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Background: Short-term studies in adolescents have generally shown an enhancement of calcium absorption by inulin-type fructans (prebiotics). Results have been inconsistent; however, and no studies have been conducted to determine whether this effect persists with long-term use. Objective: The objective was to assess the effects on calcium absorption and bone mineral accretion after 8 wk and 1 y of supplementation with an inulin-type fructan. Design: Pubertal adolescents were randomly assigned to receive 8 g/d of a mixed short and long degree of polymerization inulin-type fructan product (fructan group) or maltodextrin placebo (control group). Bone mineral content and bone mineral density were measured before randomization and after 1 y. Calcium absorption was measured with the use of stable isotopes at baseline and 8 wk and 1 y after, supplementation. Polymorphisms of the Fok1 vitamin D receptor gene were determined. Results: Calcium absorption was significantly greater in the fructan group than in the control group at 8 wk (difference: 8.5 [+ or -] 1.6%; P < 0.001) and at 1 y (difference: 5.9 [+ or -] 2.8%;P = 0.04). An interaction with Fok1 genotype was present such that subjects with an ff genotype had the least initial response to fructan. After 1 y, the fructan group had a greater increment in both whole-body bone mineral content (difference: 35 [+ or -] 16 g; P = 0.03) and whole-body bone mineral density (difference: 0.015 [+ or -] 0.004 g/[cm.sup.2]; P = 0.01) than did the control group. Conclusion: Daily consumption of a combination of prebiotic short- and long-chain inulin-type fructans significantly increases calcium absorption and enhances bone mineralization during pubertal growth. Effects of dietary factors on calcium absorption may be modulated by genetic factors, including specific vitamin D receptor gene polymorphisms. KEY WORDS Calcium absorption, vitamin D receptor, stable isotopes, inulin, prebiotics, pubertal growth, bioavailability

Background: Zinc deficiency is increasingly recognized as an important cause of mortality and morbidity. Children in developing countries are at especially high risk because of relatively low zinc intakes and poor bioavailability. Objective: We assessed the effect of 2-wk adaptation to low zinc intake (4 mg/d) on fractional zinc absorption, endogenous fecal zinc excretion, and urinary zinc excretion. Design: Sixteen healthy 9-14-y-old girls were studied twice in random order after 2-wk adaptation to diets providing either 12 mg/d (high) or 4 mg/d (low) zinc. Fractional zinc absorption and endogenous fecal zinc excretion were measured with use of established stable isotope techniques. Results: Plasma zinc was not significantly lower during the low dietary intake period (1.06 [+ or -] 0.18 mg/L) than during the high dietary intake period (1.14 [+ or -] 0.23 mg/L, P = 0.30). Endogenous fecal zinc excretion was significantly lower during the low intake period (1.08 [+ or -] 0.62 mg/d) than during the high intake period (1.82 [+ or -] 0.95 mg/d, P < 0.026), but there was no significant change in fractional zinc absorption (30.6% [+ or -] 12.4% compared with 26.6% [+ or -] 9.0%, P = 0.32) or urinary zinc excretion (0.68 [+ or -] 0.35 mg/d compared with 0.59 [+ or -] 0.24 mg/d, P = 0.30). Approximate zinc balance was significantly lower during the low-intake period than during the high-intake period (P = 0.007) and significantly (P < 0.0001) less than zero. Conclusion: Short-term zinc restriction in premenarcheal girls leads to a significant decrease in endogenous fecal zinc excretion, which was inadequate to restore normal zinc balance. KEY WORDS Stable isotopes, mineral metabolism, zinc absorption, zinc balance, zinc excretion

Adequate calcium intake is essential for skeletal integrity, particularly during the period of peak bone mass acquisition from 9 to 17 y of age. Currently, the calcium intake of many adolescent girls is below the recommended dietary allowance. The purpose of this study was to evaluate the ability of girls to respond to acute periods of inadequate dietary calcium intake. Calcium absorption was evaluated in 11 girls aged 11.6 [+ or -] 2.4 y after 10 d on both a low-calcium (7.05 [+ or -] 2.03 mmol/d) and a high-calcium (35.30 [+ or -] 2.28 mmol/d) diet. Fractional calcium absorption was determined by using oral ([sup.46]Ca) and intravenous ([sup.42]Ca) stable isotopes of calcium. During a low calcium intake, fractional calcium absorption was significantly greater (0.582 [+ or -] 0.087 compared with 0.260 [+ or -] 0.068, P < 0.0001) and urinary calcium excretion was significantly lower (1.30 [+ or -] 0.83 compared with 3.08 [+ or -] 1.98 mmol/d, P < 0.004) than values obtained during a high calcium intake. Concentrations of 1,25-dihydroxyvitamin D (combination of cholecalciferol and ergocalciferol) were greater during the low calcium intake, although the difference was not significant (108.7 [+ or -] 30.6 compared with 90.0 [+ or -] 25.1 pmol/L, P < 0.1; n = 9). Excretion of N-telopeptide was significantly greater during the low calcium intake (761 [+ or -] 508 compared with 413 [+ or -] 341 nmol bone collagen equivalent (BCM)/mmol creatinine, P < 0.02; n = 9), indicating that bone resorption was increased. These results suggest that during short periods of inadequate calcium intake, girls are able to significantly increase the efficiency of calcium absorption and decrease urinary calcium losses to conserve calcium required for bone mineral acquisition. 1996;63:579-83.