Cardiorespiratory costs of growth in low birth weight infants.

The energy cost of growth includes two components: the energy stored in new tissues and the energy expended in all energy requiring steps associated with nutrient intake and net tissue accretion. Most of the energy expended in growth is accounted for by the energy cost of tissue anabolism: peptide bonds, lipogenesis, substrate transport, etc. However, to the extent that additional work is required of the heart and lungs for growth-related increases in O2 and CO2 transport, increased energy is also expended in cardiorespiratory work. Indirect estimates of these costs can be gained by examining the effects of diet and weight gain on heart rate and respiratory frequency. We studied 66 healthy low birth weight infants, mean study weight = 2010 g, fed constant intakes of protein (2.25-3.9 g/kg per day) and energy (100-150 kcal/kg per day). These diets led to rates of weight gain ranging from 13.9 to 21.7 g/kg per day, among the diet groups. Bi-weekly 6-h assessments of energy expenditure, heart rate, respiratory frequency and state of sleep were made after full enteral intake was achieved. After adjustment of heart rate for the effect of postnatal age, heart rate during active sleep was related to weight gain (y = 0.97 x + 144, r2 = 0.15), nitrogen-energy ratio of the diet (y = 5.9 x + 139,2 r2 = 0.22), and energy expenditure (y = 0.53 x + 129, r2 = 0.13). Multiple regression analysis revealed that age-adjusted heart rate during active and quiet sleep was significantly related to a combination of the same three variables (r2 = 0.31).(ABSTRACT TRUNCATED AT 250 WORDS)