Ghrelin

Ghrelin is a hormone that stimulates appetite by using signals from the gastrointestinal tract to the brain.

For a more in-depth review, see the complete guide to proper nutrition — a fundamental guide from the expert team at Sport Zona.

What it is

Ghrelin is a peptide hormone discovered in 1999, which plays a key role in the regulation of appetite, energy balance, and body weight. Its main site of synthesis is the enteroendocrine cells (specifically P/D1 cells) in the stomach lining, but it is also produced in smaller amounts in the intestines, pancreas, kidneys, and hypothalamus. It is the only known circulating hormone that exhibits an orexigenic effect, meaning it stimulates the feeling of hunger.

The hormone exists in two main forms: acylated ghrelin (the active form) and des-acylated ghrelin (the inactive form). Acylation, a process where a lipid residue is attached to the molecule, is essential for its biological activity and its ability to bind to receptors. The ratio between these two forms can vary depending on the physiological state.

Known as the "hunger hormone," ghrelin signals the brain about the need for food intake. Its blood levels typically rise before meals and decrease significantly after food intake, making it an important player in the short-term and long-term regulation of energy homeostasis.

How it works

Ghrelin's action occurs by binding to specific ghrelin receptors, known as growth hormone secretagogue receptors (GHSR-1a), which are widely distributed in various areas of the body, including the hypothalamus, pituitary gland, brainstem, pancreas, and adrenal gland. In the hypothalamus, the main center for appetite control, ghrelin activates neurons that produce neuropeptide Y (NPY) and Agouti-related peptide (AgRP), which are potent appetite stimulants.

Ghrelin's mechanism of action includes directly stimulating appetite, as well as influencing metabolism and energy expenditure. Ghrelin has been found to:

• Increase food intake and fat accumulation.
• Stimulate growth hormone secretion.
• Influence glucose metabolism and insulin secretion.
• Affect the functions of the digestive system, including stomach motility.
• Participate in modulating the brain's reward system related to food.

Thus, ghrelin coordinates nutritional behavior with the body's physiological needs.

Why it is important for athletes

For athletes, understanding ghrelin is crucial for optimizing their nutritional regimen, controlling weight, and improving recovery. In athletes, especially those maintaining a low body fat percentage or in periods of energy deficit for weight reduction, ghrelin levels can be chronically elevated. This leads to constant hunger, making it difficult to adhere to a diet and potentially leading to dehydration or reduced immune function due to inadequate caloric intake.

Studies show that intense training can temporarily suppress ghrelin secretion, but chronic energy deficit leads to an adaptive increase in its levels. To effectively manage hunger, athletes should aim for a balanced nutritional intake, including sufficient protein (recommendations between 1.6-2.2 g/kg body weight) and fiber, which are proven to aid in satiety and suppress ghrelin. Distributing food into more frequent, smaller meals can also help stabilize ghrelin levels and prevent hunger spikes. Maintaining adequate sleep and avoiding chronic stress also have a positive impact on hormonal balance, including ghrelin.

Related concepts

• Leptin — An antagonist to ghrelin, which signals satiety and long-term energy status.
• Energy balance — Represents the relationship between calories consumed and expended, which ghrelin directly influences by stimulating food intake.
• Cortisol and nutrition — High levels of cortisol (the stress hormone) can affect ghrelin levels, exacerbating hunger and eating behavior.
• Insulin sensitivity — Ghrelin can interact with insulin and affect glucose metabolism, which is important for insulin sensitivity.