Overview

The specific requirements for the calcium intake of full-term infants, children, and adolescents have been extensively reviewed.^20,21 Dietary reference intake values for calcium and vitamin D were established in 1997 relied on bone health as the indicator in setting reference values for adequacy.⁷ That report established an adequate intake for all life stages for calcium in lieu of an estimated average requirement and recommended dietary allowance (RDA) as a result of uncertainties associated with balance studies, lack of concordance between observational and experimental data, and lack of longitudinal data to verify the relationship among calcium intake, calcium retention, and bone loss. The estimated average requirement is a term that represents the intake level for a nutrient at which the needs of 50% of the population will be met, and the RDA is the average daily level of intake sufficient to meet the nutrient needs of nearly all (97%-98%, or estimated average requirement plus 2 standard deviations) healthy individuals. In 2011, the Institute of Medicine (now the National Academy of Medicine) released a new Dietary Reference Intake report in which an estimated average requirement and RDA for calcium were set for children 1 year and older and adults on the basis of evidence on skeletal health that emerged from a combination of large-scale randomized trials and calcium balance studies.¹ The adequate intake for children 1 to 3 years of age was revised from 500 mg to an estimated average requirement of 500 mg and RDA of 700 mg. The adequate intake for children 4 to 8 years of age was revised from 800 mg to an estimated average requirement of 800 mg and RDA of 1000 mg.^1,7 These updated values, as well as the tolerable upper intake level values, are shown in Table 17.1. It is important to understand the goal of nutritional policy is not to ensure that virtually all members of a population are above the RDA, as this will lead to intakes in most members of the population above their requirements.^21,23

Many approaches can be used to assess the requirements for calcium in older children. These approaches include the following: (1) measurement of calcium balance in people with various levels of calcium intake; (2) measurement of bone mineral content, by dual-energy x-ray absorptiometry or other techniques, in groups of children before and after calcium supplementation; and (3) epidemiologic studies relating bone mass or fracture risk in adults with childhood calcium intake.¹⁸

The calcium balance technique consists of measuring the effects of any given calcium intake on the net retention of calcium by the body. This approach is commonly used to estimate the minimal requirement. Its usefulness is based on the principle that all retained calcium is used, and that unused calcium is excreted and, thus, unnecessary. In children, optimizing calcium retention from the diet should lead to the highest degree of skeletal mineralization and, thus, decrease the relative risk of osteoporosis in adults.^24,25

The substantial limitations involved in obtaining and interpreting data about calcium balance are well known. These include substantial technical problems with measuring calcium excretion and the difficulty obtaining dietary intake control in children. These problems have been partly overcome by the development of stable isotopic methods to assess calcium absorption and excretion which allow for absorption to be measured without the need for fecal collection.²⁵ Because most of these data are from studies in infants and adolescent girls, more data are needed to establish the optimal level of calcium retention at different ages. Studies have clarified that very low calcium intakes, such as those less than 600 mg/day, lead to levels of total calcium absorption and retention much lower than recommended intake levels.²⁶

A major advancement in the field was the development of methods of measuring total body and regional bone mineral content by various radiologic techniques. Currently, the technique used in most studies is dual-energy x-ray absorptiometry.²⁷ This technique can rapidly measure the bone mineral content and bone mineral density of the entire skeleton or of regional sites with a minimal level of radiation exposure. Furthermore, enhancements in the precision of the technique have made it suitable for assessing the short- and long-term effects of calcium supplementation on bone mass in children of all ages.^28,29,30,31 Nonetheless, substantial limitations in current dual-energy x-ray absorptiometry technology including the very small amount of radiation used and inability to transport the equipment have led to increased interest in the use of newer techniques, including quantitative computed tomography and bone ultrasonography.³²