Discovered in 1994, I think it's likely we do not fully understand the complexities of leptin and other metabolic hormones. But what is understood so far is the following:
Leptin is a hormone produced by the fat cells. It is present in the blood stream and its level is in direct proportion to the amount of body fat a person has. i.e. the more fat, the more leptin is produced.
Leptin's primary function is to act on the hypothalamus, the part of the brain that controls appetite and hunger levels. It tells the hypothalamus to reduce appetite (because fat stores are high), which results in decreased food intake.
On the other hand, when fat stores are low, for example, after dieting, leptin levels are reduced. This causes the hypothalamus to increase appetite. An increased appetite obviously results in greater food intake and a corresponding increase in body fat stores. More body fat results in more leptin being produced, which then tells the hypothalamus to decrease appetite, leading to a reduced food intake. This is a classic negative feedback mechanism. In theory, it should prevent us from becoming obese.
But, leptin may be one of the main hormones responsible for rebound weight gain after a diet.
Leptin is accurately described as an anti-starvation hormone because low levels lead to increased hunger. In the past it was described as an anti-obesity hormone but researchers have since discovered that obese people (who produce large amounts of leptin) are resistant to its action. This is similar becoming resistant to insulin - it's bad.
It is possible that leptin resistance may result from over-eating. When someone over-eats as a result of emotional eating or out of habit, the receptors in the hypothalamus become de-sensitised to the action of leptin. This means that the hypothalamus can't detect when leptin levels are high, so you get hungry resulting in food cravings and further weight gain. Over time obesity results.
It is also theorised that leptin may have trouble crossing the blood-brain barrier in obese people, which means it can't then stimulate the receptors in the hypothalamus. Leptin's reduced ability to cross the blood-brain barrier is thought to be due to the fact that obese people have a cerebrospinal fluid (CSF) to plasma ratio much lower than non-obese individuals.
It is also possible that poor sleeping habits may exacerbate leptin resistance because the sleep hormone, melatonin, appears to have a close relationship with leptin.
Interestingly, many overweight individuals suffer from sleep apnea, a sleep disorder caused by the epiglottis closing over the airways during sleep. Since this may cause them to wake up regularly during the night, melatonin production may become impaired, which then impacts on leptin as well.
With regards to leptin, we should aim to increase its production as much as possible without gaining fat and also try and make the hypothalamus as sensitive as possible to it.
One of the best ways to increase leptin production without gaining body fat is to have a Treat Day once a week. This will be particularly beneficial in preventing the common rebound weight gain that follows a diet.
On the Treat Day people should only have 1 or 2 meals where they eat anything they want and as much as they want (it doesn't mean you can eat anything you want all day long!). This increased calorie intake will prompt the adipocytes (fat cells) to produce more leptin, which then tells the hypothalamus to reduce your appetite.
In order to prevent or overcome leptin resistance the following strategies should be employed:
- Avoid excess sugar and bad fats.
- Perform regular, daily exercise.
- Improve sleeping habits.
- Use the following supplements: melatonin, L-carnitine, CLA and omega-3 fatty acids, but all of these are available from meat, poultry, fish and dairy, with the exception of melatonin, where supplements are necessary to raise the level in the bloodstream.