Leptin is a hormone produced by fat cells that signals the brain when the body is satiated with food. Ideally, leptin levels should be low when measured in the morning. Obese people almost always have high levels of the molecule, but their brains do not recognize its signal to feel full and stop eating.
This compound was originally discovered in highly obese mice. Study of their genetics found an obese gene, whose product did not function in these mice. Leptin was then identified as the compound that was missing in these mutants. When the mice were treated with this molecule, they lost weight. This finding provided the basis for a number of weight loss studies to see if treatment with this compound could help people to take off weight.
To provide sample to test for leptin levels, blood is drawn first thing in the morning while fasting. The blood can be stored safely for seven days if refrigerated or two months if frozen. Aside from being tested by medical doctors, blood can be sent to commercial companies to have this test performed.
Leptin levels are tested by a sensitive process called an ELISA test. Antibodies to the hormone are coupled with an enzyme that will change color if leptin is present in the blood being tested. It is possible to determine the precise amount of the compound being measured with this process. The leptin levels in women range from 3.9 to 30 ng/ml of blood, while those of men are lower at 0.5-12.7 ng/ml.
One of several things this hormone does in mammalian metabolism is to tell the brain that the body has eaten enough food. When tests show low leptin levels, it suggests the endocrine system is working properly or that appropriate amounts of this enzyme are being produced. High levels are an indication to seek advice from an endocrinologist.
When such tests are performed on obese people, they generally are found to produce a high amount of this hormone, even in early morning fasting tests. Such patients are thought to suffer from leptin resistance, in which their brains do not receive the signal that leptin levels are high. This is considered analogous to Type 2 diabetes patients who do not respond to the insulin that is present in their bodies. Some hugely obese people do not produce this hormone at all, and can benefit from injections of molecules produced by genetic engineering and do subsequently lose weight.
