In an individual in whom weight is being maintained, the energy intake of the body is balanced by its energy output, called the energy balance equation.
Alteration of either component will result in weight change. Energy input is really food intake. The absolute amount of energy (in calories or kilojoules) eaten is important. The amount and distribution of the macronutrients in the diet (protein, carbohydrate and fat) may also effect weight with low protein intake being the most important worldwide on a public health basis.
The components of the energy output side of the equation are the resting metabolic rate (RMR), energy cost of arousal, the energy cost of work and activity, and thermogenesis. This latter component is made up of shivering, non-shivering and diet induced thermogenesis. The RMR has components for the essential maintenance of the cells of the body, for homothermia to maintain the body as a whole.
On eating, there is a specific stimulation of the sympathetic nervous system which leads to thermogenesis. In addition, carbohydrate and protein eaten in excess may also stimulate thermogenesis. Fat does not elicit such a response. Weight loss may be caused by alterations in any one or more of the components of energy intake and output.
A decrease in food intake may be due to environmental factors as in a famine or it may be voluntary as in anorexia nervosa. Decreased food intake may also be caused by disease which produces anorexia, asthenia, nausea and vomiting. There also may be changes in food intake with psychological states (anxiety, stress, depression) or with ageing. Vomiting may be induced in some circumstances or caused by blockages of the oesophagus, stomach and bowel caused by disturbance in motility, strictures, tumours or other growths.
Drugs such as amphetamines and others classes which alter serotonin or block the endocannabinoid receptors in the brain can alter appetite drives and reduce eating. Other drugs may induce gastritis or nausea and so food intake may be reduced.
Once food is eaten, there may be malabsorption from the gut caused by disease of the bowel (eg. coeliac disease), altered transport of nutrients across the intestinal lining or by problems of the gut musculature and motility. Malabsorption may also be caused by inadequate digestive enzymes, bacterial or parasitic overgrowth (for example with giardia lamblia), or by a bowel which has been shortened by surgery or disease.
Weight loss may also be caused by the body's inability to use absorbed nutrients. The most common such disorder would be type 1 diabetes mellitus where carbohydrate and fat utilisation are altered. Other rarer disorders, generally inherited, are for example galactosaemia and aminoacidurias, but these tend to present at an early age.
Energy output may also be altered by increasing activity performed. This increase may be as physical work, training or leisure activity and may be voluntary or enforced. Drugs and hormones may alter RMR and/or thermogenesis (eg. thyroxine, catecholamines, nicotine and amphetamine).
Glucocorticoid deficiency may cause weight loss and asthenia. Thermogenesis is increased in the cold or by exercise and may be increased by some cancers which can also raise metabolic rate. Cancers (malignancies) may also act through humoral (hormonal) factors such as cachexin which alter metabolic processes and induce tissue breakdown. Other factors produced by cancers may alter bowel motility (eg. vasoactive intestinal peptide) and/or absorption. All these may combine to promote weight loss.
Biochemistry 2: Metabolic pathways for the conversion of major nutrients to usable energy
Essentials of Human Nutrition, 2002, 2nd Edition eds, J Mann and S Truswell, Oxford University Press Oxford New York ISBN 0 19 850861 1