Human Nutrition 3(3+0)

Lesson 8 : Physical Activity and Thermogenic Effect of Food

Factors influencing physical work capacity and efficiency

Efficiency of work and energy utilized for physical activity can be defined as gross efficiency, net efficiency and absolute efficiency, as follows:

1. Gross efficiency = Work done/Total energy expended
2. Net efficiency = Work done/Total energy expended – energy expended at rest.
3. Absolute efficiency = Work done/Total energy expended – energy expended in activity other than that associated with work viz., walking carrying a load – walking without load.

The net efficiency also called mechanical efficiency describes the efficiency with which energy is converted into mechanical work. Mechanical efficiency has been found to be around 27% and is almost the same in all individuals, irrespective of body weight or nutritional status.

Gross efficiency is also measured in terms of total amount of work done or time taken for completing a given task. The work done can also be measured in economic terms, namely materials produced in an industry or agriculture or in a mining operation. Total work output is influenced by a number of factors like body size, composition, nutritional status, social behaviour, motivation etc. Body size and body composition, as proxy for nutritional status can affect total work output.

Work output may be affected by low energy intake, and the nutritional status of the individual. When energy intake is low there is a possible behavioural adaptation to adjust work output to match the energy intake. This is well documented among populations under famine conditions or among individuals subjected to semi-starvation as in Key’s classical study on human starvation.

Body mass index, a result of chronic under nutrition right from childhood can also lead to lower work output. This has many socioeconomic implications since such undernourished workers tend to be lethargic and avoid strenuous work. Industrial and agricultural output and consequently earnings by such undernourished population will tend to be low. Efficiency of work output also depends upon the skill and training of the worker.

Nutritional status with regard to certain minerals and vitamins can also affect work output. Low levels of circulating haemoglobin due to iron deficiency anaemia or vitamin B12 / folate deficiency anaemia can compromise maximal work output due to impaired oxygen carrying capacity of the blood. In anaemia oxygen dissociation from haemoglobin increases to compensate partially for the lowered haemoglobin level and reduced O2 carrying capacity.

While, anaemia of mild to moderate severity may not affect normal level of activity which requires only about 30% of maximal O2 carrying capacity, severe anaemia may compromise even normal activity and work output.

B-complex vitamins like thiamin, riboflavin and nicotinic acid take part in energy metabolism at different points as coenzymes and their deficiencies may be expected to affect adversely energy metabolism and hence physical activity, particularly the highly intense type as exemplified by sports and athletic activities which involve expenditure of high levels of energy.

The body uses energy for three main purposes:

• Basal metabolism represents the minimum amount of energy needed to maintain a body in a resting state. The rate of a person’s basal metabolism depends greately on the amount of lean body mass, the amount of body surface, thyroid hormone levels, and other hormone levels.
• Physical activity represents energy use for total body cell metabolism above what is needed during rest (i.e., basal metabolism)
• The thermic effect of food represents the energy needed to digest, absorb, and process absorbed nutrients. This corresponds to about 5% to 10% of energy used for basal metabolism and physical activity.