Leptin
Leptin and ghrelin are two hormones that have been recognized to have a major influence on energy balance. Leptin is a mediator of long-term regulation of energy balance, suppressing food intake and thereby inducing weight loss. Ghrelin on the other hand is a fast-acting peptide/hormone, seemingly playing a role in hunger. As a growing number of people suffer from obesity, understanding the mechanisms by which various hormones and neurotransmitters have influence on energy balance has been a subject of intensive research. In obese individuals, the circulating level of leptin is increased, whereas surprisingly, the level of ghrelin is decreased. It is now established that obese individuals are leptin-resistant.
Leptin is a hormone made by fat cells which regulates the amount of fat stored in the body. It does this by adjusting both the sensation of hunger, and adjusting energy expenditures. Hunger is inhibited (satiety) when the amount of fat stored reaches a certain level. Leptin is then secreted and circulates through the body, eventually activating leptin receptors in the arcuate nucleus of the hypothalamus. Energy expenditure is increased both by the signal to the brain, and directly via leptin receptors on peripheral targets. The effect of leptin is opposite to that of ghrelin, the "hunger hormone." Ghrelin receptors are on the same brain cells as leptin receptors, so these cells receive competing satiety and hunger signals. Leptin and ghrelin, along with many other hormones, participate in the complex process of energy homeostasis. Everything we put into our body affects everything, directly or indirectly. Food is no exception. In fact, many studies suggest the effects of food are in many ways the same as the way a drug interacts with neurotransmitters and hormones. Understanding these two hormones in particular will bring a conscious awareness to what is going on hormonally in the body when we change our eating habits. Therefore, we can anticipate how we are going to feel, and also how long it's going to last and what action will change how we feel. Having this awareness and sense of control over the body gives another advantage to the dieting process, bringing known factors into play; our psychology actually shifts and makes the transition easier. Knowledge is power! Now, let's get into more detail about these two hormones.
Many instances are seen where Leptin dissociates from the strict role of communicating nutritional status between body and brain and no longer correlates with body fat levels, below is an overview of important pertinent information that will help you understand the biochemical significance of leptin:
Leptin level is decreased after fasting for 24-72 hours, even when changes in fat mass are not observed.
Leptin plays a critical role in the adaptive response to starvation.
In obese individuals with obstructive sleep apnea, leptin level is increased, but decreased after the administration of continuous positive airway pressure. In non-obese individuals, however, restful sleep (i.e., 8-12 hours of uninterrupted sleep) can increase leptin to normal levels.
Leptin level is increased by perceived emotional stress.
Leptin level is decreased by increases in testosterone levels and increased by increases in estrogen levels.
Leptin level is chronically reduced by physical exercise training.
Leptin level is increased by dexamethasone (a corticosteroid).
Leptin level is increased by insulin.
Leptin levels are paradoxically increased in obesity with an increase in leptin resistance.
Ghrelin
Ghrelin, also known as the "hunger hormone," is a peptide produced by ghrelin cells in the gastrointestinal tract which functions as a neuropeptide in the central nervous system. Beyond regulating hunger, ghrelin also plays a significant role in regulating the distribution and rate of use of energy.
When the stomach is empty, ghrelin is secreted. When the stomach is stretched, secretion stops. It acts on hypothalamic brain cells both to increase hunger, and to increase gastric acid secretion and gastrointestinal motility to prepare the body for food intake. Ghrelin also plays an important role in regulating reward perception in the ventral tegumental area of the brain (a site that plays a role in processing sexual desire and developing addictions) through its interaction with dopamine and acetylcholine.
Ghrelin is a participant in the complex process of energy homeostasis which adjusts both energy input (by adjusting hunger signals) and energy output (by adjusting the proportion of energy going to ATP production, fat storage, and short-term heat loss (all energy input is ultimately dissipated as heat)). The net result of these processes is reflected in body weight, and is under continuous monitoring and adjustment based on metabolic signals and needs, and at any given moment in time may be in equilibrium or disequilibrium.
Ghrelin and synthetic ghrelin mimetic (the growth hormone secretagogues) increase appetite and fat mass by triggering receptors in the arcuate nucleus of the hypothalamus, just like Leptin. Ghrelin-responsiveness of these neurons is both leptin- and insulin-sensitive. Ghrelin reduces the mechanosensitivity of gastric vagal afferent nerve impulses, so they are less sensitive to gastric distention. So, when Ghrelin is being secreted, you can eat a lot more and still not feel full, even though your stomach feels like it is ready to pop. This lack of sensitivity to gastric distention is especially pronounced when one is to supplement with a ghrelin mimetic (GHRP 6).
In addition to its function in energy homeostasis, ghrelin also activates the mesolimbic cholinergic-dopaminergic reward link, a circuit that communicates the hedonic and reinforcing aspects of natural rewards, such as food and addictive drugs, the response is usually unconscious and instinctual. Hypothalamic ghrelin signaling is required for reward from alcohol and palatable/rewarding foods.
So, to simplify this, leptin is responsible for feeling full when you eat food. It is associated with the ability to get to lower body fat levels and therefore somewhat responsible for the body's ability to burn fat, but more like a regulating system; it doesn't necessarily burn fat by itself. However, as you can see, there are some contradicting findings from research here, as the system is complex. I want you to understand that this hormone is responsible for feeling full, it directly affects other hormones and neurotransmitters, and obese people are leptin-resistant, much like how pre-diabetics are insulin resistant. That is why it is more difficult for an obese person to get lean that it is for someone at normal weight to lose weight. The sensitivity to leptin must be increased for the body to be capable of reading, controlling, and measuring the body fat levels to reduce them. Ghrelin is what you feel when you're hungry. It is directly related to the same receptors that “instant gratification” is related to; therefore, it has a probability of creating emotional instability and addictive behavior if the change is too much. Beyond that, this subject is debatable in many aspects.
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