SUMMARY
Current recommendations propose that an entire pregnancy requires an additional amount of energy from the diet equivalent to 335 000 kJ. This figure is mainly based on increases in the basal metabolic rate (BMR) and retention of total body fat (TBF). The BMR response to pregnancy varies considerably among women, but the factors responsible for this variability are unknown. TBF can be calculated from total body water (TBW) and information regarding the hydration of fat-free mass [the hydration factor (HF)] using the two-component model. However, the validity of this model during pregnancy has been questioned since TBW increases by 6-8 kg during gestation. This increase may affect the biological variability of HF, but no studies in this area are available. Furthermore, current recommendations propose that energy needs during pregnancy may be partly offset by reductions in physical activity, but there is little experimental evidence to support this statement. The aims of this thesis were: to evaluate whether the physical activity level (PAL) can be estimated by means of heart-rate recording, accelerometry or a questionnaire in non-pregnant women; to assess the effect of pregnancy on energy expenditure due to physical activity, on the pattern of physical activity and on the biological variability of HF; to evaluate the use of bioimpedance spectroscopy (BIS) for assessing TBW during pregnancy; and to identify factors explaining the variability in the BMR response to pregnancy.