Are you hungry?

Written in 1890 "Hunger"  is a rather difficult  novel by the 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 living organisms to seek food to survive. That physiologically several  complex,  parallel and interwoven  pathways  reinforce hunger, underscores its significance.    History is replete with instances when  severe famine has driven  people   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  internal balance for survival.    Built-in system of checks and balances ensure stable internal conditions like temperature, blood pH,  glucose levels etc. Body achieves this  through constant monitoring and warning signals that induce prompt corrective actions.  For example we feel hungry when energy level goes down,  thirsty  when  hydro mineral balance is disturbed,  we shiver  when body feels cold......

As far as hunger is concerned primitive man lived on a feasting-fasting cycle, alternating between bouts of gorging after a massive hunt followed by fasting during periods of waiting for next hunt.  At the biochemical level, homeostatic  hunger (and its alleviation) is an intricate multi hormonal, multi enzymatic process. True, gut  has the  autonomy over digestion nevertheless  inputs from the brain are also  essential. Gut microbes too  interfere with the digestion process. In this complex  drama,  two  hormones play vital reciprocatory roles:    the hunger hormone ghrelin  and the  satiety hormone   leptin.  When energy level is low,  ghrelin  secreted by empty stomach talks to hypothalamus via the vagus nerve  and  we feel  hungry. Though labelled hunger hormone, this   small peptide of 28 amino acids  was originally implicated in growth hormone release (and hence the name).  Later it was found that  besides being  the hunger hormone it has roles  in   learning, memory, mood swings,  sleep-wake cycle etc.  A very busy molecule indeed. 

With food intake,  ghrelin level drops,  glucose level in the blood goes up.  Excess glucose is ultimately converted to fat through glycolysis followed by lipogenesis.  When fat  storage reaches full capacity,   leptin, secreted by adipose tissues  contacts hypothalamus to stimulate a feeling of fullness. And we stop eating. In the reverse, when  energy is needed and food is unavailable, body first   exhausts the glucose stock  and then  switches to  burning  fat reserves.   Thus homeostatic hunger  ensures  a fine balance between the hunger hormone and the satiety hormone and  homeostasis is maintained.  Any small error, anywhere, in any of  the complex biochemical processes can lead to a host of pathophysiological conditions including anorexia, hyperphagia, obesity or Type 2 diabetes mellitus etc.    

The feasting- fasting  cycle which suited the primitive man is essentially an Eat to Live program .  That was a  long long time ago.  Now  with easy availability of food  we  have  settled for a Breakfast-Lunch-Dinner routine whether hungry or not.  It is the routine rather than the caloric need that drives us to eat.   Having  discovered the pleasures of eating, we have  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. 

What happens if we don't have ghrelin ? Somewhere along the  evolutionary pathway, Mother Nature seems to have toyed with that idea.  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.  

TAIL PIECE:

Decades later,  as if in answer to  Hamsun's "Hunger"  Franz  Kafka penned  The Hunger Artist in  1922 . The nameless protagonist  considers fasting an art  and  could go without food for prolonged periods. In his final moments he reveals his real reason for starving:  "because I couldn't find the food I liked. If I had found it, believe me, I would have made no fuss and stuffed myself like you or anyone else." 

REFERENCES:

1. Hunger by Knut Hamsun Trans. George Egerton

2. The physiology of hunger

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