Here, his field of expertise is energy metabolism, physical activity, food intake and body composition, and energy balance under controlled conditions and in daily life.

He has over 300 peer-reviewed publications and over 50 book chapters. Typical titles are: Use of the doubly labeled water technique in humans during heavy sustained exercise; Long-term effect of physical activity on energy balance and body composition; Energy intake, physical activity and body weight; a simulation model; Dietary fat and body fat: an intervention study; Validity of the assessment of dietary intake: problems of misreporting; Dietary fat oxidation as a function of body fat; and Energy balance in motion.

He was an editor in chief of the Proceedings of the Nutrition Society and he currently is a member of the Editorial Board of the journal Nutrition and Metabolism (London) and of the European Journal of Clinical Nutrition, and editor in chief of the European Journal of Applied Physiology.