|Chronic Energy Deficiency : Consequences and Related Issues (International Dietary Energy Consultative Group - IDECG, 1987, 201 pages)|
|Chronic energy deficiency and the effects of energy supplementation|
The simplest model of the effects of energy supplementation is:
Energy supplementation ® Raised energy intake ® Raised energy stores
Such a model has been described as a direct intervention model (BEATON, 1982) or a main-effect model (ADAIR and POLLITT, 1985; POLLITT, 1987). Its characteristics are a direct, proportional, linear relationship between the intervention and a specific outcome, with few confounding variables. It portrays supplementation in a dose-response manner with similar responses expected in different populations and is a variant of the clinical trial model.
In CED, the outcome variable would be a reduced incidence in CED or improved nutritional status as shown by the proxy variables of, for example, increase in growth and size in the population. However, already the outcome variables are now less specific and may be influenced by many other environmental factors, such as illness, and poor social and economic circumstances. When the outcome is a functional variable such as work capacity or performance, there are many more stages between supplementation and outcome. With a multistage model, which is also a direct and main-effect model, the effects are expected to be smaller and less specific, and more influenced by intervening and confounding variables. The multistage model has been the common conceptual approach to supplementation effects until recently.
Chronic energy deficiency is characterised by falls in body weight and fatness, falls in resting metabolic rate and habitual physical activity, and in physical working capacity. Such changes were recorded in the classic Minnesota study (KEYS et al., 1950) and are well-recognised outside the laboratory. CED has effects on behaviour and socialisation as well as on energetics, and a single outcome is obviously not representative of all the effects. In addition, some of the changes are interactive. Falls in metabolic rate reduce the extent of depletion of energy stores; reduced size and reduced activity may both influence work capacity. If the effects of supplementation in reversing some or all of the characteristics of CED are to be identified, an appropriate conceptual and methodological approach is required. However, it is the heuristics of models that are important rather than the models themselves. The development and testing of models should provide useful feedback and confirmation to guide research and planning efforts.