The helicopter takes us to this uninhabited valley, lush and distant. We land in  a clearing, and our pilot helps us unload several packages for our 4 day stay.  It only takes a few minutes, and with a cheerful wave he takes off, leaving us with the intense silence you only get in utterly remote parts of the world. We unpack, checking that we have no phone or any other sort of communication: after all, we want to be alone for the next few days.  Then we notice that there doesn’t appear to be any food.  Surely this wasn’t planned?   No matter, we say to each other in the euphoria of a new experience: we’ll live off the land. After all, our forebears did just this.  The valley looks verdant and fertile, and we can hear the noise of running water nearby.

It’s only after we have put up our little tent, and arranged our deliberately meagre belongings, that we begin to feel hungry.  We look around for something to eat.  There isn’t anything remotely edible.  Plenty of trees, but we can’t really eat leaves.  We recall that certain monkeys can, but they have specially developed guts to deal with high bulk, low calorie food.  Neither can we eat grass, as ruminants do: they have specially-designed guts as well. But even they spend much of their waking life looking for vegetation to eat; the calorie yield is so low.  Fruit or berries?  There isn’t any obvious fruit around, and the few red berries give a warning rather than an appetising signal. We realise that in the primeval world food doesn’t lie around. You have to go and get it. Know where to look. Find it. Catch it. Even compete for it………

The body doesn’t ‘know’ anything about your energy supply, or whether you need food, or whether you have enough stored energy; but the brain does.  And it does a very good job.  As we  sit on a grassy hillock in our picturesque but apparently food-free valley, dreamily thinking about hamburgers, we recall that our body weight has, for the most part, remained quite steady for the last ten years or so, as is the case for most (but not all) people…… Regulating food intake is so important that the brain gets multiple chemical messages about the food you eat.  In general, the more information the brain has, the more accurate its control.  The body makes sure that the brain has lots of information about food intake………

Obesity is the last thing on our minds as we contemplate a hungry day or so, and try to think of ways of getting something to eat in our picturesque valley, but it’s very much on the minds of health care professionals in the US and Europe.  Obesity is officially defined as having a body mass index (BMI) of 30 or over.  You calculate your BMI by dividing your weight (in kilograms) by your height (in metres) squared.  Most people have a BMI of around 20.   The proportion of people with a BMI of 30 or more has increased dramatically in the US over the past 20 years or so: about 25-30% of all adults are currently classified as obese, and the EU countries are fast catching up.  It’s an obesity epidemic – though note that the definition of obesity is an arbitrary one (BMI>30).  Who knows, it may change (upwards?) in future years…….

Scientists struggle to explain this epidemic of obesity, let alone do anything about it.  The currently fashionable explanation (more a theory than an explanation) is that we bring to our world of plenty a body genetically adept at defending itself against starvation, but with no previous need to bother about over-feeding.   Animals eat when they can, and as much as they can: the next meal may be a long way away.   So, we presume, did primeval man (or even medieval man, unless he was a lord).  If there are enforced periods of starvation, a common event in the natural world, then an array of physiological adaptations exist to defend the body against reduced energy supplies; but there was no need to take account of excess energy stores – they rarely if ever happened.  There may be other factors. Palatability is one.  Modern techniques in food manufacture are designed to make food more appetising.  How often have you, at the end of a large meal, incapable of another mouthful, accepted that Belgian chocolate?.......

There was the persistent notion that the brain might have a more dedicated chemical code for eating.   This idea had to wait for the surge of work on peptides in the 1970s and onwards.  In particular, until a peptide called neuropeptide Y was infused into the brain……. Infuse a tiny amount of neuropeptide Y (called NPY for short) into the brain,  and the animal eats and eats. If you infuse NPY  every day it puts on lots of weight.  NPY is the most powerful appetite stimulant known……. Eating is such an important activity that it would be odd if it were controlled by only one peptide.  You eat for many reasons.  And you eat different things: sometimes preferring carbohydrate, at  another time fat; your body has the ability to signal its needs to the brain.  So there are several other peptides that also stimulate eating.   There is limited evidence that some ‘code’ for certain types of food: for example, fat (that’s coded by a peptide called galanin), and, as we’ve seen, NPY inclines one towards carbohydrate…..

You will know that people from different cultures have different tastes in food.  You acquire your taste in food as you grow up. One of the recent social changes in the UK has been the ease with which people adopt new foods. It wasn’t always like this.  A previous generation regarded foreign food with suspicion and dislike.......  There are heart-rending accounts of people starving to death in a famine rather than eat the strange foods provided by an earlier generation of foreign aid agencies.  Rats are the same.  They are very wary of new food. They need to be: eating something they haven’t tried before can be a fatal business.  But damage their amygdala [part of the limbic system] , and they eat it with gusto.  A warning system has been disabled…..

Anorexia nervosa was only recognised as a discrete condition in the late 19th century, though it took longer for it to be accepted as a disorder.  Voluntary food restriction (fasting) has been known since the dawn of recorded history, particularly as part of religious observance. There has been a very marked increase in anorexia during the last  twenty or thirty years, overwhelmingly in young women, a fact that has attracted a variety of explanations..........

The point has already been made that we can only understand the function of the human limbic system, and its role in survival, together with the other properties of the rest of the brain – in our case, the cognitive abilities of our huge cerebral cortex.  Humans bring their mental versatility to their eating behaviour, as they do to everything else. The limbic system, beavering away to make sure we survive, is not alone......  First, there’s how we get food. Humans are omnivores, and eat  meat, fruit and vegetables.  This is a huge advantage, since it reduced our ancestors’ reliance on the hunt….. We also cultivate food, and this needs even more brain power.  Sowing seeds, developing cultivated strains with better yields, and breeding cattle needs at least an intuitive grasp of some fairly complex biological principles….

But  we also use food as a social instrument…..Thus the tribute to kings and chieftains was often based on gifts of food, and even today we continue the practice, taking bottles of wine to parties, sending chocolates to those we love or presents of food to those we want to influence,  doing business over lunch and courting those we fancy over candle-lit suppers…