Accretion. Since there are no data on serum Pi or phosphorus balance and limited information on bone mineral content for children aged 1 through 3 years, a surrogate indicator had to be adopted to set the EAR. An estimation of body accretion was used based on known tissue composition and growth rates. Rates of accretion of phosphorus in bone and soft tissue (for example, lean mass) during this period of growth were then corrected for predicted efficiency of absorption and urinary losses to derive an EAR using the factorial approach.
The phosphorus requirement for bone growth can be estimated from one of two methods: (1) the phosphorus content of bony tissue gained over this age range as derived from the body composition of Fomon et al. (1982), or (2) the known increments in whole body bone mineral content using the method of dual energy x-ray absorptiometry (DXA) (Ellis et al., 1997). For both approaches, a value of 19 percent by weight was used as the phosphorus content of bone. The phosphorus content of lean tissue is assumed to be 0.23 percent based on known composition of muscle (Pennington, 1994). The computations for tissue accretion are summarized in Table 5-3. The overall estimated mean value for both sexes combined is 54 mg (1.74 mmol) phosphorus accreted per day.
The value derived for phosphorus accretion in lean and osseous tissue is supported by estimates of phosphorus retention, 10 g (323 mmol)/kg body weight gained, derived from balance studies in children aged 4 to 12 years (Fomon et al., 1982), when corrected to the average weight gain for children aged 1 through 3 years. When males and females are averaged, a total of 2.36 kg of weight is gained over this period (Fomon et al., 1982); thus a total phosphorus increment of 23.6 g (768 mmol) is gained. A daily accretion of phosphorus is then predicted to be 62 mg (2.0 mmol), which is close to that calculated above for lean plus osseous tissue accretion.
The value derived for phosphorus accretion was then employed in a factorial model to obtain the EAR. It is known that phosphorus in urine increases with phosphorus intake, described by an equation derived by Lemann (1996) in adults. This approach was adopted here since no such relationship has been developed for children. Using the equation: Purine = 1.73 + 0.512 × Pintake (in mmol/day), and an intake of 310 mg (10 mmol)/day, predicted urinary