According to Tinbergen's approach, proximate causation refers to environmental stimuli that induce the formation and enlargement or depletion of adipose tissue. Many such factors are associated with lipid deposition and oxidation in humans, operating across a wide range of biological levels and indicating that body fatness is a phenotype intricately connected with many other aspects of biology. The aim of this chapter is to review the range of proximate factors and elucidate their underlying mechanisms and interrelationships. Figure 3.1 offers a schematic diagram of the different levels of biology relevant to this issue. At one extreme are the genetic and epigenetic factors that influence energy balance and so alter adiposity, acting through a number of physiological mechanisms which are themselves sensitive to environmental factors that act on them through behaviour. At the other extreme, therefore, is the impact of the broader socio-ecological niche, acting on groups of humans.

Proximal mechanisms distributed across these different levels are fundamental to evolutionary models of the functions of body fat, allowing broader hypotheses to be both generated and tested at physiological or behavioural levels. Ultimately, speculations concerning the value of energy stores during human evolution, or imputed functions of body fat for ‘funding’ specific functions, can be considered plausible only if they are consistent with the biology of these proximate mechanisms.

The energy balance equation

The energy balance equation is derived from the physical sciences and the field of thermodynamics, the branch of science that uses energy functions to describe the state of a material system and prescribe rules that govern transitions from one state to another.