It can be very difficult to contain the feeling of hunger — a feeling one is all too familiar with; the low stomach growls, hunger pains, possibly even feeling light headed. It makes it a compulsion, an instinct; it can even be a habit, but at it’s very core, it’s very simply a biochemical bodily reaction to the lack of food. It’s hardly as complicated as it sounds when it is broken down.
First of all, why does one get hungry? Why is there such a strong urge that compels us to take in food?
It all stems from the human instinct to survive, to survive and reproduce and pass on genes. It sounds like an oversimplified reason for being alive, but biologically that’s all humans are. So, from the perspective of this evolutionary need to survive, it becomes clear why such a feeling of hunger would have come to be. It turns out there is a mechanism which gives rise to this feeling, which is the biochemical bodily reaction mechanism mentioned earlier.
Every organism is constantly being regulated by chemicals within it, to help function and maintain a ‘balance’. This maintenance of balance, in the biological world, is called homeostasis, which directly translates to maintaining a sort of equilibrium.
This maintenance of balance is often controlled by molecules called hormones. They are secreted directly into the bloodstream and then carried out to tissues and organs to function. They sort of act like messengers, attempting to circulate information throughout the body.
Several studies show that a specific hormone, ghrelin, is responsible for causing hunger symptoms. It is shown to rise just before a meal, or when an individual is fasting. And as expected, it is shown to drop after a meal. This drop is a result of a surge in another hormone; leptin. An increase in leptin concentration in the bloodstream is shown to directly reduce the concentration of ghrelin in the bloodstream.
To better understand this balance; picture this hypothetical:
An individual has just woken up. They have likely not eaten for up to nine hours, and the body was in a fast. This has caused the release of ghrelin from the cells lining the stomach and pancreas (called Epsilon cells) into the bloodstream, and they have an increased appetite. Assuming the individual chooses to respond to the body’s signals, they eat. The stomach walls start to distend as food fills it and the individual begins to move toward satiety, causing the release of the hormone leptin (which is released by fat cells) into the bloodstream. In turn, the individual stops eating, as they are now satisfied or full.
The actual pathway that occurs within the organism involves a few more steps, involving a few more hormones and other biological chemical messengers.
Therefore, it is fundamentally clear that these two hormones are somehow responsible in controlling our appetites as is necessary.
Hunger is characterized by tell-tale signs like hunger pains or a growl of the stomach as mentioned in the beginning. Rhythmic expansion and contraction of the muscles of the stomach and small intestine accounts for the low ‘growl’. This muscular activity (called peristalsis) occurs due to activation of the muscles during periods of hunger.
For an empty stomach, ‘hunger contractions’ occur when the receptors lining the inside of the stomach detect the absence of food. They then cause a wave of electrical signalling triggering the hunger contractions which may last for 10–20 minutes. The signals, and hence the hunger contractions, subside after feeding.
The hormones, especially ghrelin, also respond to habit. In a study done at the Medical University of Vienna, subjects participated in an experiment involving a thirty hour fast. Their ghrelin levels did not progressively increase as fasting time was increased. They only rose during the subject’s usual meal times and fell back down even though they didn’t eat anything; an indication that ghrelin levels rose when the body expected food.
Hunger pains may also be felt which is a consequence of the expectation of food on an empty stomach; the brain triggers the release of ghrelin. Ghrelin signals the body to release stomach acids to digest food. If food is not consumed, the stomach acids begin to attack the lining of the stomach, causing hunger pains.
Interestingly, different foods can stimulate or suppress the release of ghrelin too. In a study, it was concluded that dietary fibres (or soluble fibres) like oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables showed significant rise in ghrelin levels of subjects. In other words, these dietary fibres increase appetite.
Another study showed that caffeinated coffee (with simultaneous sleep deprivation) reduced levels of ghrelin in the blood, and hence reduced appetite. Carbohydrate-rich foods have been shown to reduce ghrelin levels as well.
Several studies have also shown that ghrelin levels are inversely proportional to the body mass index of an individual. This means that people who have a high body mass index have lower hunger levels and vice versa. Similarly for leptin. The greater the ratio of fat cells, the higher the levels of leptin, causing the individual to have a suppressed appetite. This is the body’s way of regulating weight.
This may be confusing, and the question of why obesity exists is very relevant.
The answer lies in the sensitivity of the brain to ghrelin and leptin. Research has shown that many obese people have an overly sensitive receptor to ghrelin, causing increased caloric intake.
Similarly, there have been instances of mutations (genetic changes) of the hormone, leptin, which also causes increased caloric intake in an attempt to “feel full”, since the receptor for leptin is not sensitive enough. However, studies have also shown that possibly, excessive overeating can make an individual less sensitive to leptin.
Overall, several studies and research are still being done in many areas of hunger and nutrition. Food has cultural, sociological, psychological, physiological (the list goes on) factors involved. However, we do know that underlying these processes; the compulsion, instinct and habit, is caused by the hunger hormone, ghrelin.
Written by Siddharth Murali and Deesha
