|Causes and Mechanisms of Linear Growth Retardation (International Dietary Energy Consultative Group - IDECG, 1993, 216 pages)|
|Nutritional influences on linear growth: A general review|
The preceding analyses have shown that dietary quality is important for the linear growth of preschool-children between 18 and 30 months of age. However, in Fig. 1 it is apparent that the period of greatest growth faltering started very early, at about 3 months after birth, and was essentially complete well before the weaning period. This early growth faltering is common to most developing countries (Martorell & Klein, 1980; Waterlow, Ashworth & Griffiths, 1980). Between about 22 and 40 months of age, linear growth in the group as a whole is undergoing a slight 'catch-up' relative to rates in well-nourished reference children.
Fig. 2 illustrates the relative importance of nutritional factors and their relationship to linear growth failure during the first two years of life. The length Z scores are based on smoothed data from the Mexico Nutrition CRSP, but the values and pattern of change are similar to those in the Egypt and Kenya Nutrition CRSP, and in other populations (Martorell & Klein, 1980; Waterlow, Ashworth & Griffiths, 1980). The relative importance and timing of the nutritional factors is hypothetical, based on a review of the literature.
The main points illustrated in Fig. 2 are that: early growth faltering may be related to sub-optimal fetal endowment with nutrients during pregnancy so that stores are low at birth; this may be compounded by low levels of nutrients in breast milk of the same mothers the peak prevalence of diarrhea and morbidity occurs after growth faltering begins; and in the post-weaning period, when most children consume a diet similar to the rest of the household, there is a relatively faster rate of growth with some 'catch-up'. Interestingly, Martorell & Klein (1980) showed that during this period in the longitudinal INCAP study there was almost no beneficial effect of the nutrient supplements. Again, the major impact of malnutrition, and the greatest response to supplements, occurred prior to 18 months of age.
The role of perinatal and maternal nutrition in linear growth faltering therefore deserves more attention; there is almost no information on this topic in human populations. Maternal nutritional status is likely to affect fetal endowment of minerals and vitamins, and to predict the age at which stores of these nutrients become depleted postnatally. The size of the infant at birth is known to be a strong predictor of size at 6-8 months and resistance to growth faltering in the early postpartum period (Butte et al., 1992), though the reason for this is not understood. In addition, mothers who have sub-optimal nutritional status during pregnancy are likely to do so in lactation.
It is now generally accepted that maternal capacity to produce a normal volume of milk is relatively unaffected by moderate maternal malnutrition, so that inadequate milk production is unlikely to be a significant factor in early growth faltering. This was confirmed in a recent study by Butte et al. (1992) to determine whether growth faltering of Mexican-Indian infants was attributable to low intakes of breast milk. Compared to a well-nourished reference population in Houston, Texas, the Mexican infants were significantly shorter (even at 4 weeks of age) and their length velocity was significantly slower between 12 and 24 weeks. Supplemental feeding and intakes of other fluids were very low, negligible at 4 months, and unrelated to length or weight velocity. Compared to their Houston counterparts, the growth-faltering Mexican infants actually had higher or similar intakes of energy, higher intakes of protein and carbohydrate and lower intakes of fat from breast milk. The authors concluded that either a different growth-limiting nutrient was deficient in breast milk, or the infants had higher nutrient requirements. An alternative explanation might be poor endowment of micronutrient stores at birth.
There is little available information concerning the important question of whether low levels of specific nutrients in breast milk might contribute to infant growth faltering. Zinc (Karra et al., 1989) and iron (Lönnerdal, 1986) concentrations in breast milk seem to be relatively unaffected by maternal intake (including supplements) or status. In contrast, milk vitamin concentrations are relatively strongly associated with maternal intake and stores, especially in the case of water soluble vitamins (Kirksey, 1986; Allen, in press b). Vitamin A in the form of fortified MSG marketed through ordinary channels in Indonesia, increased the concentration of retinol in breast milk by about 16% but had no impact on infant growth (Muhilal et al., 1988).