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Human energetics

Grazia Borrini and Sheldon Margen



The science of human energetics studies the human body as it consumes, transforms, and reproduces energy. It was developed through the application of concepts and principles of energy and energy flows to living organisms. Two main approaches to this science are explained.


Energy flow through the human organism can be treated as a purely mechanical phenomenon that follows the rule of energy conservation like any other physical system. In mechanistic terms, the body can be seen as a "rigid" user of energy that transforms food and radiation into heat and mechanical work at a fixed rate.

This approach to human energetics is a useful first approximation, but it does not explain phenomena such as homeo-stasis (regulation of the energy content of the body under different energy inputs and outputs) or the influence of socioenvironmental conditions and psychological factors on energy use by the body.

Energy flows in the human body can also be studied as the interaction between the human organism and its surroundings. In this approach, the context and the continuously changing response of the individual play the major role and offer the most meaningful information.

Whereas the first approach emphasizes measurable amounts of energy input and output, the second approach emphasizes variability and change in the energy flow and focuses on structure, process, and context: The combination of the two approaches offers a powerful tool for understanding human energetics.

In terms of measurable aspects of the energy flow through the human body, inputs of food and radiation and outputs of mechanical work and heat are mediated by the body's metabolism (the sum of an enormous number of microprocesses that transfer and use energy within the cells of the body). At the macroscopic level, metabolism is influenced by muscular activity, psychological state, drugs, physical properties of the environment, and dietary habits.

There is no generally valid relation between intake and expenditure. Energy intake of human subjects can vary from day to day and from week to week without significant fluctuation in body size or activity. Combined changes in dietary intake, muscular efficiency, and metabolic efficiency may account for homeostasis (the maintenance over time of body size and composition).

The Sukhatme-Margen hypothesis is a fundamental feature of a nonmechanistic model of energy interaction between the body and the environment. This hypothesis states that the body can maintain homeostasis within a wide range of energy intakes. Interaction between the body and its environment is regulated by factors intrinsic to the individual, its unique genetic background and development, and its variable environment (Sukhatme and Margen 1982).

The quantification of energy requirements for individuals is called into question on the basis of several issues, both empirical and theoretical. Energy requirement, as referred to by the Food and Agriculture Organization of the United Nations (FAO), is that energy intake able to meet the needs of an average individual of defined sex, age, health status, and activity level. It is assumed that the quantity of food eaten by healthy people living a normal life represents their requirement.

Rather than requiring a single fixed value of energy intake, an individual's energy balance is regulated in a flexible way by a continuously changing interaction with the environment. The capacity for regulation varies widely for an individual and among individuals.

The most important product of the flow of energy through the human body is not work output but the process of life. The relation between energy intake and work output must be considered within the context of the biological and cultural environment of each community. This relation also involves political and ethical considerations. Better health and nutrition should be promoted because they are, first of all, good in themselves.

Energy needs, means, and wants are shaped by the local environmental, social, and cultural reality. The context is essential to define energy requirements and assess whether these requirements are being met. A more appropriate definition of energy requirements would allow planners to direct resources toward more productive interventions and to target feeding programs to those who need them the most.

The human body, behaviour, and culture exhibit a flexible energy interaction with the environment. This flexibility should be carefully managed to protect the interests of the individual and the community. Because energy requirement is both a biological and a political issue, the roles of researchers and experts should be changed. Rather than directing research processes to understand, define, and make decisions, they should be providing resources to the community to allow it to achieve its own goals.

Full advantage could then be taken of our understanding of the complexity of human energetics. Concepts such as "fixed energy requirements" that are divorced from context and values should be renounced. Energy needs can best be defined in the process of community organization and should involve not only bread, but dignity and justice.


Suggestions for Further Research

Research should seek to understand

· The processes of energy regulation and adaptation, with particular reference to variations in energy intake,

· The degree to which genetics and the environment control energy regulation, and

· The nature of functional changes involved in energy regulation and adaptation. The most important research recommendation is that the people for whom research is being carried out be involved in the research process.