The prevalence of overweight in the developed world and the increased mortality and morbidity risk of overweight people stimulate research into the imbalance between energy intake and energy expenditure. Little information is available about the 24 hour energy expenditure and energy requirement of lean and overweight people. This thesis describes experiments on 24 hour energy expenditure, measured in whole body indirect calorimeters, energy intake and energy requirements of lean and overweight female subjects.<p/>The experiment described in chapter 2 was performed to gain an insight into the reproducibility of measurements of 24 hour energy expenditure in human subjects. Ten female subjects were measured twice over three successive 24 hour periods, under similar conditions in the calorimeter, with an interval of between 2 and 24 months. One woman lost 13 kg during the interval; her data were excluded from analysis. The data from this experiment indicate that the variability in 24 hour energy expenditure within subjects, between periods of measurements, is small under similar conditions and after sufficient adaptation to the calorimeter (within subject coefficient of variation ca 2%). The results are discussed with regard to the length of the trial and the number of subjects required to test for a difference in 24 hour energy expenditure.<p/>Chapter 3 deals with the energy requirement of lean and overweight women. The 24 hour energy expenditure of 29 lean and 18 overweight women, after 5 days on an experimental diet, was measured over 3 successive days by indirect calorimetry. The observed mean 24 hour energy expenditure was considered to be an estimate of the energy requirement as energy balance was close to zero for both groups of women. The energy requirement of the overweight subjects was higher than that of the lean subjects. However, the energy requirements of the overweight subjects, relative to fat free mass and body weight, were respectively similar to and lower than those of the lean subjects. A multiple regression analysis was performed to predict 24 hour energy expenditure (energy requirement) from variables related to the 24 hour energy expenditure. The 24 hour energy expenditure of the adult female subjects was best predicted by body weight and body fat percentage. The data on energy requirements of lean and overweight subjects are compared with data of other investigators.<p/>Chapter 4 deals with the comparison of metabolizable energy intake, during normal life, with the observed mean 24 hour energy expenditure in the calorimeter. The metabolizable energy intake was measured using a 7 day weighing record method. This method was assumed to provide an estimate of the <em>usual</em> energy intake required to maintain body weight (energy requirement). The 24 hour energy expenditure measurement was also considered to give an estimation of the energy requirement. The latter estimate was considered to be the most accurate. The estimated metabolizable energy intake was validated against the observed 24 hour energy expenditure for the groups of lean and overweight women. The 7 day weighing record method provided an accurate estimate of the <em>usual</em> energy intake of lean women. However, the use of this method to estimate <em>usual</em> energy intake of overweight women remains questionable.<p/>Chapter 5 describes the effect of alternating daily energy intake on energy metabolism. Theoretically one might expect that alternating the energy intake would increase 24 hour energy expenditure in comparison with a constant daily energy intake. This would result in a zero energy balance at a higher level of energy intake. The experiment was performed with two groups of subjects, each consisting of 7 females and one male. One group consumed first a diet providing a constant daily energy intake, and then a diet with an alternating daily energy intake (one day low, one day high). The other group followed the reverse protocol. Twenty-four hour energy expenditure was measured on both diets and energy balance was calculated. The result, that alternating daily energy intake does not affect energy balance, is discussed with regard to the expected theoretical effect. It is suggested that alternating energy intake might affect energy balance, if it is alternated over periods longer than one day.<p/>Chapter 6 deals with the effect of an eight week low energy intake on the energy requirements and energy metabolism of overweight women. The experiment initially involved 14 women, but because of questionable adherence to the experimental protocol by two subjects, the data of only twelve subjects are presented. Twenty-four hour energy expenditure of these subjects was measured four times. The first measurement was made when they were consuming an experimental diet that provided approximately the energy required to maintain body weight (100% diet); this was followed by a measurement after 5 days on a weightreducing experimental diet (4.2 MJ/d diet). The subjects then consumed a prescribed diet of 4.2 MJ/d during 6 weeks. After this period they were measured while still on the 4.2 MJ/d experimental diet, and then again after 5 days of refeeding on the 100% diet. The 24 hour energy expenditure of the overweight women decreased by 9% within the first week on the 4.2 MJ/d experimental diet.<br/>After 8 weeks on a low energy intake the 24 hour energy expenditure on the 4.2 MJ/d experimental diet had declined by 15%. After one week of refeeding with the 100% diet, 24 hour energy expenditure was still 10% lower than the initial value. Energy requirement of the subjects was calculated before and after weight reduction, using data on the energy balance and metabolizable energy intake on the 100% and 4.2 MJ/d diets. Energy requirement of the subjects after weight reduction had decreased by 1.2 MJ/d, which was more than would be predicted from the change in body weight and body fat percentage by the equation presented in chapter 3. It remains to be established whether this lower energy requirement than predicted (adaptation) is a consequence of the long term low energy intake or whether it is a temporary phenomenon or an indication of a low energy requirement that may have caused overweight.<p/>The general discussion follows in chapter 7. In this chapter suggestions for future research on energy expenditure and energy requirement are presented. The chapter also considers some implications of the results of the experiments presented in this thesis for the dietary management of overweight.<p/>An appendix describes in more detail the experimental diet and the calorimetric measurements.