Hormones are molecules that change how cells and the body function. Food intake and metabolism are controlled by numerous nerves and hormones. Insulin and leptin are examples of two hormones with profound effects on nutrient metabolism. Because they regulate various cellular processes, imbalances of these hormones are often associated with disease.
Proteins and Hormones
Both leptin and insulin are protein hormones. Proteins are relatively large molecules, made up of smaller molecules known as amino acids, and are synthesized by cells and coded for by DNA. Proteins have numerous functions in the body, such as providing strength, structure, forming channels for other molecules, as transport molecules or enzymes which speed up chemical reactions, or as hormones. Leptin and insulin are released into the blood and travel downstream throughout the body to signal their target cells. Once the hormones bind to receptors on the target cells, and the cells react according to the type of hormone and cell.
Function and Location
Insulin is released into the circulation from the pancreas in response to a meal. The meal raises the levels of the blood sugar glucose, derived from the breakdown of carbohydrates. Insulin signals cells throughout the body to absorb glucose, which they then use along with other nutrients and oxygen to create adenosine triphosphate, or ATP, the main energy carrier of the cell. Leptin is synthesized and released mainly by fat cells. It acts on receptors in a part of the brain known as the hypothalamus, and functions to decrease the appetite. Levels of leptin are proportional to the amount of fat in the body.
Effects
Insulin exerts a number of effects in addition to the cellular uptake of glucose. It also increases amino acid uptake by cells, increases DNA replication and protein synthesis. It causes the liver to store glucose as the molecule glycogen increases fat synthesis in fat cells, decreases the breakdown of fats and protein, and increases blood flow and stomach acid. In addition to decreasing the appetite, leptin may play a role in development. It is required for healthy male and female fertility, and changes the structure of bone by increasing the strength of bones.
Dysfunction
Leptin was originally thought to be a link between food intake, fat and energy use. These findings, however, were based on animal models. In humans, the picture becomes more complicated. Levels are effected not only by the level of fat in the body. They are also effected by exercise, stress, sleep deprivation and the sex steroids estrogen and testosterone. In contrast, Type I and II diabetes directly involve insulin. In Type I diabetes, the pancreas does not make insulin. In Type II diabetes, the body's cells fail to respond to insulin.
References
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci, et al.; 17th Ed 2008
- "Physiology"; Linda S.Costanzo; Fourth Ed 2008
Member Comments