We need to eat to live and for most people the feeling of hunger is a regular and very familiar occurrence. But the mechanisms that control hunger are a very complex mixture of the physiological and the psychological. There is the intake of necessary energy in the form of calories and there are fat deposits and general metabolic rate. There are also external, psychologically mediated factors such as the sight or the smell of a favourite food. This might well tempt us to eat even though we have recently eaten.

The complexities of the mechanisms that control eating and satiety (the cessation of eating - we do need a mechanism to stop us eating as well as one that prompts us to eat) are not surprising. We need a wide variety of foods to survive and so must have a mechanism that allows choice to be made, that allows food preferences to develop usefully. We need a system that automatically turns us away from poisonous foods and that allows us to follow a balanced diet.

To take an obvious example of built-in food preferences, it is built in to most people to prefer sweet- to bitter-tasting substances. This is adaptive. Most poisonous substances are bitter and we reject such substances by spitting them out and registering disgust (a primary emotion). By contrast, generally, sweet things contain more calories and thus have a higher food/energy value than bitter things. In this sense, they are better for us.

This difference also provides an interesting example of how modern culture can clash with the built-in exigencies of evolution. Food manufacturers have capitalised on our preferences for sweet things and advertise in such a way as to constantly tempt. Unless we guard against it, we are thus in the position of easily eating far more sweet stuff than is good for us, the results being tooth decay, obesity, increased risk of heart disease, and so on. In one sense this all follows from the exploitation of an inherited taste preference which was (and potentially still is) of adaptive, evolutionary value.

Not all food preferences are built in. Our bodily system is flexible enough that some preferences can be acquired through learning. For example, first tastes of coffee or alcohol or even yogurt or blue cheese are frequently not much enjoyed. But they can be rapidly acquired and last throughout life. Similarly, if on the first occasion you taste, say, a peanut butter sandwich, something independently occurs to make you nauseous, then there is a high likelihood that you will have acquired an instant aversion to peanut butter sandwiches.

Internal and external influences

Bodily control of eating occurs both in the central nervous system and the peripheral nervous system. There are centres in the hypothalamus that determine the starting and finishing of eating. However, there are also peripheral mechanisms involved. The glucostatic theory suggests that the level of blood sugar is important and the lipostatic theory suggests that the amount of fat in the body is crucial. Set-point theory has it that the number of fat cells in the body is set either genetically or very early in life and remains constant. So, a person with a higher set point has more fat cells and will tend to eat more than a person with a lower set point. This person will find it relatively hard to diet or to lose weight.

Similarly, there are peripheral mechanisms that help us to stop eating once we have started. Sheer stomach distension is the most obvious example. We feel less like eating if our stomach is full than if it is empty. Feedback from the full stomach goes directly to a nucleus in the hypothalamus, a nucleus that is known to be involved in the cessation of eating.

Bodily control is not enough to account for why we eat and why we stop eating. External factors also come into play, stimuli from the environment that we become aware of through our sense organs. So, the sight and smell and taste of food influence whether or not we feel like eating and how much we eat. Also involved are such things as the time of day, the company we are in and whether or not we are stressed or anxious. We tend to eat more in company than when alone and most people tend to eat less when highly anxious than not, although some people eat far more then.

As Stanley Schachter (1964) showed many years ago, our eating is under the control of a mixture of internal physiological cues and our interpretations of external cues. The balance between the importance of these cues varies from person to person. So, some people are more under external control for their eating and others more under internal control. It is never the extreme; both sets of cues are always involved but the balance varies. The 'externals' are more likely to be overweight or constantly dieting than the 'internals'.

Schachter demonstrated this in many ways. For example, he found that people who are overweight are more likely to overcome the effects of jet-lag than those of normal weight - they simply find it easier to eat at the same time of day as everyone around them. Or, counter-intuitively, he found that overweight Jews are more likely to stick to a fast day than Jews of normal weight. No doubt, this finding would extend to non-Jewish people as well.

Eating disorders

In Western society, currently, slim is beautiful, although there is a little evidence that this might be changing and the more robust look of 30 years ago is beginning to be in favour once more. However, the pressures of society (fashion, television, the corporate world, etc.) have been huge for people (mainly, but not solely, women) to stay slim or to become slim.

Reasons for being overweight (however this is defined - usually by life assurance companies dealing with actuarial risks to life) are genetic or because of overeating, or both. Increased calories will lead to increased weight, but two people taking in the same amount of calories may well gain weight at different rates, or one might gain and the other not. We cannot control our personal genetics so the only possibility is to control how much we eat and how much we exercise - that is, the balance between calories in and calories out. And the problem for most overweight people is that they are more under external control and they tend to eat more when experiencing any sort of increased emotional arousal.

People vary enormously with respect to how restrained their eating is. Some constantly restrict their intake and others never do; most of us fall somewhere between these extremes. There is one huge irony involved in severe food restriction or dieting. People who train themselves to ignore feeling hungry or feeling that they want to eat also tend not to be able to recognise their feelings of being full. So they tend to eat a great deal once they start. Thus severe dieting often leads to an increase in weight. For example, the person who manages to eat nothing all day will often then eat more in the evening than she (it is usually 'she') would have, had she eaten moderately throughout the day.

In the extreme, when eating goes wrong, the disorders of anorexia nervosa and bulimia can develop. There are many reasons for this, some of them stemming from patterns that are laid down in early childhood, from family background, from personality, and so on. These disorders can be extreme and life-threatening and those who suffer from them need a great deal of help to control or overcome them. It is not, as some people think, a matter of simply starting to eat or starting to eat sensibly, in other words, of 'pulling their socks up'. It is far more complex than this.

How to lose weight

Bear in mind that one of those unpleasant rules of life is that it is easier to put weight back on than it is to lose it. This means that if you want to lose enough weight to alter your body shape, then you will have to make a permanent change in your life-style. And this can be expensive. Eating less and running on the spot are probably not enough. A balanced, restricted diet and the membership of a gym or equipment to use at home or even running shoes can cost a great deal. So, if you start, start seriously.

It is important to put controls in place. For example, if your downfall is dairy food, then don't have any in the house. If you live with other people, either family or flat-mates, then you might have to enlist their aid in this. Or, to take another example, don't go to the supermarket when you have not eaten all day. Go soon after you have eaten and you will be less tempted to buy items that you 'should' not. Also, the type of food we eat is important to consider. How hungry we feel depends on what we eat as well as on how much we eat. So, it is better to eat those types of foods that are less easily changed to fat, for example.

Cutting down intake is not enough, however. We have to use up more calories than we take in and it helps in doing this to establish an exercise programme. Here there is another irony. At first, restricting intake seems easier than exercising, if you are used to doing neither. But rapidly, exercising becomes easier than restriction. The first meal is easier to miss than it is to take the first run, but the tenth meal is harder to miss than it is to take the tenth run.

Finally, one of the most important points to realise in attempting to lose weight successfully is to be reasonable. If you go to extremes, if you try to restrict and exercise too much and too fast, the chances are that you will fail. You'll binge and make matters worse. And this type of lack of success is very dispiriting; you might even turn to food as a source of comfort!

It is important to make reasonable changes to one's habits, one's typical way of being around food. So, for example, it might be important to lay down a rule never to eat between meals, and to only eat at certain places at certain times and never at others (in front of the television, for instance), and never to miss a meal entirely. It is only through approaching weight loss in a reasonable way that you will be likely to succeed. It is possible to make permanent changes to one's eating and exercise habits but to attempt to do so in an unreasonable way is likely to end in failure.

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Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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