Unit 2: Nutritional Requirements of the Preterm Infant

As with other nutritional requirements, models have been used to determine mineral requirements of the preterm infants, normally using the factorial approach. The goal is to match the high fetal intrauterine accretion rates of calcium (Ca), phosphorus (P) and magnesium (Mg) that normally occur in the third trimester of gestation and are important for skeletal development (Mimouni et al. 2014).

Two approaches are combined to derive recommendations for mineral supplementation. First, data from body composition analyses of third-trimester aborted fetuses provide estimates of mineral accretion rates for calcium (Ca), phosphorus (P) and magnesium (Mg) (Widdowson et al. 1951). The accretion rates for Ca, P and Mg were calculated to be approximately 100-120 mg/kg/d, 50-65 mg/kg/d and 3-5 mg/kg/d respectively. 

Secondly, intestinal absorption coefficients of these minerals derived from either balance studies or more recently, stable isotope studies in preterm infants are then used to estimate how much supplementation is necessary to achieve this accretion. Unfortunately, the estimated coefficients of absorption show a large variability. There is a range between 40 to 70% in the coefficient of absorption of calcium. For phosphorus it is between 60 to 95% and magnesium around 40%.  Such are these variations, that the intake recommendations by different experts also vary considerably (Mimouni et al. 2014) (Please refer to table 5 on the next page).

Deficiencies of Ca, P and Mg can result in osteopenia of prematurity. Factors that put preterm infants at particularly high risk of mineral deficiencies include (Mimouni et al. 2014):

• low mineral stores due to interrupted gestation (during the third trimester 80% of minerals are accreted in bone);
• low enteral intakes immediately after birth;
  
• difficulty in infusing adequate concentrations of minerals parenterally;
  
• use of certain medications including loop diuretics and corticosteroids;
  
• aluminum toxicity from contamination of parenteral feeding solutions with aluminum, which interferes with bone mineralization;
  
• immobility (common in very ill preterm infants) (Koo & Mimouni 1995).
• vitamin D deficiency,  frequent in many preterm infants, further impedes intestinal mineral absorption.