I briefly checked in to Twitter during a self-imposed hiatus (because, let’s face it, I’m an addict and when there’s something really cool on io9 I have to share it with my fellow Twitizens) and noticed a tweet by article82, that echoed the sentiment of a lot of folks this time of year:
I'm again famished. Is there a reason I get hungrier when I eat more food over the course of a few days?—
Brian Colquhoun (@article82) December 28, 2012
It’s almost endemic to the festive season what with the abundance of large traditional meals, office parties and a general feeling of eat, drink and be merry. Hyperbolic headlines aside, the British Dietetic Association estimates that the average caloric excess at this time of year is an additional 500 cals per day, which depending on how early the “holiday” eating begins (and continues consecutively) can potentially add up to an extra 5lbs to contend with when the new year starts.
But why do we overindulge, or rather, despite sating ourselves through hedonic eating (that is, for pleasure and not just for nutrition) why do we keep topping up?
Short, lazy, oversimplified answer might be: blame your hypothalamus (naughty, naughty hypothalamus, it always wants something…)
The longer, non-definitive, more complicated, multifactorial explanation involves the hypothalamus, some melanocortins, dopamine, leptin, serotonin, ghrelin, reward centres of the brain that relate to addiction, light-dark cycles, alcohol consumption and host of other external, behavioural influences that will vary from person to person, culture to culture. And maybe tryptophan. Simple, no? (/sarcasm)
OK, there are a lot of crazy words and concepts there, so I’ll try to boil it down a bit and focus on the most prominent bits**, we’ll start with the hypothalamus: The pleasure centre, the passion palace, the seat of seduction (ooh er missus) of the brain.
What’s going on under the hood?
The hypothalamus (meaning, cleverly ‘under thalamus’) resides in the diencephalon of the forebrain in us human types. For such a tiny thing (most descriptions say it’s approximately the size of an almond) it has a huge influence and in some instances, immense control over a myriad of physiological functions, among these are impulses relating to hunger, thirst and sexual desire. The regulating of hunger falls under the remit of balancing the body’s energy needs and keeping the brain powered with glucose. Appetite is largely managed by two parts of the hypothalamus, the ventromedial which tells you when you’re full or satisfied and the lateral hypothalamus from where the sensation of hunger originates .
The ‘need to feed’, in and of itself is more intricate than that, but appears to be triggered by neurons in the hypothalamus that detect a drop in the hormone leptin, which is released from fat cells called adipocytes. The inverse appears to be true that with high leptin release levels, the desire to eat gets turned off. There are some theories floating around that when humans are well-fed, the sensitivity to leptin levels becomes dulled, which could be one piece of the ‘overeating’ puzzle .
But it’s so yummy!
I bake pies. Conveniently, Mr Schrokit eats pies. And he always has the same reaction to them after eating: MOAR (sic). Now, knowing what I put in them, after a couple of slices, I can’t imagine the leptin levels in his bloodstream are lacking, and one would assume that some anorectic peptides (oversimplified translation: the brain’s in house appetite suppressants) would be saying, “no really, mate, we’re good *burp*. Pardon”. But parts of his midbrain, like the ventral tegmental area, are all, “hey, dude, the party doesn’t have to end, there is more pie, after all…”.
This response to pleasurable and tasty food is not unlike that of potentially addictive drugs, triggering brain reward circuitry by being both rewarding (in taste) and reinforcing (it feels good, why NOT have more), by releasing dopamine from the midbrain.
To quote Mr Schrokit in one of his finer moments, “Dopamine monkey wants more dopamine”. Indeed. While similar to drug addiction, the kind of dependence and tolerance overeating creates, develops over extended periods of time, there is some evidence to suggest that it can also be triggered by binge eating events, and from diets rich in high fat foods  .
What? My cooking’s not good enough for you?!
Apologies, I appear to be channeling my dearly departed Gran (who perfected ‘guilt’, so the rest of you are merely pretenders). She was also a feeder! Aside from taking it as a (joking) affront on her cooking, having grown up during the great depression, she couldn’t imagine why anyone would ever waste food. If she cooked it, you were damn well eating it (no, not the brussell sprouts again…). So even though you were stuffed full to your eyeballs, nan (or mum or auntie) gave you that “look”, and thus you had the second, third etc. helping.
Maybe the holidays with your family is really frelling*** stressful :-( and that triggered a cascade of ‘emotional eating’ . Maybe your resolve to not have that umpteenth mince pie was weakened by that last glass of mulled wine, and you threw caution to the wind. Whatever the reason, your environment, peer pressure and the mood you’re in can very much have an impact on your eating behaviour. You could just simply be relaxed, feeling deserving of some gastronomy fuelled indulgence… vowing to tackle the Crimbo Bulge on January 2nd, or was it the 22nd? Or maybe it was July 2nd, anyway, screw it, it’s the holidays, baby!
Clear as mud?
Far from a simple pat explanation, it’s clear that the physiology, pharmacology and pyschology of eating is no easy matter. And no wonder finding cures for weight gain, eating disorders and obesity (or the behaviours that lead to these states) is both confounding and compelling. Again, I’m aware I’ve glossed over a good deal of detail in favour of digestible amounts of info, but like i said, if you’d like seconds…
And just cause it’s the holidays, a picture of a brain I drew:
** this is simply to say, yes, it’s very very intricate and complicated, and there are a lot of details/specific pathways I’m leaving out in my summary, however, if anyone wants more, I’m happy to go deeper and geekier for you. Yes, I’m that giving ;-)
*** yeah, I’m finally watching Farscape
 Holt, R IG, Hanley, N. A., (2007) Endocrinology and Diabetes 5th Edition, Blackwell Publishing, pp 254 – 255
 Bear, M.F, Connors,B.W., M.A., Paradiso; (2007) Neuroscience: Exploring the Brain, 3rd Edition, chapter 16 pp 510 – 515
 Berridge, K. C., Ho, C.-Y., Richard, J. M., & DiFeliceantonio, A. G. (2010). The tempted brain eats: Pleasure and desire circuits in obesity and eating disorders. Brain Research, 1350(C), 43–64. doi:10.1016/j.brainres.2010.04.003
 AlsiÃ, J., Olszewski, P. K., Levine, A. S., & Schiöth, H. B. (2012). Feed-forward mechanisms: Addiction-like behavioral and molecular adaptations in overeating. Frontiers in Neuroendocrinology, 33(2), 127–139. doi:10.1016/j.yfrne.2012.01.002
 Wise, R. A. (2012). Dual Roles of Dopamine in Food and Drug Seeking The Drive-Reward Paradox. Biological Psychiatry, 1–8. doi:10.1016/j.biopsych.2012.09.001
 Doğan, T., Tekin, E. G., & Katrancıoğlu, A. (2011). Feeding your feelings: A self-report measure of emotional eating. Procedia – Social and Behavioral Sciences, 15, 2074–2077. doi:10.1016/j.sbspro.2011.04.056