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close this bookActivity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)
close this folderThe energy requirements of growth and catch-up growth
View the document(introductory text...)
View the documentAbstract
View the document1. General concept of growth
Open this folder and view contents2. Outcome variables
View the document3. General principles relating nutrients to growth
View the document4. Hierarchy of metabolic functions
View the document5. Normal growth
Open this folder and view contents6. Catch-up growth
Open this folder and view contents7. Factors affecting net energy accretion
Open this folder and view contents8. Extent to which colonic fermentation of carbohydrates contributes to energy requirements in childhood
View the document9. Conclusions
View the documentAcknowledgements
View the documentReferences

3. General principles relating nutrients to growth

If growth represents the net increase in the energy content of the body as lean tissue, then all the individual components that constitute lean tissue have to be available in the appropriate amounts for tissue formation to take place. This leads to the concept of the first limiting nutrient as that component of tissue which is available in least amounts relative to the demands for tissue formation. The rate of tissue growth is determined by the availability of the limiting component. All other nutrients can only be utilised at a rate determined by this rate of growth, and any individual components that are available in excess can not be used efficiently and will be wasted.

By application of this concept, it becomes clear that the basic principle that determines the overall rate at which the process of growth takes place will be the availability of the limiting nutrient. This principle can be applied equally to the entire process, or to individual parts of the process. One point which has attracted increased attention over the past decade is that, given the composition of most mixed diets, energy is more likely to be limiting for normal maintenance and growth than any specific nutrient. It is generally accepted that the requirement for protein increases as the demand for lean tissue synthesis goes up. For most nutrients it is accepted that the requirement for the nutrient increases with the rate of growth, although the basis upon which the magnitude of the increase is determined is not always clear.

During deliberations leading to the definition of requirements and safe levels of intake, it is accepted implicitly that the requirement for a nutrient can only be determined in a situation where the requirement for all other nutrients has already been satisfied. Sufficient care has not always been taken to ensure that this constraint is adhered to in experimental situations, even if this were possible. Of greater importance is the tendency to forget or to ignore the potential confounding effects that nutrient-nutrient interactions might have at the level of marginal or inadequate intakes of specific nutrients. In 1945, KLEIBER reviewed the data that related the efficiency of energy utilisation to the adequacy of the diet in respect of specific nutrients. He made the general observation that energy is used less efficiently in the face of a specific deficiency of many vitamins and nutrients:

"A diet is deficient in any nutrient whose addition decreases the calorigenic effect of the ration. A ration is deficient in any food constituent whose addition increases the total energy efficiency of energy utilisation."

Although there has not been a great deal of experimental work that seeks to test this observation formally, there is a substantial body of information in the literature to indicate that it represents a general rule of considerable importance to our appreciation of the availability and utilisation of energy within different population groups. There are many animal studies in which the effect of specific nutrient deficiencies has been explored. Almost invariably when a deficient diet is offered there is a consequent reduction in food intake. One necessary control for this experimental situation is to pair-feed a control group on a complete diet to the intake of the deficient group, to correct for any influence that total intake might have on the interpretation of the results. The general observation is that the pair-fed animals gain weight at a rate that is significantly greater than the deficient animals, although not as great as the group fed ad libitum. The details of the intensity of the response varies with the nutrient, but the overall consequence is the same (Table 1). This leads to an important, outstanding question: in the normal population, to what extent might the apparent variability in the energy requirements for maintenance or growth be accounted for by a marginal or inadequate availability of specific nutrients? Similar considerations apply to the intake of protein, with respect to the quality of the protein and the availability of individual amino acids. There is evidence that analogous considerations may be extended to the relative proportions and the quality of dietary carbohydrate and fat.

Table 1. Animals reared on a diet that is deficient in a specific nutrient fail to grow and lose their appetite. When a control group of animals is pair-fed to the intake of animals on a deficient diet the pair-fed animals gain weight at a rate that is significantly greater than the deficient animals

Specific deficiency

Species

Duration

Body weight (g)

Reference




Deficient

Pair fed


Vitamin A

mouse

12 wk

39

47

AHMED et al.

Copper

rat

8 wk

259

317

BROWN et al.

Iron

rat

5 wk

219

241

BEARD (1987)

Potassium

rat

2 wk

50

95

DORUP and CLAUSEN (1989)