and the Brain

It is thought that brains of the obese people are perhaps not sensing how much fat is stored in their bodies which leads to increased appetite. Two studies published in Science journal examined brain pathways that control feeling of fullness and hunger in adult mice. By examining the actual brain nerves, researchers saw that mice that make no leptin have strong brain circuits that signal them to eat and weak circuits that signal them to stop eating.

Just six hours after giving additional leptin to the animals, their brain circuts changed becoming more like those in normal rats. Two days after being treated with leptin, the mice lost their huge appetites.

Could it be that obese people are leptin resistant from birth? Or is it the way they train their brains to resist the hormone signals? The specific role that leptin plays in obesity is still unclear.

Sources:

Leptin and its potential role in human obesity. J Intern Med. 1999 Jun; 245(6): 643-52. Review.

Short- and long-term changes in serum leptin dieting obese women: effects of caloric restriction and weight loss. Journal of Clinical Endocrinology and Metabolism 1998 Jan; 83(1): 214-8.

Trophic Action of Leptin on Hypothalamic Neurons That Regulate Feeding. Science, 2004 Apr; 304 (5667): 108-110

Concurrent reductions of serum leptin and lipids during weight loss in obese men with type II diabetes. Am Journal of Physiology 1999 Aug; 277(2 Pt 1): E277-82.

Plasma leptin and insulin levels in weight-reduced obese women with normal body mass index: relationships with body composition and insulin. Diabetes. 1999 Feb; 48(2): 347-52.

Proceedings of the National Academy of Sciences 2004 Feb; 101: 2058-2063. News release, University of Texas Southwestern Medical Center at Dallas. Find out more on Australian Fitness: