Leptin is a polypeptide hormone secreted by the white fat cells. It is a key hormone involved in appetite regulation. Although it is secreted from several tissues, the major site is the adipocyte (white fat cell). The name leptin is derived from the Greek word ‘leptos’ which stands for ‘thin’. Leptin is a hormone which profoundly affects many aspects of energy metabolism and weight control in the body. The major site of action for this hormone is the hypothalamic region of the brain. The hypothalamus is the seat of the satiety center which ultimately decides when one should stop eating.
There are two types of hormones which can interact at the satiety center. One type of hormones stimulates the satiety center while the other type inhibits. Ghrelin and neuropeptide Y are examples for the former while leptin and α- MSH are examples for the latter. Both these actions have specific motives or, purposes behind them. When somebody is fasting or, consuming less amount of food, the hormone ghrelin is released from the stomach region and it stimulates the appetite center to promote eating. In fact, ghrelin has been called the hunger hormone. Under these circumstances, the circulating levels of leptin fall.
When enough food has been consumed, leptin provides the ‘stop eating’ signal. This is basically achieved by displacing the appetite stimulating neuropeptide Y from the hypothalamus. Leptin also increases the secretion of α- MSH from the pituitary gland and this hormone acts to suppress the appetite. Obesity is assuming epidemic proportions worldwide. While altered lifestyles and dietary habits are the major contributors, there are people who are obese due to changes in the levels of these appetite regulating hormones. For example, excess production of ghrelin or, under production of leptin may lead to obesity.
Insulin is a hormone which is produced and released from the pancreas. This hormone is concerned with the oxidation of sugar molecules. One of the actions of leptin is to promote the actions of insulin. Thus, insulin is able to function better in the presence of adequate quantities of leptin. Leptin also promotes the burning of fat in the body. Chronic insomnia has several deleterious consequences. One of the outcomes of sleep deprivation is the fall in the blood levels of leptin and this, in turn, can lead to the onset of obesity. Sleep deprivation also leads to an increase in the levels of the hunger hormone ghrelin. Thus, a fall in leptin and an increase in ghrelin together contribute to the obesity seen in chronic insomnia.
In obese individuals, there is a decline in the entry of leptin into the brain. This, somehow, affects the longterm memory. Obesity also leads to decreased sensitivity to leptin. This has been called leptin resistance. Fall in leptin levels is also associated with decline in mental abilities. Decreased leptin may promote dementia. Inactivity leads to obesity which produces leptin resistance. On the other hand, increased physical activity promotes leptin action. Even walking for a few minutes may help to increase the sensitivity to leptin.