The smell of roasting meat or frying onions, the sight of attractive cakes or salads, even listening to someone discussing a good menu, are all sufficient to give us an appetite, even when we have eaten only a few hours before. But for those who are really hungry, because they are almost starving, no such stimuli are necessary. Thirst is a more acute sensation, probably one of the most powerful experienced by the body. We must have water to live and can survive usually not more than a few days with none at all. Thirst can be as a feeling as pain and is capable of consuming every thinking moment.
Hunger – the desire to take in food – is an extremely complex mixture of human behaviour and physiological need. The desire to eat is not controlled simply by the need to keep the body’s internal furnace well stocked with fuel. Hunger is a basic instinct. We want to eat from the moment we are born. The hunger pangs one feels when deprived of food occupy all thoughts, and starving animals (including human beings) will go to extraordinary lengths to find something to eat. We soon learn to alleviate the unpleasant feelings of hunger by eating and being satiated with food is a pleasant experience. Appetite is a refinement of the basic hunger drive. People learn to express choice in what they eat. Preferences for some foods and aversions to others be-come incorporated into memory. Colour, shape, aroma, taste and texture all form part of the complex behaviour of appetite. Eating can be a very pleasurable experience, as good chefs realize. Appetite is also influenced by other factors, especially social behaviour. You may well eat because the clock tells you it is time to eat, or because you see other people eating or saying they are about to have a meal. All this information must come together in order for you to begin eating. The coordinating centre for this activity is located in a region of the brain called the hypothalamus, which is between and at the base of the cerebral hemispheres.
Over the last few decades an enormous amount of work has been done to investigate how the hypothala-mus acts in controlling feeding behaviour. Animal experiments have shown that the area known as the lateral hypothalamus is responsible for initiating the feeding response. When rats have had this area of their brains damaged they perform some of the tasks associated with a preliminary investigation of their food – but they do not eat. Another area, the medial hypothalamus, contains the so called ‘satiety centre’. If this area is destroyed in an animal it does not know when to stop feeding and can gain weight remarkably quickly. The other major influences on these parts of the brain are through naturally-occuring chemicals called neurotransmitters, which convey messages within the brain and around the body. Their chief function is to bridge the gaps (synapses) between nerve endings and in this way carry a nerve impulse from one neuron to the next.
Some drugs affect hunger and appetite. Opiates such as morphine can create a sense of satiety; so can cannabis. Other drugs such as amphetamines can work in both ways: small doses of amphetamines may suppress appetite (these drugs are sometimes used in treating obesity) whereas larger doses increase appetite. The use of amphetamines as appetite sup-pressants is, however, discouraged because these drugs may become addictive in some people. Studies on animals and the effects of drugs may one day throw light on why some people have great difficulty in losing weight. Scientific understanding of the mechanisms that make us eat is much more advanced than the application of this understanding in dealing with obesity.
What makes you want to eat?
The empty stomach experiences powerful muscular contractions and secretes gastric juices, and these give rise to hunger pangs and rumbling of the stomach. These signal the brain that it is time to eat. Many other factors then come into play including smell, colour, shape and texture of food as well as previous experiences of the food on offer. The brain integrates this information to make you begin eating. The smell or thought of food can activate the salivary glands and make your mouth water. At the level of body chemistry, sensors monitor the amount of glucose in the blood and, if the amounts fall below a certain level, the brain is informed of the need to initiate feeding behaviour. In ordinary terms, you feel hungry. There must also be some sort of in-built measure -communicated to the hypothalamus – of the body’s long-term nutritional needs or reserves, because food intake is generally balanced overall to keep an adult’s body weight more or less stable over several years. Also we do not have to be hungry to want to eat. Even when we have eaten one course of a meal and feel full, we can usually tackle the next course. If the presentation and taste of the food is different from what we have just eaten, our appetite returns. This observation has been confirmed in experiments with animals. Dogs e.g. can be induced to overeat by changing their food periodically during one huge meal. ‘The salted peanut effect’, as it was called by an American psychologist, is another way of getting people to eat when they are not necessarily hungry. A small morsel of food can stimulate you to eat more -eat one peanut or potato crisp and you want, and eat, more. Again, this conclusion has been confirmed experimentally. Dogs already sated on one meal can be persuaded into eating more by ‘priming’ them with a different food – that is, by putting a small morsel of it into their mouths. They then go on to eat another whole (but different) meal. Even the aromatic smell or appealing sight of food can re-stimulate the appetite of someone who has just eaten. People on slimming diets are often particularly susceptible to this type of temptation; if they give in, many eat more than someone who is not on a reducing diet.
What stops you eating?
There does not appear to be one particular signal -from the gut, for example – that makes you want to stop. There appear to be several levels of control as far as stopping eating is concerned. You may find it difficult to refuse an appealing sweet after a full main course, but the guilt of taking up unnecessary Calories may help. Another factor which suppresses the desire to eat is distension of the stomach and the gut. Various gut hormones are involved in the process of digesting food. Some of these hormones are also thought to affect the satiety centre of the hypothalamus. All those mechanisms are meant to prevent overeating. It is not possible to eat so much that the stomach bursts. Before this, nausea develops, which eventually results in vomiting.
Probably the most prominent eating disorder, following obesity, is anorexia nervosa. This usually affects teenage girls, whose will to eat is almost totally suppressed. The girl mentally distorts her body image. Although she hardly eats at all, and as a result is very skinny, she perceives her body as very fat. She is really in danger of starving herself to death. Some doctors believe it is a phobic state, with the victim in fear of achieving adult female body weight and shape and its associated social demands. A related disorder is bulimia nervosa, also known as dietary chaos syndrome. The victim has a morbid fear of becoming fat coupled with an overwhelming desire to eat. The eating is often done in secret, with the victim gobbling up large quantities of easily-consumed foods such as cream cakes and sweets. Then, overwhelmed by guilt, the victim induces vomiting to avoid gaining the Calories from the food. The loss of body salts through continuous vomiting can cause death. Both anorexia nervosa and bulimia nervosa need urgent professional treatment, usually by doc-tors and psychiatrists.
You do not need to stop drinking to feel a great thirst or become dehydrated. People who have lost large amounts of blood experience thirst, as do those who are suffering from burns or diarrhoea – especially the almost continuous kind caused by disorders such as cholera, in which enormous quantities of water are lost from the body. If the fluid loss is not made up quickly the victims die. Diarrhoea kills more babies in Africa than any other single health problem. People who have been deprived of water for as little as 24 hours remark on how unpleasant their mouths taste during the period of deprivation and how wonderful the water tastes when they drink it again. Interestingly, this novelty ‘taste’ of water disappears very quickly.
The body can withstand a fluid loss of up to 20 per cent of total body weight before death is imminent. The early signs of dehydration are severe thirst and a dry mouth. As more water is lost, the victim com-plains of drowsiness and a sensation of furriness or cotton-wool in the mouth, and spirits are low. He or she is soon unable to walk and finds difficulty in speaking because the tongue swells. Near death, the skin cracks and the victim suffers deafness and dim-med vision.
Trying to maintain the correct body fluid composition is a battle which life on this planet has fought since it function is the regulation of the water and salt content of the body. The consequence of a shortage of water is a low blood volume which results in an inadequate perfusion of the body tissues. The water and salt contents are strongly related. Salty food for example increases the salt content of the blood. By means of special salt-detecting sensors in the brain, the production of antidiuretic hormone (ADH, also called vasopressin) is stimulated. This hormone signals the kidney to slow down their release of water. In this way the blood is diluted and the salt content becomes normal again.
The kidneys are also able to influence the amount of salt that is excreted. Serious diarrhoea will result in great loss of salt and water. Because of the lowering of the blood volume the filtering action of the kidneys is also impaired. In response, these organs act as endocrine glands by releasing a hormone called renin into the blood. By means of another series of chemicals, the adrenal glands are stimulated to produce aldosterone. This hormone reduces the excretion of salt and – coupled with this – water by the kidneys. In this way the blood volume is brought to an acceptable level again. Thus the regulation of the water content of the body is closely related to the control of the salt content. The reverse is also true.
Left the sea. Water is the largest single constituent of the body. The actual amount varies from one individ-ual to another and with time, but it generally forms between 45 and 70 per cent of total body weight. Older people have relatively less water in their bodies than children. Survival involves making sure that you do not lose more water than you take in. The kidneys, whose job it is to make urine and conserve water, play a key role in this.
How do you know when you need a drink?
Curiously enough, a dry mouth is only one of the signals telling you that your body is short of water. Scientists discovered this in a series of experiments known as ‘sham drinking’. People and animals were given water through their mouths but the water was diverted away from their stomachs (and instead went out through a pipe). Bathing the mouth with water alone was not sufficient to relieve the sensations of thirst (although desert travellers say that keeping stones in the mouth or sucking citrus fruits stops some of the feelings). Later, scientists found that giving fluids by injection directly into a vein relieved the feelings of thirst. At first this was interpreted as showing that all body cells could sense a lack of water. They now believe, however, that the majority of water sensors are found in the brain, called the hy-pothalamus. Other important sensors are in the kid-ney and around the heart.
What stops you drinking?
It is unlikely that the water you drink is absorbed quickly enough to activate a chemical messenger, such as a hormone, that could tell the brain to stop drinking. Animals deprived of water over a period drink their fill in two or three minutes, but the physiological changes that take place after drinking can be detected only about ten minutes later. It is more than likely that you are told to stop drinking by your stomach. The sheer distension caused by the intake of fluid is the signal that you are replete. Over-drinking does occur in some animals and in humans, and when this happens the kidneys compensate by producing very dilute urine.
Thirst and physical disorders
Severe thirst can be a sign of illness rather than part of a normal physiological process. It may sometimes occur in mental illness even when there is no need for fluid intake. The most common causes of excessive thirst are the two forms of diabetes. In diabetes melli-tus (sugar diabetes) there is not enough of the hormone insulin in the blood to reduce the level of glucose when it rises too high. The excess glucose is removed from the body in the urine, carrying with it large quantities of water. The fluid loss that results causes excessive thirst. In diabetes insipidus, the brain is unable to tell the kidneys to reduce the levels of urine production because of a deficiency of anti-diuretic hormone. The development of excessive thirst should always be reported to your doctor.