The Importance Of Energy Balance

Body weight is the integrated product of a lifetime’s energy intake, offset by energy needs. Throughout the last century there has been a trend towards increased body weight and increases in body mass index, a measure of weight relative to height. Data from the annual Health Survey for England shows that the average gain in weight of the adult population over the last 10 years has been approximately 0.35 kg/year, which is primarily adipose tissue, with modest concomitant increases in lean tissue.

At an individual level, excess weight gain may occur gradually, almost imperceptibly, over many years or, in intermittent episodes of more pronounced positive energy balance, perhaps related to holidays or festive periods when usual diet and activity habits are distorted. However, spontaneous weight loss is rare, except in association with pathological processes. This asymmetry in energy balance is underpinning the rise in obesity.

Energy balance is the product of both innate and discretionary processes. Energy expenditure consists predominately of three components; resting energy expenditure, thermogenesis and physical activity. Resting energy expenditure is a product of an individuals size, shape and body composition and accounts for 50–80 per cent of energy needs. Additional energy (approximately 10 per cent) is expended in the thermogenesis accompanying digestion and processing of food, or for thermoregulation. Only the energy expended in physical activity is discretionary and thus modifiable. Energy intake is more complex. Undoubtedly there is a complex regulatory system in place that underpins eating behaviour, but this is an imperfect system, easily disrupted by specific food properties, social or environmental cues and subject to a high degree of cognitive control.


The nature of these various mechanisms to control energy balance is the subject of intensive research. With a day-to-day coefficient of variation of energy intake of around 23 per cent and physical activity about 8 per cent, it is readily apparent that most humans do not regulate energy balance on a daily basis. Indeed some, for example in The Gambia, regulate their weight across an entire annual agricultural cycle as a period of plenty following the annual harvest is gradually followed by diminishing food reserves and escalating physical activity during the production of the new crop, a process driven by climatic imperatives. Yet despite this innate capacity to cope with fluctuations in energy balance the rising tide of obesity throughout the world suggests that these mechanisms are increasingly failing to constrain excess weight gain in children and young people and to maintain a stable body weight in adults.

In situations where changes in lifestyle are imposed there are profound effects on body weight. Obesity is almost unknown in animals in the wild, but rapidly develops in captivity. Small animals housed in laboratories, fed standard chow are able to maintain a healthy body weight, yet when given access to highly palatable, refined diets, rich in fat and sugar, they rapidly gain excess weight and become obese. In humans living in economically developed societies dietary choices and physical activity are discretionary components of an individual or family lifestyle. This chapter will consider how these lifestyle habits can determine the risk of obesity.