Many studies in the field of obesity deal with the balance between energy intake and energy expenditure. However, it is necessary to consider not only the energy balance, but also the fact that the "composition of expended energy" must on the average match the composition of the food ingested. Failure to satisfy this condition will cause depletion of some body component(s) and this can be expected to generate food intake signals if such a depletion can be perceived (which is likely only in the case of carbohydrate; this contention is to be tested by animal studies). Because the contribution of fat to the fuel mix oxidized after the overnight fast barely exceeds the fat content of a typical "mixed diet" (40% of calories as fat), we suggest that the adult human organism will not readily oxidize a fuel mixture as rich in fat as the diet (i.e., not without generating definite hunger feelings), unless the daily routine includes sufficient physical activity of the type which tends to lower the Respiratory Quotient (RQ). Taking advantage of the rapid response provided by mass spectrometric gas analysis, multiple RQ measurements will be performed, under conditions representative of our society's life-style and eating habits, to generate a body of knowledge on the composition of expended energy and its variability (particularly as regards the carbohydrate-to-fat ratio). This "survey" will also serve to establish if a net transfer of calories from carbohydrate to lipid ever occurs (i.e., non-protein RQ greater than 1.0), or whether lipogenesis is so limited that carbohydrate has only the effect of sparing fat from oxidation. The long-range objective is to derive recommendations designed to prevent increases in the body's fat content, using as leverages a reduction in the fat content of the diet and physical activities selected for their effect in lowering the RQ. Because such guidlines can be more specific than recommendations directed at maintaining the overall energy balance, it may become easier to select the components of a successful strategy for weight maintenance.