Title: Information Needed To Assess Individual Energy Requirements
Key words: energy expenditure, calorific intake, food intake, BMR, basal metabolic rate, energy status, calorimetry, PAL, physical activity level, diet, high fat intake, thermogenic, bodyweight, obesity, post-prandial, thermogenesis, cold, shivering, sweating, pregnancy, lactation, menstruation, thyroid, epinephrine, adrenaline, convalescence, metabolic demand, diabetes, arthritis, cancer, drugs, caffeine, nicotine, beta-blockers, heredity, weight
Date: Sept 2006
Author: Morgan, G.
Information Needed To Assess Individual Energy Requirements
Ideally, daily energy expenditure in the human body should closely match that of food intake. Great efforts have been made over the years, working with large populations, to determine the factors bearing upon this ratio, which is obviously of interest to health professionals given the present concern with obesity in the developed countries. Assessment of the energy status could either be carried out by measuring the calorific intake of the food imbibed or by measurements of the energy expended. In practice it has been found to be more practical to adopt the latter approach, by using such methods as indirect calorimetry, as such an approach is able to provide data on energy requirements for specific work tasks and thus is of greater analytic usefulness.
For a large population sample the following two concepts have been introduced to help predict the average daily energy requirements for an individual:
1. BMR. The basal metabolic rate is the energy expenditure at rest with the subject awake under controlled conditions. BMR is dependent on age, sex, body weight but, even allowing for these factors, is only accurate to plus or minus 17% (Garrow et al. 2000).
2. PAL. The physical activity level is a measure of the overall energy expenditure of the body during the course of the working day. It quantifies the excess energy expended over and above the BMR, such that the total energy expenditure = BMR x PAL. It is the sum total of the energy expenditure of individual activities (e.g. light work or heavy exercise) and is expressed as a ratio of the BMR to indicate the degree of the excess energy burden.
Such assessments are very useful in assessing the energy requirements for the average individual engaged in standardized activities. Unfortunately the following factors that are difficult if not impossible to quantify may have a significant effect on the overall requirements. These are:
a) Diet. A high fat diet leads to a relative preponderance of adipose tissue, a low thermogenic tissue, thus influencing the BMR. Obesity has effects on both the BMR and PAL over and above those attributable to increased bodyweight. In addition, frequent eating or overeating lead to post-prandial thermogenesis and an increase in the daily energy requirements.
b) Climatic considerations. Both cold, and the sweating associated with shivering, lead to an elevation of the bodyís energy requirements.
c) Hormonal factors. Later pregnancy and lactation are associated with increased energy demands but irregular menstruation or thyroid dysfunction also cause aberrations.
d) Anxiety and stress. Mediated through epinephrine (UK = adrenaline) which increases the metabolic rate.
e) Illness. Convalescence, especially post-operatively, is associated with increased metabolic demands. Certain illnesses associated with age, such as diabetes, arthritis or cancer also are influential and are mediated by metabolic, dietary, psychological and other factors.
f) Drugs. Everyday drugs such as caffeine and nicotine increase the metabolic rate. Certain medicinal drugs also have an effect, e.g. beta-blockers, may lead to a slight decrease in energy expenditure.
g) Heredity. Genetic factors are thought to affect the BMR by a factor of plus or minus 10% (Garrow et al. 2000).
Taking into account the above factors, in practice it is difficult to predict energy requirements for a particular individual with great accuracy. Data collection is always imprecise and statistical averages do not take into account genetic, dietary, sociological and other factors. Dual-labelled water analyses, which more accurately assess the energy expenditure over a 2-3 week period may in the future allow more accurate estimates to be made but are unlikely to be widely available. On an individual level simple weight measurements over time will continue to be the best means of assessing the energy input-output status of a specific individualís body.
1. Millward J (2001) Lecture Notes. Surrey University
2. Garrow JS, James WPT, Ralph A (2000) In: Human Nutrition and Dietetics, 10th edn. Churchill, London
3. Bender DA (1997) Introduction to Nutrition and Metabolism, 2nd edn. London