6.2. Use of weight/increment in body fat

The advantage in exploring the protein requirements for weight gain is that the outcome indicator weight can be measured with considerable accuracy and reliability. However, the danger and disadvantage are that there is a tendency to assume that all weight gain has the same composition. This pitfall is often avoided for the relative proportions of lean and fat tissue gained, but the variability of lean tissue composition has not been as widely appreciated. Total body water has been used extensively as an index of lean tissue mass; the assumption being that the relationship between water and lean tissue is relatively invariant. PATRICK et al. (1978) have shown that, during the early phase of rapid weight gain, children recovering from severe malnutrition demonstrate a significant increase in the relative hydration of the body, which progressively tends towards the normal as recovery proceeds (Figure 2). Therefore, the use of total body water to derive values for lean body mass or fat mass is particularly liable to error during the early period of most rapid weight gain, although values derived from measurements of total body water may be of use when taken over the entire period of catch-up growth. This observation may explain why ASHWORTH found that the ratio of observed to theoretical weight gain was greater than 1 during early catch-up growth.

Figure 2. Total body water was measured in severely malnourished children on admission, while they were receiving a maintenance intake of energy and nutrients, and at times during recovery on a high-energy diet. Expressed as a percentage of body weight, body water was significantly greater in children with oedema (Kw+) than in the same children when they had lost their oedema (Kw-), or in children who had never had oedema (M). During early catch-up growth there was a significant increase in total body water as a percentage of body weight for all groups of children (RG1). Total body water tended towards normal as recovery proceeded, RG2, RG3, R (PATRICK et al., 1978).