Preterm Infants

Calcium and phosphorus accretion rates increase exponentially during the third trimester in utero. Decreased calcium intake is common in preterm infants and may be less than the postconceptional requirement. This decrease places preterm infants at risk of osteopenia and rickets. It is a common problem in infants with weight less than 1500 g who have relatively low intakes of calcium and phosphorus that do not meet the needs for bone growth and mineralization. The frequency of osteopenia is also increased in preterm infants who require long-term parenteral nutrition or who require medications, such as diuretics and glucocorticoids, which may adversely affect mineral metabolism.³³ In small preterm infants fed parenterally, the danger of calcium-phosphorus precipitation in the solution limits the amount of these minerals that can be administered intravenously. As a result, prenatal retention rates of calcium and phosphorus are not achieved in preterm infants, although if optimized in intravenous solutions, should be adequate to prevent severe osteopenia or rickets. In situations in which fluids are being restricted, this may be more difficult to achieve.^34,35 A summary of current recommended intakes of calcium and phosphorous in the preterm infants is shown in Table 17.1

The presence of osteopenia can be assessed by direct radiologic evaluation.²² Increased lucency of the cortical bone with or without epiphyseal changes is characteristic of significant osteopenia. Frank rickets is identified using standard criteria for older infants and children including cupping and fraying at the epiphyses. Although the presence of a fracture can be the presenting sign of osteopenia or rickets, most infants with decreased bone mineralization, including some with severe rickets, do not have fractures. Fractures can occur in preterm infants, however, as part of caregiving by family or medical staff even without the presence of obvious osteopenia or rickets. AAP recommendations for monitoring preterm infants for calcium and phosphorous deficiency are summarized in the text box.²²

Human milk is relatively low in calcium and phosphorus relative to in utero accretion rates of these minerals. Although minerals are well absorbed from human milk (60%-70%), the net retention of calcium and phosphorus are far below the rates in utero, which leads to the development of under mineralized bones. Supplementary calcium and phosphorus are needed to sustain optimal calcium balance in preterm infants. Currently, human milk fortifiers (for human-milk-fed infants) and special formulas with added minerals are marketed in the United States and many other countries for feeding preterm infants. Use of these products has led to net calcium retention comparable to that achieved in utero.³⁶ After preterm infants with weight less than 1500 g are discharged from the hospital, there may be benefits to providing a higher mineral intake than is available from human milk or from routine cow milk-based formulas.^1,37,38,39 This is particularly true for infants who require oxygen or fluid restriction after hospital discharge. Multiple strategies are in clinical use for this situation without clear identification of an optimal approach (see also Chapter 5: Nutritional Needs of the Preterm Infants). A recent randomized control study has shown the benefit of continuing human milk fortifier in preterm infants after hospital discharge.⁴⁰

Full-Term Infants and Children

The optimal primary nutritional source during the first year after birth for healthy full-term infants is human milk. There is no available evidence to show that exceeding the amount of calcium retained by the exclusively breastfed full-term infant during the first 6 months after birth or the amount retained by the human milk-fed infant given complementary foods during the second 6 months after birth is beneficial to achieving long-term increases in bone mineralization. Cow milk-based formulas contain more calcium than does human milk. Relatively greater calcium concentrations are found in soy formulas and specialized formulas, such as casein hydrolysates, to account for the potential lower bioavailability of the calcium from these formulas relative to cow milk-based formula.¹⁶ Of note is the fact that the fractional absorption of calcium from some formulas is similar to that of human milk.^41,42 Thus, the much higher calcium content in such formulas may lead to greater net calcium retention in the formula-fed infant than in the breastfed infant.^43,44,45,46 Studies comparing the bone mineral content of full-term infants during the first year after birth have generally found a slightly greater value for those fed infant formulas than those fed human milk, likely because of the usual greater net calcium retention, as noted previously.^43,44,47 However, there are no data suggesting that such a difference is maintained through adolescence, and there is no evidence at present that these differences lead to clinically significant differences in bone mass.⁴⁸ Longer-term studies are needed to evaluate these issues, but at the present time, the bone mass of the breastfed infant remains the reference standard for appropriate bone mineral mass accumulation in infancy.

One should be cautious about using the adequate intake guidelines by the National Academy of Medicine to determine the appropriate intake of calcium for formula-fed infants. The adequate intake guidelines are specific to breastfed infants, and the adequate intake value for calcium does not hold for infants who are not breastfed. The concentrations (of calcium, phosphorus, and the calcium-to-phosphorous ratio) in infant formulas are set by statute (the Infant Formula Act of 1980), and there is no specific science-based rationale for specific adequate intakes of calcium for formula-fed infants.⁴⁹ The National Academy of Medicine did not make any specific recommendations in this regard in its 2011 guidelines.¹

Few data are available about the calcium requirements of children before puberty.²⁰ Calcium retention is relatively low in toddlers and slowly increases as puberty approaches. The benefits of calcium intakes above the RDA are uncertain. High levels of calcium intake may negatively affect other minerals, especially iron, although adaptation to this effect occurs and the intake of calcium containing beverages such as dairy should not be restricted solely for this reason.⁵⁰ Because these minerals are important for growth and development and may be marginal in toddlers and preschool-aged children, more data regarding the risks and benefits of a calcium intake above the RDA are needed before it can be recommended prior to puberty.

In 2011, the National Academy of Medicine differed from its previous calcium recommendations. Instead of using an adequate intake for calcium intakes, they determined that there was sufficient evidence for an estimated average requirement and RDA for calcium. Shown in Table 17.2 are the proportion of infants and children below the estimated average requirement, which defines the deficient proportion of the population. The prevalence of inadequate dietary intakes is determined by the estimated average requirement cut-point method. This represents the proportion of the population with intakes below the median requirement. In the case of calcium, data from the National Health and Nutrition Examination Survey 2003-2006 revealed that children 1 to 3 years of age at the 50th percentile for calcium intake consume approximately 955 mg/day and that about 5% of that population has an intake below the estimated average requirement of 500 mg/day, which would be considered inadequate.¹

Perhaps of most importance in young children is the development of eating patterns that will be associated with adequate calcium intake later in life. As such, it is important that families learn to identify the calcium content of foods (see Appendix E) based on the food label and incorporate this information into their food-buying habits. The most readily available food source of calcium (70%-80% of calcium content in US diets) is from dairy products, and the Dietary Guidelines for Americans recommend 3 to 4 servings a day.⁵¹ The food label currently provides the amount of calcium as a proportion of the Daily Value, which is 1000 mg; thus, a 20% Daily Value on the food label equates to 200 mg per serving. The US Food and Drug Administration has recently required that the new revised food labels for older children and adults also include the actual amounts of calcium per serving (see also Chapter 47: Federal Regulation of Food Labeling).

Preadolescents and Adolescents

Most research in children about calcium requirements has been directed toward 9- to 18-year-old females. The efficiency of calcium absorption is increased during puberty, and most bone formation occurs during this period. Data from balance studies suggest that for most healthy children in this age range, an intake of 1300 mg/day will support optimal bone growth.^1,52

Numerous controlled trials have found an increase in the bone mineral content in children in this age group who have received calcium supplementation.^{7,35,53,54,55,56} However, available data suggest that if calcium intake is augmented only for relatively short periods (ie, 1 to 2 years), there may be minimal or no long-term benefit to establishing and maintaining maximum peak bone mass.^35,57,58 Even long-term increased intake of calcium may only lead to relatively small benefits in bone mass,⁵⁵ although calcium supplementation may be more beneficial in some groups of children, such as children who reach puberty early or are of greater height.^55,59,60 The implications of such findings for dietary guidance are unclear. In general, available data emphasize the importance of a well-balanced diet in achieving adequate calcium intake and in establishing dietary patterns with a calcium intake at or near recommended levels throughout childhood and adolescence.²⁰

In addition to calcium intake, exercise is an important aspect of achieving maximum peak bone mass. There is evidence that childhood and adolescence may represent an important period for achieving long-lasting skeletal benefits from regular exercise.⁵³ Low bone mass may be a contributing factor to some fractures in children.⁶¹

Although virtually all data regarding the importance of calcium intake has focused on the bone health benefits, emerging evidence, both in adults and in some studies performed in children, suggest that calcium intake may be important in both blood pressure and weight regulation. However, some but not all evidence supports the conclusion that children who have an adequate intake of calcium are more likely to have an optimal weight for age.^{56,62,63,64,65,66,67}

It is recommended that pediatricians actively discuss issues of bone health with families during routine visits. Recommended ages for such discussions are 2 to 3 years of age, 8 to 9 years of age, and then later during adolescence. The 1997 adequate intake for children 9 to 13 years old was revised in 2011 from 1300 mg to an estimated average requirement of 1100 mg and RDA of 1300 mg.^1,7 An emphasis should be placed on preventing inadequate calcium intake, encouraging weight-bearing exercise, and ensuring adequate vitamin D status.²⁰

Adolescent Pregnancy and Lactation

At birth, the fetus contains approximately 30 g of calcium. This represents approximately 2.5% of typical maternal body calcium stores.¹⁶ Evidence suggests that, in adult women, much of this 30 g comes from increases in dietary calcium absorption during pregnancy.⁶⁸ A similar increase in calcium absorption during pregnancy occurs in adolescents.⁶⁹

During lactation, a period of 6 months of exclusive breastfeeding would lead to an additional 45 g of calcium secreted by the mother. Although some of this is accounted for by decreased urinary calcium excretion during lactation, there is extensive evidence demonstrating a loss of maternal bone calcium during lactation.^70,71,72 In adult women, however, bone remineralization occurs after weaning, and neither pregnancy nor lactation is associated with persistent bone loss. Because of data demonstrating that calcium supplementation is not effective in preventing lactation-associated bone loss or enhancing postweaning bone mass recovery,⁷² dietary recommendations do not suggest increases in calcium for healthy adult women who are pregnant or lactating above the 1000 mg/day RDA for nonlactating adult women.¹

The situation for pregnant and lactating adolescents is less clear. Guidelines do not recommend an increased intake above the age-appropriate maximum for adolescents (1300 mg/day) who are either pregnant or lactating.⁷ Shorter femur length in fetuses of pregnant African American adolescents with low dairy intake compared with those with higher intakes has been observed.⁷³ This is consistent with earlier similar data demonstrating a lower neonatal bone mineral density associated with low calcium intake during pregnancy in adults.⁷⁴

At the present time, the available evidence supports the recommendation that the benefits of breastfeeding greatly outweigh any demonstrated risks to adolescents in terms of achieving either optimal growth or peak bone mass.^69,75 No available data suggest that calcium intakes above the recommended amounts are beneficial to pregnant or lactating adolescents. However, it should be noted that these recommended intake levels are far above those typical of the diet of even most nonpregnant adolescents.