Abstract

* Department of Human Nutrition, University of Southampton, Bassett Crescent East Southampton S09 3TU, U.K.

One of the primary determinants of the rate of growth is the energy available to the organism. Energy is required for the synthesis of new tissue, and is also deposited in it. The density of the energy stored varies with the type of tissue, from 5.6 kJ/g for lean to 35 kJ/g for adipose tissue. The energy available for growth appears to be preferentially deposited as lean tissue, provided that all other nutrients (amino acids, minerals and vitamins) are available in sufficient amounts. The availability of the first limiting nutrient will determine the overall rate of utilisation of all other nutrients and hence the overall rate of growth. The absence of a single nutrient may lead to a grossly inefficient utilisation of the available energy, which may be dispersed as heat or deposited as adipose tissue.

Catch-up growth is a more complex process than simply an accentuation of normal weight gain. During catch-up weight gain, energy intakes may be as high as 4.5 times basal expenditure. At high rates of weight gain, there is a limitation in the deposition of lean tissue, with excessive deposition of adipose tissue. As the rate of lean tissue deposition increases, there is a disproportionate increase in the demand for nutrients relative to energy. To sustain very high rates of lean tissue deposition, requires a dietary intake with a high P:E ratio.

Understanding of the relative effect which the quality of dietary energy has upon the rate of weight gain is poor. Fat and carbohydrate exert different effects at the highest rates of weight gain. The effect of complex carbohydrates is virtually unknown. The metabolic activity of the microflora of the lower bowel is of much greater significance to growth than has been appreciated in the past.