For part II of this blog series, we are going to look closer at the intricate world of your body's metabolic dance. Imagine this dance as a well-choreographed performance where hormones like ghrelin, leptin, insulin, and incretins, along with the brain, lead the way. There is a delicate balance that keeps this dance in perfect harmony.
Before starting, understand that at its core, hunger is a primal survival mechanism deeply ingrained in the evolutionary history of our species.[1] The body is hard-wired to protect its energy stores, specifically, fat. To say it in a different say, the body has hard-wired mechanisms in place to ensure that the body's primary focus is to obtain energy when there are signals that suggest there is a need, to store energy for later use, and to resist weight loss.
Ghrelin: The Hunger Hormone
Ghrelin is a hormone produced in the stomach that signals hunger to the brain. It acts as a signal from the body to the brain indicating the need for nourishment and prompts the initiation of the appetite movements. It serves to ensure that energy intake is timely and harmonious. It is affectionately known as the "hunger hormone". I remember it like "GHRRRelin" = a "GRRROWling" tummy. An increase in ghrelin is triggered by an empty stomach, usually between meals, and tells the brain that it's time to eat. Other factors such as reduced calorie intake, lack of sleep, and certain stressors can also stimulate ghrelin production. These last points are important to remember when we think of obstacles in weight management. We'll come back to these in a later blog post.
How do we get our hunger hormone levels to drop? Ghrelin levels typically decrease after eating, especially in response to the intake of nutrients. Consuming a meal signals to the body that it has received sufficient nourishment leading to a reduction in ghrelin production. Carbohydrates send the quickest message to the brain to stop ghrelin release, but that message lasts for the shortest time. It takes proteins a little longer to tell the brain to stop releasing ghrelin but its message will last the longest. That is why you can be hungry again in a shorter time after a carbohydrate-heavy meal than after a protein-heavy meal. Additionally, increased stomach distension (a stretch in the stomach) after food intake, contributes to the decline in ghrelin levels. A nutrition and diet hack we often share is that high-fiber foods such as fruits, whole grains, and legumes are known to activate stretch receptors in the stomach leading to the decreased release of ghrelin from the stomach. Meals containing high-fiber foods tend to make you feel more full and satiated longer between meals than those that do not.
The Satiety Hormone, Leptin.
When you eat more calories than your body needs for energy, especially from foods high in fats and sugars, the extra calories are turned into fat and stored as energy ready to be released when needed. This process of storage, called lipogenesis, happens after a meal with a caloric surplus and contributes to the accumulation of body fat. Leptin is a hormone produced by these fat cells. In this context, you have to remember that we are talking about fat as energy. Fat = Energy.
Before a meal, when the body's energy stores are relatively lower (presumably the body's fat stores are also lower), the body's leptin levels are also lower. As you eat and accumulate energy, particularly by storing fat, leptin is released into the bloodstream. Leptin then travels to the brain, specifically the hypothalamus, where it signals that the body has enough energy stored, resulting in decreased appetite. It acts as a satiety hormone, helping regulate body weight by promoting a feeling of fullness and signaling to the brain that it's time to stop eating.
Leptin acts as the counterbalance to ghrelin. It gives us the harmonious transition from hunger to satiety. It signals to the brain when the body has enough stored energy, leading to a decrease in appetite and increase in energy expenditure.
Insulin. A Storage and Metabolism Hormone
Now, let's talk briefly about the role of insulin because we are going to talk about this a little more in a later blog. After a meal, insulin is released by the pancreas and plays a pivotal role in glucose (sugar) metabolism. It acts as a key, allowing the entry of glucose into cells (and out of the bloodstream) where it's either converted into energy or stored as glycogen or fat. This process helps regulate blood sugar levels, preventing hyperglycemia. Insulin is critical in energy balance and it is vital for metabolic health and preventing conditions like diabetes.
Incretins: Metabolic Regulators
Lastly, a quick word about the incretin hormones, GLP-1 and GIP. Incretin hormones are gut peptides that are secreted after nutrient intake and stimulate insulin secretion. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are the known incretin hormones secreted from the small intestine. They coordinate the dance of digestive harmony, enhancing insulin's performance and ensuring a balance between blood sugar control and appetite regulation. Specifically, these hormones help stimulate insulin release from the pancreas in response to food intake, which again, promotes effective glucose utilization. In addition, incretins have also been shown to work in the brain to reduce appetite and increase satiety. GLP-1 works in the stomach to slow down the emptying of food from the stomach which contributes to a sense of fullness and it also works in the liver to decrease glucose production and increase glucose storage to help maintain balanced blood sugar levels and manage hunger. Incretins play a pivotal role in this overall metabolic dance of signals and hormones, and in a later blog post, we will see how incretins have been a critical focus of the pharmaceutical industry and the new anti-obesity medications.
Putting it together. The Full Dance: A Mealtime Performance
It's 11:00 am and your stomach grumbles. Your ghrelin levels must be increased because it's been several hours since breakfast and you suddenly realize you are hungry and it's time to have lunch. As you sit down and eat, your body's hunger hormone, ghrelin, starts to decrease, signaling the satisfaction of your appetite. Simultaneously, your gut is releasing incretins like GLP-1 and GIP in response to food intake. These incretins not only stimulate insulin secretion but also slow down gastric emptying, prolonging the feeling of fullness.
As you continue to enjoy your meal, insulin levels rise, facilitated by the incretins, to help cells absorb the incoming glucose for energy or storage. This insulin surge also aids in suppressing ghrelin further, reinforcing satiety signals. Meanwhile, leptin, the long-term satiety hormone, plays a background role, modulating overall hunger and energy balance.
After finishing your meal, ghrelin remains suppressed, maintaining the feeling of fullness. Incretins continue to support insulin action, ensuring efficient glucose utilization and preventing blood sugar spikes. This coordinated effort of ghrelin, leptin, insulin, and incretins orchestrates a seamless transition from hunger to satiety, optimizing nutrient absorption and metabolic balance during and after the meal.
Conclusion: Harmony in Motion
In the mesmerizing choreography of your body's metabolic dance, each hormone plays a vital role, ensuring a delicate balance between hunger and satisfaction. The brain orchestrates this performance, allowing you to move through the rhythm of meals with grace and harmony however, an imbalance in this metabolic dance can throw things off-key. In the next blog post, we'll explore what happens when this harmonious symphony encounters disruptions and how it can impact your weight management journey.
Dr. Cardona is dual board certified in family medicine and obesity medicine and is the founder and physician owner of Cardona Direct Primary Care and RefineMD Aesthetics. Currently accepting new patients. (904) 551-4625. Visit www.cardonadpc.com/weightandwellness for information on our obesity management services.
Russell, Sharman Apt. Hunger: An unnatural history. Basic Books, 2006.
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