Although it is somewhat complicated, it is fascinating to understand how our body can tell when we are hungry and when we are full. There are two hormones that dictates our hunger and satiety. They are Ghrelin and Leptin.
Ghrelin is known as the "hunger hormone" which is produced by gastrointestinal tract. As the meal time gets closer blood ghrelin level increase and suggest us that it is time to eat.
Leptin is the opposite from ghrelin and it is hunger inhibitor or "satiety hormone" which is produced by adipocyte or fat cells and small intestine. Leptin seems to take about 15-20 minutes to take an affect. This is a reason why we should eat slow for leptin to signal us that we had enough food rather than eating until we are stuffed which will be too much.
More than Hunger Hormone
Ghrelin as mentioned earlier is known as the "hunger hormone". However, ghrelin's role is much more than that.
One of the role is ghrelin regulates glucose homeostasis by inhibiting insulin secretion. Which means, when our glucose level decreases, ghrelin seems to signal pancreas to slow down or stop secreting insulin to avoid hypoglycemia or low blood sugar.
Ghrelin also seem to have impact on energy homeostasis meaning it may regulate energy intake and usage. In a mice study, using ghrelin promoted increase in fasted energy expenditure along with decrease cholesterol, triglycerides and improved insulin sensitivity(1).
It is possible that during fasted state, ghrelin can signal body to start using fat as energy source, which makes sense because during fasted state our resting metabolic rate higher than when we are consuming food. This will also shows that ghrelin can indirectly create heat (thermogenesis) in a process of increase in fasted energy expenditure. This is most likely due to heat produced while metabolizing fat into energy source called ATP (adenosine tri-phosphate).
Ghrelin can also indirectly increase muscle mass by increasing food intake, regulate bone formation, and cardio-protective affect. When you look at it, being hungry is not a death sentence. It is ok to feel hungry and not eat for a while.
Satiety Hormone
Leptin is a hormone for us to know when we had enough food and stop eating. Without this hormone it is difficult to know when we are actually full and may end up eating more than we should. It is also a key factor to understanding obesity pandemic across the globe.
Leptin in small increase can suppress appetite and lead to decrease in body weight, however, in obese population this affect does not seem to take place. There is a condition where leptin's ability to suppress hunger or increase in body's energy use is decreased called leptin resistance. Main symptoms of leptin resistance are consistently hungry and increase in food intake despite adequate or excess amount of body fat (2). Possible reason is transportation of leptin from blood to brain across blood brain barrier. Since leptin is secreted in adipocytes (fat cells), obese individual may secretes excessive amount of leptin due to excess amount of fat, which may cause to decrease the permeability at the blood brain barrier, which leads to not able to signal satiety.
There is a functional and anatomical relationship between fat cells and immune cells. This also leads to assumption of leptin may have affect on immune system. Leptin is a proinflammatory, which means excess amount remain in the blood may cause inflammation and affect our immune system.
Appetite Signals Are Important
It is important to understand why we have both hunger and satiety signals, and it is also important to understand what they mean.
There has been several clients of mine who came to discuss about they're not getting hungry. Ghrelin being secreted in gastrointestinal tract, the person may have digestive issue and may not be able to secrete ghrelin properly. Solution that was discussed was consuming water soluble fiber, such as oatmeal, apples, beans, to help rejuvenate gut bacteria along with replenish gut bacteria colony with probiotic and prebiotics and seems to be working well.
From weight management perspective, you should learn to regulate ghrelin frequency to your dietary needs, however, if you have tendency to undereat, you shouldn't ignore your hunger signal and may need to force to eat even if you are not sensing hunger to avoid health complication.
Leptin or satiety signals are also important. When you are getting full, it is important to place your fork down and stop eating. I do understand the feeling of wanting to finish everything on the plate, however, to avoid obesity and other health complications it is important to learn to save it for next meal. Since leptin is secreted by adipocyte or fat cells and small intestine, it is ideal to maintain body fat under control and improve your digestive health.
Kota Shimada
Reference:
(1) Pradhan, G., Samson, S. L., & Sun, Y. (2013, November). Ghrelin: Much more than a hunger hormone. Current opinion in clinical nutrition and metabolic care. Retrieved November 11, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4049314/
(2) Gruzdeva, O., Borodkina, D., Uchasova, E., Dyleva, Y., & Barbarash, O. (2019, January 25). Leptin resistance: Underlying mechanisms and diagnosis. Diabetes, metabolic syndrome and obesity : targets and therapy. Retrieved November 11, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6354688/
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