Written in 1890 "Hunger" is a rather difficult novel by Norwegian Nobel laureate Knut Hamsun. The novel carries autobiographical overtones of a starving individual and sketches his physical and mental trauma. Hunger is an existential necessity contrived by evolution and compels organisms to seek food to survive. That physiologically complex parallel and interwoven pathways reinforce hunger underscores it significance. History is replete with instances when severe famine has driven societies to consume things they normally would have absolutely avoided.
Physiologically, hunger is an homeostatic signal for energy. Homeostasis is the process of maintaining a stable, balanced internal environment for a living system to function correctly. From single-celled organisms to complex multicellular beings like humans, all living systems require this internal balance for survival. Built-in system of checks and balances ensure stable internal conditions like temperature, blood pH, and glucose levels etc. Body achieves this through constant monitoring, corrective signals and prompt remedial actions. For example we feel hungry when energy level goes down, thirsty when hydromineral balance is disturbed, we shiver when body feels cold......Primitive man lived on a feast-fast cycle, alternating between massive gorging after a successful hunt followed by involuntary fasting during periods of food scarcity.
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At the biochemical level, homeostatic hunger and its alleviation is an intricate multi hormonal, multi enzymatic process with hypothalamus calling the crucial shots. In this multistarrer drama three hormones play vital roles: , the hunger hormone ghrelin and the satiety hormones GLP-1 and leptin. When energy level is low, ghrelin secreted by empty stomach talks to hypothalamus via the gut-brain axis and stimulates hunger. Though labelled hunger hormone, this small peptide of 28 aminoacids is a very busy molecule as it is associated with learning, memory, mood swings, sleep-wake cycle etc. With food intake ghrelin levels drop , glucose level in the blood goes up and GLP-1 secreted by the intestines instructs pancreas to release insulin. When sufficient fast is stored leptin, secreted by adipose tissues contacts hypothalamus to stimulate a feeling of fullness. There needs to be a fine balance between the hunger hormone and the satiety hormones. Any small error anywhere in this complex biochemical process can lead to a host of pathophysiological conditions including anorexia, hyperphagia, obesity or Type 2 diabetes mellitus etc. Metabolites released by gut microbes too influence appetite hormones, and communicate with the brain via the vagus nerve. During those times, when body needs energy and food is unavailable, body first exhausts the glucose stock and then easily switches to burning fat reserves.
Though we might still retain some elements of the feasting- fasting program within us, with easy availability of food we have settled for Breakfast-Lunch-Dinner routine whether hungry or not. We have also discovered the pleasure of eating and thus succumbed to hedonic hunger. Also called hyperphagia, hedonic hunger can be triggered by a variety of confusing and contradictory cues. For example we eat when we feel happy, we also eat to drown our sorrow. We eat when excited, also when bored. Some people eat when over stressed while others eat when relaxed. Even after a full meal, sensory stimuli such as sight, smell or just thought of tasty food can work up our appetite. Surprisingly the neurocircuits for homeostatic and hedonic hunger overlap and ghrelin is implicated in both. Ghrelin can influence brain's reward pathways, and spike the release of dopamine, the "feel good molecule". The hedonic urge is powerful enough to override the homeostatic signals of satiety. Precisely for this reason, ghrelin's role in addiction is under intense scrutiny.
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What happens if we don't have ghrelin ? Somewhere along the evolutionary pathway, Mother Nature seems to have had such a thought. A recent study published in Royal Society Open Biology Journal reports that 32 serpentine species either lack the gene for ghrelin or it is nonfunctional. Authors explain this as an evolutionary advantage bestowed on snakes to fast and survive in unfriendly terrains where prey availability is uncertain.
REFERENCES:
1. Hunger by Knut Hamsun Trans. George Egerton
3. Overlapping brain circuits for homeostatic and hedonic feeding
4. The role of ghrelin in addiction: a review
2. Loss of hunger hormone has made snakes masters of fasting
