The relationship of biochemical and dietary parameters to growth in preadolescent black and white girls

The relationship of growth to selected biochemical and dietary parameters was studied over a 2-year longitudinal period in 124 preadolescent girls age 9 ± 0.5 years at the beginning of the study. Approximately 50% of the girls were white and 50% were black. Approximately 50% of the girls from each race were from low-income families ($1,200 or less/person/year), and 50% were from high-income families ($2,00o or more/person/year). Thus, these girls fell into four different race-income groups: low income white (LIw), high income white (HIW), low income black (LIB) and high income black (HIB) with approximately 30 girls in each group.

The changes in body height, body weight, subscapular skinfold, bone density of the phalanx and bone mineral of the radius and ulna were the parameters used as indicators of growth. Dietary parameters measured included intakes of energy, protein, calcium and iron, while the biochemical parameters studied included blood hemoglobin and hematocrit, urinary excretion of creatinine, nitrogen, inorganic sulfate and hydroxyproline. Measurements were made at the beginning of the study and at yearly intervals.

Average dietary intakes calculated from 24-hour recalls showed energy and protein to be approximately 80 and 180%, respectively, of the Recommended Dietary Allowances (RDA) for the four race-income groups at each age. There were no significant differences in the mean intakes of energy and protein between the groups at either 9, 10 or 1l years of age. Mean values for calcium intakes were greater than ll0% of the iv

RDA at 9 and l0 years of age for the four groups. At 1l years of age, mean calcium intakes were 78 and 92% of the RDA for blacks and whites, respectively. Mean values for iron intakes for all groups combined at 9, 10 or 11 years of age were 95, 97 and 57% of the RDA, respectively.

Among the dietary variables, the correlations between energy and protein, energy and calcium and protein and calcium were larger for the low-income than for the high-income girls. Iron intakes were highly correlated with energy, protein and calcium intakes in LIW and LIB but only with energy and protein in HIB and only with protein in HIW.

Average body heights ranged from 133.4 cm in LIB at 9 years to 148.8 cm in HIB at 1l years of age. Average body weights ranged from 30.2 kg in HIW at 9 years to 43.4 kg in HIB at 11 years of age. Blacks tended to be slightly taller and heavier than whites. There was a significant increase with age in the average body height and weight for the four race-income groups. Mean values for subscapular skinfold showed a significant increase with age in the four groups. There were no significant differences in subscapular skinfold among the four raceincome groups within each age. Mean values for bone density of the phalanx and bone mineral of the radius and ulna were significantly greater in blacks than whites at each age. There was a significant increase with age in the mean values for bone density of the phalanx and bone mineral of the radius and ulna in the four groups.

Significant positive correlations were seen between body height and body weight in LIW, HIW and HIB but not LIB. Subscapular skinfold

vi was highly correlated with body weight in all four groups and with body height in LIW and HIB. There were no race effects on subscapular skinfold. Significant positive correlations existed between bone density of the phalanx and bone mineral of the ulna and between bone density of the phalanx and bone mineral of the radius in blacks but not in whites. Bone mineral of the radius and bone mineral of the ulna were significantly correlated in all groups.

Average blood hemoglobin values ranged from 12.1 to 13.4 g/d1 over the three ages for the four race-income groups. Whites had significantly higher mean values for hemoglobin than blacks at each age. Average values for hematocrit ranged from 39 to 47% over the three ages with whites having significantly higher mean values than blacks at each age. Significant positive correlations were seen between hemoglobin and hematocrit for LIW, HIW and LIB. Average hematocrit values showed a significant increase with age in both races. Mean values for urinary creatinine ranged from 0.115 to 0.208 g/dl over the three ages for all groups. Average values of urinary creatinine tended to be greater in the high-income than the low-income group within the races. Average values for urinary nitrogen/creatinine were not significantly different between the four groups within either age. There was a significant decline with age in mean values for nitrogen/creatinine in the groups. Average excretion of inorganic sulfate/creatinine ranged from 0.177 to 0.322 mg/g for the four raceincome groups with a decline (approximately l/3) in average values from 9 to 11 years of age. Mean values for urinary hydroxyproline/ creatinine ranging from 0.072 to 0.128 g/g did not differ significantly between the four groups at 9 or l1 years of age.

Urinary creatinine was significantly correlated with bone mineral of the radius in LIB and with dietary energy and protein in LIW. Urinary nitrogen/creatinine showeda significant negative correlation with body height in LIW and with bone density of the phalanx in HIw. Urinary inorganic sulfate/creatinine was significantly correlated with nitrogen/creatinine in LIW, LIB and HIB but not HIW. Urinary hydroxyproline/creatinine and urinary nitrogen/creatinine were significantly correlated in LIW and HIB but not LIB and HIW.