The impact of energy turnover on mechanisms relating to energy balance and energy metabolism in humans is still not fully understood. The objective of the present thesis is to partially clarify these mechanisms by identifying specific problems going along in individuals with high (e.g. endurance athletes) and low energy turnovers. In a first step, a systematic review assessing the energy expenditure, energy intake, and body composition in endurance athletes across the training season was conducted. We found that men and women endurance athletes showed important fluctuations in energy turnover across the training season. Secondly, the extent to which the physical activity levels (PAL) are associated with compliance to dietary micronutrient intake recommendations was explored. Results of the present thesis indicate that individuals with low energy turnover have a higher risk for the development of insufficient micronutrient intake, as those with higher PAL. We further showed by using a linear model that an increase in PAL up to levels of 2.0 and a concomitant linear increase in energy intake in order to cover for increased energy demands would reduce the prevalence of micronutrient deficiencies. Finally, our work demonstrated that for the valid assessment of maximum metabolic equivalent of task and energy expenditure the resting oxygen consumption should be measured or, if not possible, estimated by use of published validated equations. To conclude, the energy and nutrient requirements of individuals with high energy turnover (viz. endurance athletes) are not static over the training year and strongly dependent on daily PAL. In addition, promotion of physical activity is important not only for weight management but also for adequate energy and micronutrient intake. In general, caution must be taken when interpreting energy expenditure data, where the calculation is based on an estimated or a standard resting oxygen consumption value.