As the new lunar year approaches, many Chinese mothers will once again slave away in their kitchens to cook the annual Chinese New Year family dinner. Thomas Wolfe once said, “There is no sight on earth more appealing that the sight of a woman making dinner for someone she loves” – and I am pretty sure that the efforts of many mothers will be appreciated by their families dining together during the festivities.
Many people would also overindulge – at some annual family dinners in the USA, it is estimated that the average calorie consumption is somewhere around 4,500 calories per person, and there is little to suggest that Chinese families would consume any less calories for their Chinese New Year dinner.
This amount of food far exceeds even the generous guidelines for normal people, except perhaps for athletes or sumo wrestlers, so it might be interesting to know what happens when people eat so much more food than they need. And that’s because the digestive system processes such excessive amounts of food rather differently from normal meals.
The urge to eat
The handling of food by the body is actually quite a fascinating and complex mechanism. It normally starts with a hormone called ghrelin produced by the ghrelinergic cells in the intestinal tract.
This hormone is issued when the stomach is empty and it acts on the hypothalamus in the brain, causing a primal, desperate need to eat food.
Hence, sometimes I do feel sorry for the poor people who get long jail sentences for petty thieving of food due to hunger – it’s because their brains would have compelled them to find and eat food. It is much more excusable than the thieving done by corrupt politicians or professional conmen who really do deserve to be locked up for long periods, because their criminality is planned.
The stimulus to eat isn’t always initiated by ghrelin. In modern society, the desire to eat is more often stimulated by other senses such as smell, sight and even hearing.
Metabolic signals also induce eating – low blood glucose levels cause glucoprivation and low blood lipid (or fat) levels cause lipoprivation and both conditions also trigger the need to eat.
How much can you stomach?
Regardless of the stimulus, food is taken in orally and both mechanical and chemical digestion commence.
Both digestion processes start at the mouth where teeth grind and tear (masticate) food into smaller bits while mixing with saliva and enzymes – then these partially-digested lumps of food (called bolus) are pushed through the oesophagus into the stomach via a muscular pulsing action called peristalsis.
The stomach can be viewed as a large, kidney shaped balloon hung between two cords – the top cord is the oesophagus (linked via the cardia) and the bottom link is the small intestines (via the pyloric canal and the duodenum).
Normally, the capacity of the adult human stomach is rather limited, around 0.45 to 0.75 of a litre – and usually it would send hormone signals (via curtailing ghrelin and producing leptin) to say it is full when it holds about 1 litre of food.
And at this point, most sensible people will simply stop eating and walk away, letting the Enteric Nervous System (ENS) do its job of managing the digestion processes.
Eat till you burst
So it gets interesting when people consume more than 1 litre of masticated food. At this point, two important things should be noted: (i) the stomach is distensible up to 4 litres in capacity, beyond which it can rupture – a very dangerous situation indeed, and (ii) food doesn’t flow into the small intestines from the stomach in a continuous stream – the stomach has to store the pre-digested food (called chyme) and release it into the intestinal tract in batches only when the tract is ready.
So what causes people to overeat?
Well, put simply, it is mostly willpower (or greed), unless they have some eating disorder or a reaction to some hormones – and this is more common than you think.
It has been suggested that humans have evolved to overeat periodically, presumably to have more food to convert into fat for storage in the body when food is plentiful, which was quite rare in Palaeolithic times.
The evidence is an adaptation of the gastric parietal cells which line the walls of the stomach – it’s known as canaliculus.
A canaliculus is a deep infolding which can be unfolded to expand the surface area of the stomach walls, thus exposing more gastric cells for the secretion of digestive juices into the food.
The mechanism by which the canaliculus unfolds is rather ingenious – the gastric parietal cells are organised as a thick furrowed membrane and it is stretched by an array of little tubes called tubulovesicles shrinking and pulling open the canaliculi, a little like stretching deep wrinkles.
When the food is gone, the tubulovesicles return to their normal shape and closes the canaliculi.
The hunger hormone, the fullness hormone and the rumbling belly
Ghrelin production starts when the stomach shrinks to close to its minimum size, inducing a sensation of hunger – if this sensation is ignored, the stomach may make a rumbling sound.
The noise is the result of the digestive muscles initiating a form of peristalsis called the migrating motor complex to try to sweep up any remaining undigested food into the intestinal tract. The vibrations caused by this muscular wave make the empty stomach vibrate and “rumble”.
Many modern people may be overeating due to leptin tolerance.
Leptin is a hormone produced when the stomach reaches its expected filled capacity of around 1 litre.
Before leptin appears, the production of ghrelin has usually already stopped – in fact the production of ghrelin pretty much stops once the hormone gastrin starts to work on the food that has been eaten.
Hence that explains the awesome sense of relief when you snaffle the first few mouthfuls, switching off those skittish hunger pangs.
Gastrin is the hormone that stimulates the secretion of hydrochloric acid in the stomach, along with an enzyme precursor called pepsinogen.
Pepsinogen is a fascinating and rather complex compound, worthy of another article but we must press on and just classify it for the moment as a “digestive juice”.
Leptin is intended to send a signal to the brain when the stomach is filled to its optimal digestive capacity and hence there is no need to eat any more.
But many people have developed a tolerance to leptin – and this is possibly because they have been ingesting a lot of sweet food containing fructose, which inhibits the action of leptin.
A tolerance to leptin means that people never feel satiated – to stop eating, they have to rely on other more insistent signals from the digestive system, such as a cocktail of digestion-related hormones such as cholecystokinin, secretin, gastric inhibitory peptide, enteroglucagon, etc.
These hormones turn off various stomach actions, possibly when the intestinal tract is congested – therefore the stomach is overloaded and cannot pass more food into the small intestines.
Combined together, these hormones can induce a sense of nausea or a need to vomit – and that’s exactly what some people are forced to do.
It isn’t elegant, it isn’t a nice feeling – so remember that just because you can eat a lot, it really doesn’t mean that you have to.
So what else happens when you overeat?
Quite a few things actually, and none of them too charming. In summary, the body has to handle a significant workload to digest a distended stomach full of food.
As a start, the heart has to pump harder to service the organs involved in the digestive process. The flow of blood and oxygen is often diverted from the brain to service the digestion organs – and this explains the sleepy, slow feeling you feel, a condition sometimes called postprandial somnolence.
You are highly unlikely to solve a complex puzzle after a heavy meal – so don’t overeat, especially if you need to drive or keep your wits after the meal.
Worse than this, there are statistics that indicate that heart attacks are between four and seven times more likely in the two hours after a heavy meal – so, if you have heart issues, perhaps it is time to reconsider your portion sizes.
After a heavy meal loaded with fats, studies have suggested that this may lead to blood clotting more easily – another possible explanation for the heightened risk of heart attacks, especially for people with high blood pressure.
Why high GI food makes you happy
As an aside, there is a common misconception that eating high GI (glycaemic index) foods means that glucose is absorbed more quickly into the body.
Blood glucose is always absorbed at a constant rate regardless of concentration – and therefore eating a lot of high GI foods just means that there is (much) more glucose in the blood.
One effect of insulin is to shoo the excess glucose away from the blood (where it can become toxic) into the skeletal muscle tissues, thus maintaining blood sugar at a constant safe level.
A side effect is an enhanced uptake of other amino acids such as valine, leucine and isoleucine into the skeletal muscles as well – but crucially, not tryptophan.
This is significant in that more tryptophan is therefore left in the body – this amino acid can cross the Blood Brain Barrier (BBB) into the brain and get converted into two feel-good neurotransmitters: serotonin and melatonin.
In the brain, serotonin induces a feeling of contentment and melatonin causes sleepiness – this is another reason why many people find cakes and other high GI foods so comforting.
The gall bladder is profoundly involved with the digestive processes and squirts out bile along a narrow bile duct into the duodenum – bile is required for the digestion of fats.
However, due to the very narrow diameter of the bile duct, it is possible that gall stones may get forced into the duct during heavy digestion, causing extreme pain and symptoms not unlike a heart attack.
Never trust a fart
Less dangerous but possibly also less socially acceptable than gallstones is the flatulence that usually accompanies overeating.
Due to the sheer volume, often the chyme is not fully digested and can therefore ferment in the gut – causing a build-up of noxious gases.
Although somewhat inconvenient to people in the vicinity, this is not usually an issue until you are over the age of 60 or so.
The reason is that many muscles weaken with age and that includes the anal sphincter – so while a senior citizen might think about squeezing out a surreptitious fart, much more can appear than intended. As Jack Nicholson once famously said: “Never trust a fart”, with rather good reason.
A normal meal takes usually between two and three hours to exit the stomach. But due to the traffic jam conditions in the intestinal tract, a heavy meal may actually take between eight and 12 hours to leave the stomach.
The road to gluttony
Due to the protracted time needed to process a heavy meal, the normal sleep pattern is often affected.
Also, the distension of the stomach means that it is likely that eating patterns are also troubled – you would tend to feel hungrier and want to snack more often while the body adjusts back to a normal dietary regime, and this may take quite a few days.
But actually, some people never return to their old eating patterns after a few heavy meals – they have started on the road to gluttony.
It is a road I had travelled myself – and while it was fun for quite a long time, with the benefit of hindsight, it was really not worth the hassle.
Just enjoy your food and listen to your own body – it plays a lovely sophisticated symphony if only you are aware of what it is doing.
Julia Child once said, “People who love to eat are always the best people”, but she should have added, “the best people don’t need to eat that much.”