The body mass index (BMI) has served public health and clinical medicine well in the recognition of obesity. However, its use has generated some instructive paradoxes and misunderstandings which argue for the appreciation of body compositional disorders (BCD) as such and, in particular, for the parallel evaluation of muscle mass with a definition of 'orthosarcal' conditions to enable the early detection of sarcopenia. Across the life-span, and with gender and ethnic differentials, BCD is basic to the full spectrum of nutritionally-related disorders and diseases. In the case of metabolic diseases like diabetes, muscle, fatness and its distribution, and even bone seem to play pathogenetic roles. Optimal body fat and distribution are relevant to child development, maternal health and healthy ageing, with much more to learn about the mechanisms. The economic and societal costs of obesity tend to increase progressively with the BMI, but the health outcomes, at least for mortality, are J-or U-shaped. With some established chronic diseases, like diabetes, renal failure or cardiac failure, overfatness may be protective; sometimes this may be because contaminant fat-soluble endocrine disrupters are segregated in fat tissue. This means that some of the relatively favourable survival in the elderly who have more body fat is at the expense of the health care system. Younger children with chronic energy deficiency, on the other hand, may succumb before expenditure saves them. In these respects, our species is more vulnerable than we have thought. Fortunately, a better understanding of BMI and health is emerging.