Role in Weight Control For The Hormone of Darkness
Melatonin is a simple molecule derived from the amino acid tryptophan. It is produced by all living organisms. In the human beings, it is secreted from the pineal gland which is a small pea sized gland located in the center of the brain. This gland is outside the blood-brain barrier and therefore melatonin is able to enter the circulation easily. Melatonin is a hormone and the target cells are located in the brain and since it is a hormone, its biological actions are felt only when melatonin binds to specific receptors. Melatonin is also released from the gastrointestinal tract, retina, lymphocytes, bone marrow cells and epithelial cells. But, the melatonin from these cells acts as a paracrine hormone (acting on nearby cells) and has no effect on the brain.
The release of melatonin is photoperiodic in nature. The presence of light inhibits the secretion of melatonin. The release of melatonin starts in the evening and reaches a peak at midnight and thereafter, it falls. Therefore, it has also been called ‘the hormone of darkness’. This secretion of melatonin at night has been observed in both diurnal and nocturnal animals. This hormone is concerned with the regulation of the sleep cycle and has been therapeutically used to induce sleep.
Melatonin is a powerful antioxidant and is therefore, is capable of removing the harmful free radicals produced in the body. Many of the antioxidants like vitamin E do not undergo permanent chemical transformation and are regenerated through repeated cycles of oxidation followed by reduction. However, melatonin is not regenerated after its antioxidant activity and therefore, has been called suicidal antioxidant.
Melatonin may also be concerned regulation of body weight. Studies have indicated that melatonin can prevent weight gain. It is known that as people age they tend to put on weight. One of the reasons for this gain in weight is that metabolism slows down with ageing. It has also been shown that as people age, the melatonin levels drop. Thus, there is a correlation between melatonin levels and body weight. Animal studies have indicated that rats with a mutation in the gene responsible for the production of leptin receptors, show insulin resistance, increased cholesterol levels, weight gain and hypertension. Leptin is the hormone which is responsible for providing the ‘stop eating’ signal at the hypothalamus of the brain. When these rats were treated with melatonin, there was reduction in body weight, LDL cholesterol and triglycerides. At the same time, the HDL cholesterol levels increased.
There are people with the habit of staying up late. This may be due to work pressure or, it may be for watching TV. Another study has shown that people who sleep late tend to put on weight. Such people tend to consume more of fast foods and less of fruits and vegetables as compared to people who go to bed at the normal time. They also like to drink calorie rich beverages. When people stay awake for lengthy periods, they do so in the presence of light. Light inhibits melatonin secretion. Thus, when melatonin secretion is decreased, there is a tendency to eat more, leading to gain in body weight.
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