We hear a lot about leptin in the weight loss industry, mainly because of its effects in animal research.

In 1994, it was discovered that a protein hormone called leptin, which is released from fat cells and monitored by the brain, was deficient in a certain strain of genetically mutated obese mice.

Normal mice had a gene which caused fat cells to secrete leptin, but the mutated obese mice lacked that gene. When the leptin-deficient mice were injected with leptin, their weight eventually returned to non-obese levels. Soon it was discovered that leptin could increase metabolic rate, or energy expenditure, in mice.

Those in the weight loss industry soon learned about this research, and began claiming that leptin was the cure for obesity.

What works for mice, however, doesn’t always work for humans. Researchers quickly learned that even an 80% increase in leptin levels in humans doesn’t change the body’s resting metabolic rate [Chan JL, 2007]. Changes in leptin levels, in fact, don’t seem to have any effect on resting metabolic rates in humans [Rosenbaum M, 1997; 2002].

Leptin, in humans, seems to be correlated to the amount of food we eat. The more we eat, the more leptin we can find in our bloodstream.

Leptin levels can be increased when an individual overeats for several days, but those levels return to normal within hours [Kolacznyski JW, 1996].

Leptin appears to be a marker of both fat mass and caloric intake. Leptin levels are highly correlated with body fat levels even in cases of severe calorie restriction (as in those found in anorexic women). The lower the body fat, the lower the leptin levels [Bossu C, 2007].

Research shows there are substantial differences in the physiological actions of leptin in mice and humans [Ahima RS, 2000], which could explain why there are so many confusing theories about leptin in the diet industry.

Leptin is complicated. In different situations, its levels rise and fall, and explanations for the changes aren’t always logical. Long term endurance exercise and resistance exercise can reduce leptin levels. So can fasting, increased testosterone levels, and increased catecholamine levels. Injected anabolic steroids can also decrease leptin levels.

In all of these situations, an increase in fat burning occurs in spite of decreases in leptin levels.

While leptin is an important hormone, it is not all that it is made out to be by the fitness industry.

While short term fasting usually involves an acute decrease in leptin levels, the consistent increase in growth hormone ensures that fat loss remains elevated during that time. Even when leptin is injected into fasting individuals, more fat isn’t burned and growth hormone levels aren’t decreased [Chan JL, 2008].

Short term, flexible intermittent fasting combined with resistance training remains one of the most effective and simple ways to reduce body fat.

Testosterone is important for men’s health and well-being. It also prevents osteoporosis, helps determine how much muscle mass a man will possess, and affects a man’s libido.

As men age, testosterone levels decrease.

Although most sports consider it to be a form of “doping,” and although it’s been banned by the International Olympic Committee, some athletes use testosterone in the form of anabolic steroids to improve performance, build muscle and decrease body fat.

Testosterone can be consumed via intramuscular injections, transdermal gels and patches, and implantable pellets.

Bodybuilders and fitness competitors often use testosterone doping, and it’s largely responsible for the unrealistically large proportions of professionals in these sports.

Testosterone levels can also be changed by workouts, nutrition intake – and even by a man’s mood.

High protein and high fiber diets can decrease testosterone levels, and diets with low protein and higher saturated fat tend to increase levels, but fasting can also affect levels.

Short fasts don’t seem to have an effect on normal testosterone levels. Testosterone levels tend to be highest – 20-30% higher than levels found in the evening – the morning after an overnight fast.

Longer fasts can decrease testosterone levels slightly. By the third straight morning of fasting, morning serum testosterone levels are usually notably reduced [Röjdmark S. 1987], however measurements are still within a normal range for healthy adults [Merck Manual 1992].

With the Eat Stop Eat program, fasting is only maintained for 24-hours, so serum testosterone levels never drop below normal. Eat Stop Eat also requires you to follow a resistance training program, which also increases testosterone levels [Ahtiainen JP, 2004].

With Eat Stop Eat, you can still reduce body fat and build muscle mass without lowering your testosterone levels.

Many people think the growth hormone is responsible for helping children grow taller, but the truth is that adults utilize the hormone all throughout life to maintain their overall health.

I’d go so far as to say that growth hormone should be renamed “fasting hormone”, since growth hormone becomes especially important when an individual is fasting. During fasting periods, this hormone regulates metabolism and maintains muscle mass.

When a person fasts, his or her growth hormone interacts with an enzyme called hormone-sensitive lipase – or HSL, letting fat cells know when to release stored fat that the body can use for energy.

Contrary to what some believe, low insulin isn’t what causes fat to burn during fasting periods. In fact, scientific research has proven that very little fat is burned when insulin levels are low if growth hormone levels are also low.

If a person is attempting to get rid of body fat, a combination of low insulin levels and high growth hormone levels is the best approach.
An added bonus: The growth hormone has beneficial effects on muscle mass, which is also important when an individual is attempting to lose weight by fasting.

Some athletes have used supplemental growth hormones to enhance performance. The hormone is so effective at reducing body fat and building muscle mass that is has been banned by organizations such as the NCAA and the International Olympic Committee in its supplemental form.

An individual’s natural growth hormone is released every three to four hours, with the biggest release occurring approximately an hour after her or she falls into a deep sleep. Levels of the hormone are also increased during periods of intense exercise. Research has shown that as little as ten minutes of intense exercise can have a profound effect on the amount of growth hormone levels in a person’s body.

How can you get the most out of your growth hormones?

First, follow a flexible intermittent fasting method such as Eat Stop Eat. Brief periods of fasting can increase growth hormone levels. Secondly, if you want even more GH release, be sure to get at least ten minutes of intense exercise every day and get plenty of sleep each night, since both of these practices allow for optimal release of the growth hormone.

Healthy growth hormone levels will help your body burn fat and maintain muscle mass.