Body weight energy balance

To lose weight, people must take in less energy than their bodies require. When this happens, the body bums fat to meet its energy requirements. People can modify the energy balances of their bodies by eating less and by exercising more. 

The first law states that energy is neither created nor destroyed, which applies to the human body as to any other system. Thus the body mass represents a balance between energy intake (i.e. food) and that expended in various processes in the body, especially physical activity. Thus the law is particularly relevant in weight-reducing diets to overcome obesity or to maintain normal body weight. The subject of obesity is discussed in Appendix 1.5. The general principles of energy intake and expenditure are now discussed. 

Concepts of metabolic processes may be distorted if they are based solely on energy. We have already shown that protein production and the accumulation of lipids differ between the three races of European anchovy, but the energy equivalents of the body weights alter identically in all three (Shulman and Urdenko, 1989), even though the metabolic patterns differ. No fish farmer will consider only the caloric content of the feed, but will concentrate on a proper balance of constituents. Only considerations of both matter and energy will give a true understanding of all the processes involved. 

Normal values range from 22 to 28. Values > 30 are indicative of an overweight condition. Obesity is always the consequence of a disturbed energy balance caloric intake exceeds consumption. Almost unavoidably, excess body weight and obesity lead to diseases and a shortened lifespan. 

It is generally accepted that the primary etiology of obesity concerns problems of energy balance as a consequence of nutritional excess and physical inactivity with less than 1% of cases associated with endocrine dysfunction (13). Changes in body weight or body composition depend on the relationship of energy intake to energy... 

Positive energy balance produces increases in body weight, body fat, or both. Weight loss occurs when expenditure exceeds intake. While the energy balance relationship is clear, application of the concept has not proven to be straightforward or easy. 

This chapter will review the relative contributions of exercise and food intake to changes in body weight and more specifically body composition. The emphasis of this chapter is on exercise as a modality for fat reduction and fat-free weight maintenance with the focus on aerobic exercise which has greater potential to modify body composition due to larger effects on energy balance. The first section reviews the effects of aerobic exercise on body composition in humans. The second section addresses techniques for measurement of body composition and limitations of these measurements in humans. The third section examines the use of experimental animals for studies of exercise and body composition, and the fourth section examines the interactions of diet and exercise. 

Lay publications are inundated with fad diets designed to reduce body weight "quickly and easily" using a variety of caloric restrictions (68). Most individuals attempt to produce a negative energy balance and reduce body weight by decreasing food intake. 

Figure 5. Relationships between milk production, dry matter intake, body weight, and mean energy balance of dairy cows during a typical lactation cycle. Reprinted with permission from Ref. 9.

Weight gain is widely believed to be a common consequence of HRT, and the desire to avoid obesity is a major reason why some women decline treatment. However, the potential effects of HRT need to be distinguished from effects that could be due to changing lifestyle or ageing. The effects of short-term hormone replacement and age on alterations in weight, body composition, and energy balance have therefore been studied in a prospective... 

---