Ghrelin and Its Role in Sports Nutrition - Part 1

Dear BLOG readers, dear PEAK customers,

The bodybuilding community is known for discussing all kinds of hormones and neurotransmitters. It’s interesting to explore what can significantly influence performance, muscle growth, or fat loss. You can find plenty of articles and forum posts about testosterone, insulin, growth hormone, adrenaline, and thyroid hormone.

One hormone, however, has been largely overlooked to date—at least, in my research, I haven’t yet found any relevant studies that have examined this thoroughly interesting hormone in connection with sports nutrition.

GHRELIN

Since ghrelin also plays a role in my HBN (Human-Based Nutrition) dietary system, I couldn’t resist taking a closer look at this previously little-known hormone and presenting the results to you in the following article. You’ll be amazed at the impact this hormone has and the conclusions that can be drawn regarding currently popular—and in some cases, established—dietary approaches.

Enjoy!

What Is Ghrelin?

The hormone ghrelin is referred to in English as “Growth Hormone-Releasing Peptide” and consists of 28 amino acids. It is initially synthesized in the form of a prohormone and later cleaved into an active peptide.

Where is ghrelin produced?

Ghrelin is produced in several tissues throughout our body. The majority of ghrelin-producing cells are located in the gastric mucosa. (Immunoreactive) ghrelin cells are also found in the ileum, duodenum, jejunum, and cecum. Additional ghrelin cells have been detected in the pancreas, the kidneys, the placenta, the Leydig and Sertoli cells of the testes, and the cardiomyocytes of the heart muscle. Although ghrelin production in the brain plays a relatively minor role, ghrelin-producing cells have also been detected in the hypothalamus.

Ghrelin-producing cells are classified into two types, depending on which signals they respond to by synthesizing ghrelin.

Conclusion:

Ghrelin is produced in numerous structures throughout our body. It can therefore be assumed that it influences a wide range of systems and processes in our body.

What does ghrelin do?

Effect on Hormones

Growth Hormone (GH)

As a peptide hormone, ghrelin primarily acts on the pituitary gland, where it stimulates the release of GH via its own signaling pathway using so-called GHS receptors. Ghrelin binds to these receptors, thereby triggering the release of GH. Ghrelin is transported to these GHS receptors via the bloodstream.

This type of GH synthesis occurs independently of GHRH-mediated synthesis.

Illustration: Growth Hormone (GH)

Other Hormones

Under the influence of ghrelin, levels of ACTH, cortisol, aldosterone, epinephrine, and, in certain cases, prolactin increase.

Within physiological ranges, ghrelin does not affect thyroid hormone levels.

Regarding testosterone, studies have shown a decrease in LH and testosterone levels approximately 1 hour after exogenous administration of ghrelin to young male subjects. While this demonstrates an effect, it is not significant within the range of physiological ghrelin levels.

Conclusion:

Ghrelin stimulates growth hormone production but also affects other hormones.

Assessment:

The effect of ghrelin on growth hormone levels should not be overestimated. Despite generally lower ghrelin concentrations at night, that is precisely when growth hormone production peaks!

Appetite Stimulant

Ghrelin promotes food intake by influencing the nervous system and by stimulating the cholinergic-dopaminergic reward system. This leads to increased release of neuropeptide Y and a protein called “agouti-related,” both of which have a strong appetite-stimulating effect.

Ghrelin acts as an antagonist (opposite) to leptin, which tends to suppress feelings of hunger. The two hormones complement each other through their opposing effects and jointly regulate energy balance.

Another hormone called obestatin has been identified as an antagonist of ghrelin, but according to initial studies, it appears to have only a minor influence on body weight development.

Illustration: Ghrelin and Food Intake

Conclusion:

Ghrelin serves the evolutionary purpose of survival by prompting food intake.

Interesting fact:

Studies of patients with nighttime food cravings have revealed elevated ghrelin levels.

Glucose Metabolism

Ghrelin modulates the sensitivity of glucose-sensitive neurons in the central nervous system, influences insulin secretion and hepatic glucose production, and inhibits the activity of glucose-sensitive neurons in the brain. Endogenous glucose production is reduced through a decrease in adiponectin synthesis. 

Adinopectin increases cellular insulin sensitivity, which is consequently reduced under the influence of ghrelin.

Ghrelin thus mediates hyperglycemic effects (increases blood glucose levels) through endocrine pathways, but also via a direct effect on hepatocytes, which stimulate glycogen synthesis, gluconeogenesis, and glucose production in the liver.

Conclusion

Ghrelin raises blood glucose levels and reduces insulin sensitivity.

Assessment:

This makes sense, because at times when our body is craving food, it does not want to replenish muscle or liver glycogen, but rather ensure that there is sufficient glucose in the blood to supply glucose-dependent systems.

Fat Metabolism

Ghrelin influences fat metabolism through its effects on skeletal muscle, adipose tissue, and the liver.

Liver

In the liver, it promotes the formation of new fat tissue (lipogenesis) and triglycerides (the stored form of body fat). Thus, under the influence of ghrelin, there is an increased risk of developing fatty liver disease.

Adipose Tissue

Under the influence of ghrelin, the amount of AMPK—a fat-reducing protein—decreases, shifting fat metabolism toward an anabolic state. Lipogenesis (the formation of new adipocytes) is stimulated through an increase in insulin-dependent glucose uptake.

Skeletal muscle

In skeletal muscle, ghrelin increases mitochondrial enzyme capacity, leading to a reduction in triglyceride levels and simultaneous storage in the liver—a process that, in turn, contributes to the risk of developing fatty liver disease.

Conclusion:

Ghrelin inhibits lipolysis and promotes lipogenesis.

Immunomodulation

It is now known that ghrelin also influences the immune system. Among other things, it acts as a regulator of pro-inflammatory cytokine release, thereby exerting a positive immunomodulatory effect. In vitro, administration of ghrelin was found to reduce reactive oxygen species (free radicals).

Conclusion:

Ghrelin has a positive effect on the immune system.

Interesting fact:

Ghrelin’s regulatory influence also extends to cell proliferation and differentiation, as well as to programmed cell death (apoptosis).

Stomach

Studies on the effect of ghrelin on gastric acid secretion and gastric motility yield anything but clear-cut results. Some suggest a stimulating effect, others an inhibitory effect, and still others no effect at all.

The general assumption is that ghrelin has a stimulating effect on gastric acid secretion and gastric motility to prepare the stomach for food, which in turn leads to insulin secretion from the pancreas.

Despite this assumption and an increase in insulin levels, the hyperglycemic effect of ghrelin has also been documented and is sometimes attributed to a reduction in insulin sensitivity.

Conclusion:

Ghrelin likely increases gastric motility and gastric acid secretion and raises insulin levels.

Heart

Studies suggest that ghrelin increases cardiac output. It has been shown to promote myocardial contractility, protect against heart failure caused by a heart attack in the heart muscle, and act as a vasodilator by improving the bioavailability of NO (nitric oxide).

Conclusion:

Ghrelin has a positive effect on our heart and can lower blood pressure.

5 more……

1. Ghrelin is involved in the formation of osteoblasts (bone-forming cells) and thus promotes bone metabolism.
2. Ghrelin has an inhibitory effect on the sympathetic nervous system.
3. Animal studies show a link between ghrelin levels and alcohol consumption. Higher ghrelin levels led to increased alcohol intake compared to control animals. Blocked ghrelin receptors resulted in significantly reduced alcohol intake compared to control animals. This is due to ghrelin’s stimulating effect on the cholinergic-dopaminergic reward center, which also increases cravings for addictive substances such as ethanol.
4. High ghrelin concentrations presumably enhance memory performance and promote learning ability. Apparently, ghrelin can improve communication between nerve cells in the brain. Consequently, the best time to learn would be during the day and on an empty stomach.
5. Ghrelin has been identified as a sleep-promoting substance.

Summary

Ghrelin is a hormone that does much more than simply stimulate appetite. It is produced in countless locations and therefore influences a wide range of processes in our body, as shown in the accompanying diagram:

Illustration: Effects of Ghrelin

Of particular interest to bodybuilders is that ghrelin inhibits lipolysis, promotes the formation of new fat cells, raises blood sugar levels, reduces insulin sensitivity, and generally makes you feel hungry.

If you ask me, these aren’t really properties that we as bodybuilders value.

Part 2 will cover the natural production of ghrelin, ways to influence it, some additional connections, and finally, an evaluation within the context of sports nutrition.

Until then, best regards

Best regards

Yours,

Holger Gugg

www.body-coaches.de