calcium absorption human nutrition and metabolism age-dependent changes bone calcium deposition rate calcium absorption in healthy girls exchangeable bone calcium human milk human nutrition prematurity zinc Africa Community and International Nutrition NONE Nutritional Models bone mass bone mineral metabolism bone mineralization bronchopulmonary dysplasia calcium calcium metabolism children compartmental model development and pediatrics dietary requirements fetal growth fortification fortified beverage growth infant formula infant nutrition infants iron iron absorption maternal nutrition and optimal feeding practices micronutrient deficiency mineral requirements minerals minerals requirements nutrient interactions nutritional supplementation premature infants puberty puverty rickets stable isotope stable isotopes stable isotopis sunshine undernutrition vitamin D