Introduction

Body weight and height are two simple anthropometric measurements fundamental to a physical description of an individual or population. Both measurements possess the virtues of being precise (highly repeatable), accurate (close to the true value) and valid (representing what they are thought to represent). By themselves they provide useful information on the mass and size of the human body. It has been proposed that together they provide information on the composition of the body, particularly the adiposity of the body. As different levels of fatness and energy stores in an individual or population are associated with different levels of morbidity and mortality, there is a need for a simple, non-invasive method for assessing fatness and energy stores. This need has been the impetus to the investigation and use of weight: height indices in nutrition. Body mass index (BMI; weight/height², kg/m²), has become the index of choice as it is more highly correlated with % fat and less well correlated with stature than other weight: height indices.

If individuals and populations were of similar shape and composition, mass would be proportional to height cubed (H3). This is rarely observed in practice. Different exponents have been found in different populations for reasons that are not altogether clear. The implication of an exponent of 2 or close to 2 rather than 3 is that individuals range from the short and broad to the tall and narrow.

This paper examines the relationships between BMI and body composition in different populations where low BMIs might be expected. The extent to which low BMIs are influenced by differences in size and shape is also assessed. There are meagre data on BMI and measured body composition in Third World populations and the more voluminous data are indirect. The paper concludes with a description of the contribution of variations in shape to BMI differences within and between populations.