Retenção de cálcio ósseo em meninos com baixa ingestão dietética de cálcio

Abstract

Bone mass expansion and daily calcium retention in bones vary by pubertal stage, being higher in pubertal stages III and IV. During this period, the daily calcium retention in bones can reach around 290 and 360mg/d in boys with calcium intake over 1000mg/d. Little is known about the daily calcium retention in boys with low calcium intake. The aim of the present study was to determine daily bone calcium retention over a period of six months in boys with low calcium intake. Twenty seven boys in pubertal stages I (8.5 ± 0.6 years, n=9), II (10.8 ± 0.7 years, n=9) and III (13.0 ± 1.0 years, n=9) participated in the study. Blood samples, bone measurements and body composition using dual energy X-ray absorptiometry (DXA) were performed at baseline and after 6 months. Food intake was estimated by three 24-h recalls throughout the study and analyzed by the software Nutrisurvey. Analysis of PTH-i, 25(OH)D and IGF-I in serum were assessed by chemiluminescent immunoassays. The daily calcium retention was calculated assuming that calcium represents 32.2% of the whole body BMC and considering the ratio between change in total body calcium content and days elapsed between measurements. Calcium intake was similar between groups (684±265mg/d, 618±406mg/d e 597±332mg/d in stages I, II e III, respectively) and corresponded to about 55% of the recommended intake. Z-scores for whole body and total proximal femur BMD were greater than -2.0 at the beginning of the study and after six months. The daily calcium retention in bones was lower (P <0.05) in boys at pubertal stage I (110 ± 38 mg/day) compared to those at pubertal stages II (189 ± 64 mg/day) and III (225 ± 102 mg/day). Approximately 75% of the boys studied had serum 25(OH)D concentrations lower than 50nmol/L (n=6, n=8, n=7 in pubertal stages I, II and III, respectively) at the beginning of the study. Serum 25(OH)D was positively correlated with BMD and BMC of the whole body and femur at the beginning and end of the study. The IGF-I was higher in pubertal stage III compared to II (288 ± 135 vs. 193 ± 79 ng/mL, P<0.05). In the whole group, IGF-I was positively correlated with whole body and femur bone area (r>0.49; P<0.05) at the beginning of the study and after six months. Serum IGF-I at the beginning of the study was the only biochemical index associated with daily calcium retention (r=0.40, P=0.04). Despite the low calcium intake and vitamin D insufficiency, the boys had adequate bone mass for age. Daily calcium retention rate was similar to the observed in boys with adequate calcium intake, except for boys at pubertal stage III. Only in these boys, calcium intake was positively associated with daily calcium retention (r=0.80, P<0.01). Therefore, only for those boys in pubertal stage III, recognized as a period of very high calcium demand for bone mineralization, calcium intake together with an insufficient vitamin D status appeared to limit bone acquisition or at least reduce the velocity of bone mass accumulation