Heat treatment of infant formula: effect on postprandial serum alpha-amino-nitrogen concentrations in very-low-birth-weight infants

Several different types of heat treatment are used in the production of commercial infant formulas. The extent of heat treatment can have different physicochemical effects on the milk proteins and will affect their solubility and digestibility. In the present study, 40 very-low-birth-weight infants were randomized at the age of 2 weeks to be given one test feed of either fresh, human milk protein-fortified human milk, a conventional canned sterilized liquid formula, a spray-dried powder formula, or an ultra-high-temperature (UHT)-treated liquid formula. The mean volume of the nasogastric test feed was adjusted so that protein intake was the same in each study group (0.45 g/kg). Venous blood samples were taken preprandially and at 30, 60, and 120 min after the test meal. alpha-amino-nitrogen in serum was assayed by the ninhydrin reaction. The concentration of alpha-amino-nitrogen rose rapidly after the fortified human milk feed, reaching a peak at 30 min. After the peak was reached, the serum values progressively declined, reaching baseline values at 120 min after the start of the test meal. The areas under the curve for all formulas were significantly different from that found for fortified human milk. At 30 min, all formulas had significantly lower alpha-amino-nitrogen values than fortified human milk; at 60 min, all formulas had significantly higher mean concentrations than fortified human milk. At 120 min, none of the formula-fed infants had reached baseline values; in particular, canned sterilized formula was still significantly above baseline values. By extrapolation, it was estimated that baseline was not reached before -180 min after the test meal. These results demonstrate that heat treatment of infant formula has significant effects on in vivo amino acid absorption in very-low-birth-weight infants.