Hunger and eating
Food 49
Energy use and storage 49
Control systems 49
Gastric contractions 50
Dual-centre set-point theory 50
The gastrointestinal tract 52
Multiple controls 52
Psychological influences on eating 53
Palatability 53
Sensory-specific satiety' 54
Learning what to eat 54
Learning what not to eat 56
Learning when to eat 57
Disorders of eating and weight control 57
Obesity 57
Anorexia nervosa 59
Bulimia nervosa 60
Summary 60
Further reading 60
Food
Food serves a number of requirements of the body. Each of the three main food groups (macronutrients), carbohydrates,proteins and fats,provides energy. In addition, food provides the body with a variety of other nutrients. Proteins provide the key building blocks of tissues, amino acids, some of which cannot be synthesised by the body. Fats (in general, lipids) are also important in numerous physiological processes and some fats, the essential fatty acids, cannot be synthesised in the body. Most vitamins, and all of the minerals that are essential for the proper functioning of the body, must be ingested in the diet. By the process of digestion food is broken down mechanically and chemically into simpler substances that are absorbed from the small intestines and used by the cells of the body.
Energy use and storage
The tissues of the body obtain most of their energy from the metabolism of glucose or of free fatty acids. Energy' requirements are continuous, but food intake is occasional, so most of the energy' in ingested food is stored. Immediately before eating, during digestion (in response to the secretion by the stomach and intestines of gastrointestinal hormones) and during absorption (following stimulation of glucoreceptors in the liver by increased blood glucose), insulin is released from the pancreas. Amongst its metabolic effects, insulin causes glucose to be stored in the liver in the form of a more complex molecule, glycogen. As circulating glucose is used by the tissues another pancreatic hormone, glucagon, causes the reconversion of glycogen to glucose and its release into the bloodstream. Blood glucose levels are maintained within quite narrow limits by the dynamic negative feedback loops provided by these hormones.
Only a small proportion (about 800 Calories) of ingested energy' is stored in this immediately available way', mostly in the muscles, but some in the liver. Most glucose is converted into fatty acids and hence into fats in the liver and in adipose tissues. Most of this fat is kept as a longer term store in the adipose tissues, and in an average-weight person may amount to some 140,000 Calories. All of these storage processes are promoted by insulin. When glycogen levels in the liver fall, this stored fat begins to be mobilised, again under the influence of glucagon, by conversion into fatty acids and glucose.
