These measure energy intake or output by a variety of methods including food and physical activity diaries and questionnaires. All these methods are based on a number of assumptions and many are only very rough estimates of energy expenditure. Methods based on heart rate diaries are also not very accurate.
Body weight is the most important component of total energy expenditure, mainly because of the association between resting metabolic rate and fat free mass (FFM). Because bigger people have more total body mass and therefore also FFM, total energy expenditure will be greater. On the other hand, even the lightest female measured at rest in a metabolic chamber over a 24-hour period will expend at least 1200kcals, suggesting that a minimum dietary intake of 1200kcals is necessary for normal daily functioning.
Doubly labelled water. A more recent technique for energy expenditure measurement, which is likely to become more common in the future, is the doubly labelled mater technique. This was first used with humans in the early 1980s and involves loading a person with water which has been labelled with two stable isotopes, deuterium and a form of oxygen, and then measuring the differential elimination of these from the body through the urine. From this information, scientists can calculate the rate of use of oxygen and production of carbon dioxide over the period measured to a high degree of accuracy. The method is simple, safe and non-intrusive. At present its main limitation is the expense of the labelled water and the mass spectrometer needed to analyse the output.
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