We all feel it. That sensation of emptiness followed by an ache, an urge to eat, to supply the body with food and the nutrients within. It is innate, uncontrollable and likened to being ravenous or starving, but where does hunger* actually come from? And more importantly, what is our body trying to tell us when we feel the daily ache/urge/need/emptiness?
*Please note that hunger can also be a term used to describe the 795 million people around the world that do not have enough to eat on the daily basis. Knowing this and wanting to be respectful, this article is simple about the mechanism behind the feeling of hunger, not a situation that society typically denotes as hunger or rather World Hunger. Please do read more about World Hunger, as it is a topic definitely worth your time. Check out The Hunger Project and The World Food Project!
For now, let us take a look at where the feeling of hunger comes from and how we can all get back in touch with such cues.
The evolution of hunger
Hunger is an innate response that is intertwined and incorporated into everyone’s biology. Just like we all get that ‘gut feeling’ when our subconscious is trying to convey something to us, hunger is an innate behaviour built from within. A mechanism that is moulded by hormonal pathways in our bodies that move throughout the bloodstream from the stomach to the hypothalamus in a complex negative feedback mechanism.
I could have described the evolutionary theories that try to recreate the path Homo Sapiens have forged over the past millions of years, but really that would just be my inner Zoology degree trying to break free but really hunger is a physiological response from within calling for brain fuel.
To be more specific, a 2014 review study examined the origin of grehlin receptors in not only mammalians but also in vertebrates. Interestingly, grehlin receptors are found in both types of animal, concluding the origins of hunger or rather one of the hunger hormones is so deep within our biology it is without a reason of a doubt, innate.
There are two major hormones responsible for controlling hunger – leptin and ghrelin. Let us take a look at how this mechanism works and the role each hormone plays in hunger.
Leptin is a hormone produced by fat cells as well as the heart, skeletal muscle and stomach, with the main responsibility to ensure we maintain a balance when it comes to calorie intake. The prevention of under eating and notably over eating is at the core of leptin. It acts on the hypothalamus to control how much we eat. Studies show that without leptin abnormal energy homoeostasis results, meaning the body is unable to elicit the correct signals to prevent both over and under consumption. There is an inverse relationship (negative feedback loop) that exists between the number of fat cells and the action our brain takes to initiate the hunger cues. An increase in fat cells produces more leptin. This signals to the brain that we have had enough to eat and to stop eating.
Interestingly, there is an inverse relationship (negative feedback loop) that exists between the number of fat cells and the action our brain takes to initiate the hunger cues. An increase in fat cells produces more leptin. This signals to the brain that we have had enough to eat and to stop eating.
You might by now thinking that the more body (i.e obesity), the less leptin and therefore the less we eat – but this is not always the case due to the phenomenon called leptin resistance. With so much leptin circulating out bodies, the brain becomes immune to the trigger and in these cases, leptin does not stop a person from eating. This has a complete adverse reaction on the body, as we gain more weight and leptin resistance increases, the brain tells us to eat more and expend less energy as a means to conserve body fat for the times of famine and starvation (that innate reasoning again!).
However, when people diet and lose fat, the leptin levels decrease… yay! Well actually, no. The brain thinks the body is in starvation mode and this results in a lack of motivation to exercise and those pesky cravings become more and more apparent. People often fail such diets because of many reasons (one of them is leptin, more specifically leptin resistance) and often gain their entry ticket into the yo-yo dieting show.
Ghrelin is secreted primarily from the mucosa of the stomach lining and about a third from the small intestine. Its main responsibly is to promote hunger. Funnily enough, ghre- is a root word from indo-European meaning growth, one of the other functions of ghrelin, as well as energy balance and GI motility. Considering, the neuroendocrine nature of this hormone, the stomach today is more seen to have an endocrine function rather than simply a mechanical digestive role. When ghrelin is secreted from the stomach, it acts on the hypothalamus to induce appetite plus speeds up the release of gastric secretions and small intestinal contraction. (2004) Ghrelin, appetite, and gastric motility: the emerging role of the stomach as an endocrine organ. FASEB J. 18 (3) 439-456
Even though ghrelin was discovered seven years post leptin, ghrelin has been gaining more and more attention in previous years as hunger is not the only ‘condition’ ghrelin is responsible for.Leite-Moreira & Soares (2007) Physiological, pathological and potentially therapeutic roles of ghrelin. 12 (7-8) The express, excretion and acceptance of ghrelin by receptors in the body have been shown to play a role in many diseases including cachexia, anorexia, obesity, diabetes, polycystic ovary syndrome and gastroparesis (just to name a few). Leite-Moreira & Soares (2007) Physiological, pathological and potentially therapeutic roles of ghrelin. 12 (7-8)
Unleash your hunger cues
It is not simply just about the stomach but there is an ever growing connectedness between the gut and one’s mind.
Interestingly, there are external and internal cues to eat. The internal involves the action of the hypothalamus whereas external cues include stress, smell or the sight of food. Many of which trigger a memory associated with the smell, taste or look of food. Albeit, today most people rely on external cues to guide their eating behaviours rather than the trusty hypothalamus. Simply getting back in touch with your internal motivations to eat can really assist with over-eating prevention.
References [ + ]
|1.||⇪||(2004) Ghrelin, appetite, and gastric motility: the emerging role of the stomach as an endocrine organ. FASEB J. 18 (3) 439-456|
|2.||⇪ab||Leite-Moreira & Soares (2007) Physiological, pathological and potentially therapeutic roles of ghrelin. 12 (7-8)|