The Unpopular Hormone: Leptin

Leptin is a hormone released by adipocytes in our adipose tissues. There is a direct relationship between body fat levels and leptin levels, the more body fat, the more leptin circulating in our bloodstream. The reason leptin scales with body fat are because leptin is produced in our fat cells. Leptin’s primary functions include: appetite control and metabolism.

The effects of leptin were found in a study in the 1950’s. In this study mutant obese mice were randomly created. These mutant mice were obese, diabetic, and showed reduced levels of activity, metabolism and body temperature. It wasn’t until 1994 that leptin was discovered through studying these mice. After leptin was found, the obese strain of mice was found to produce zero leptin. The obese mice were then given synthetic leptin and most of their problems (besides diabetes) were reversed.

Leptin was then claimed as the cure for obesity! Unfortunately humans and mice are not the same. As I stated at the beginning, high body fat levels result in high amounts of leptin. So obese humans have large amounts of leptin, humans with low body fat levels have smaller amounts of leptin. It has been hypothesized that obese humans may be leptin resistant. The body or brain isn’t receiving the proper signals from leptin to suppress our appetites despite the fact that leptin levels are high. Another theory, the insufficiency theory shows that perhaps the leptin levels in our bodies are high, but they are not reaching the brain properly. So even though we have plenty of leptin in our bodies, not enough of it is reaching the brain to signal the release, and synthesis of metabolic hormones.

Let’s give a brief overview of how leptin affects our brain to suppress appetite. Leptin is a regulatory hormone, so its levels will regulate the release of other hormones related to metabolism. Leptin counteracts neuropeptide Y and anandamide (two feeding stimulants). Leptin also promotes the synthesis of α-MSH, an appetite suppressant. Leptin also increases the quantity of various catecholamines like epinephrine (adrenaline) and norepinephrine (noradrenalin). These catecholamines burn fat by breaking fat down into fatty acids for use in our metabolism.

As noted before leptin concentrations are proportional to our body fat levels. Leptin levels are also acutely affected by our caloric intake. If we are in a caloric deficit, leptin levels will be lower than baseline levels. A caloric excess will cause an increase of leptin levels over baseline levels. I would assume the leptin would be increased to blunt appetite.

As you can tell, leptin has many important functions. It’s incredibly important when it comes to weight loss. As I stated, you don’t want your leptin levels too high, as that means you have some form of leptin resistance. You don’t want it too low either, otherwise hunger will increase and metabolism will decrease. Martin Berkhan states in this article that individuals who maintain low levels of bod fat percentage may be able to increase leptin sensitivity. Although there is no research supporting that claim, he has tons of experience in maintaining low levels of body fat successfully.

I’d like to use myself as an example now. I’ve been intermittent fasting for around 2 months now. I haven’t gotten my leptin levels checked, nor do I know my baseline levels. I can say though, that I rarely have a huge appetite despite eating 3 meals a day and fasting for 16 hours out of the day. You would assume with everything that I’ve talked about here that my leptin levels would be low and my appetite would be raised along with slower metabolism. This just isn’t the case as I’ve now dropped 15 lbs and am rarely hungry.

In the link above, Martin Berkhan talks about how IF’ing causes peaks and valleys in leptin concentrations in response to food intake. He uses this study as an example. The study showed that during a daytime fast during Ramadan, serum leptin levels remained neutral! So during fasting leptin levels drop and when you consume food they rise again. This is why Martin’s protocol calls for frequent refeeds to boost leptin levels.

Check out Martin Berkhan at and Lyle McDonald at for more information on leptin and fat loss in general.

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