Injecting leptin into humans does seem to have a remarkable ability to decrease appeite and reduce food intake, but these changes seem to be seperate from raising leptin via eating strategies, and some people and companies marketing the benefits of Leptin do so using research where Leptin was injected into humans.
In fact, acute changes in leptin levels do not seem to have any influence nor are they influenced by changes in resting metabolic rate in humans [Rosenbaum M, 1997; 2002]
However, while the link between leptin and metabolic rate (the amount of calories you burn) is extremely weak, this does not mean that Leptin is not important to your fat loss efforts.
In humans Leptin seems to be correlated to the amount of food we eat, and the amount of body fat we are currently storing on our bodies.
The more food, and the more body fat, the more leptin in our blood stream.
Overeating for several days can increase leptin levels, however these levels return to normal within hours after the overeating is stopped [Kolacznyski JW 1996].
Even in severe conditions of calorie restriction (like those found in anorexic women) Leptin levels are highly correlated with body fat levels. In other words, the lower the body fat, the lower the leptin levels [Bossu C, 2007].
Research has shown consistently that there are substantial differences in the physiological actions of leptin between rodents and humans [Ahima RS, 2000] and this may explain why there is so much leptin-confusion in the diet industry.
The reality is Leptin is a complicated hormone.
It rises and falls in different situations, which are not always logical. Both long term endurance exercise and resistance exercise can cause reductions in leptin levels, as can fasting, as well as increased testosterone levels and increased catecholamine levels.
Even injected anabolic steroids can decrease leptin levels.
In fact, there is a very interesting relationship between Leptin and Testosterone. In men this relationship is 'negative' meaning as leptin goes up testosterone goes down, and as testosterone goes up leptin goes down. In Women the relationship is 'positive' which means when Leptin goes up testosterone goes up, and vice versa.
Probably the most interesting phenomena is the relationship between leptin and fasting. Even a short fast of 72 hours can decrease leptin by more than 80%. However, When people fast AND have their leptin levels kept high by taking leptin injections, there is no change in the fat loss or metabolic rate.
What this tells us is that while leptin is still extremely important in body fat regulation, it's acute (short-term) changes may not be as important as maintaining overall healthy levels of Leptin.
The fact that leptin is highest in obese people has made some people think there is a 'leptin resistance', similar to being 'insulin resistance' where the body is no longer able to react to leptin the way it should.
Unfortunetley, Since the term "leptin resistance" is a term so broadly-applied and context-depndent that as of February 2012 there is no nuiversal, clinically useful definition or diagnoses for "leptin resistance".
One problem with the concept of leptin resistance is the suggestion that leptn actions may be at a naer maximal level at the concentrations found in most people, and that obese people even with their ultra high levels of leptin may already be 'maxed out' and thus more not seeing any additional benefit from the extra Leptin (Krol E, 2007).
This suggestion is strengthened by the fact that the people with the highest leptin sensitivity are those with low or normal levels of body fat (Bluher S, 2009).
Leptin is an important hormone, but it is not the ‘master regulator’ of fat burning that it has been made out to be by the fitness industry, at least not in the short-term.
The bottom line is that while short term fasting and intermittent fasting typically involves an acute decrease in leptin levels , the consistent increase in Growth Hormone ensures that fat loss remains elevated during periods of fasting. In fact even when Leptin is injected into fasting individuals it doesn’t improve fat burning or decrease GH levels [Chan JL, 2008]
Most likely small acute changes in leptin do not negatively affect your ability to lose fat, however chronic low or high leptin levels can lead to leptin resistance or immune issues.
Using short-term flexible intermittent fasting combined with resistance training remains one of the most effective and simple ways to lose weight and reduce your body fat.
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Brad Pilon is a nutrition professional with over eight years experience working in the nutritional supplement industry specializing in clinical research management and new product development. Brad has completed graduate studies in nutritional sciences specializing in the use of short term fasting for weight loss.
His trademarked book Eat Stop Eat has been featured on national television and helped thousands of men and women around the world lose fat without sacrificing the foods they love. For more information on Eat Stop Eat, visit www.eatstopeat.com
