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Letter |
Nutrition Research Division
Health Products and Food Branch
Health Canada
Ottawa, Ontario, CANADA
Agostoni et al. [1] presented very useful and interesting data on the free amino acid (FAA) compositions of human milk and some commercial infant formulas sold in Europe. Apparently all the infant formulas except one (Isomil; soy based) studied in their investigation were based on cows’ milk. All the infant formulas were fortified with taurine, while Isomil also contained supplemental methionine.
Their data for total FAA in human milk (3019.7 ± 810.1 µmol/L) obtained from Italian mothers were similar to the values reported by us [2] for total FAA in milk of Canadian mothers (3397 µmol/L in preterm milk, 3069 µmol/L in term milk, 3069 µmol/L in mature milk). Although we did not determine FAA in milk-based infant formulas, our value for total FAA in cows’ milk (1061 µmol/L) was also comparable to the FAA data for infant formulas reported by Agostoni et al. [1].
It is well known that human milk contains a substantially higher level of free taurine compared to cows’ milk, and therefore milk-based infant formulas are fortified with taurine to simulate the amino acid composition of human milk in some jurisdictions, as was the case with formulas studied by Agostoni et al. [1]. Compared to cows’ milk and milk-based infant formulas, human milk is also a rich source of free glutamic acid and glutamine. Based on this difference, Agostoni et al. [1] have suggested the fortification of infant formulas with free glutamic acid and glutamine. However, unlike taufine (which is only present in free form), glutamic acid is present in large amounts in protein-bound form in cows’ milk and milk-based formulas. For example, total (free and protein-bound) glutamate + glutamine contents of mature cows’ milk and human milk have been reported to be 20.8 and 19.0 g/100 g total amino acids, respectively [3]. Similarly, total glutamic acid contents of a milk-based infant formula sold in Canada and human milk have been reported to be 18.3 and 17.5–17.8 g/100 g protein, respectively [4, 5].
In view of the high level of total glutamic acid in milk-based infant formulas, the need for additional crystalline glutamic acid, as suggested by Agostoni et al. [1], would require documentation of the beneficial effect of free glutamic acid during early postnatal development, especially in the case of term infants.
REFERENCES
Department of Pediatrics, San Paolo Hospital, Milan (C.A., E.R.), ITALY
Istituto Superiore di Sanità, Rome (B.C., C.B., E.S.), ITALY
We agree that more data are needed on the functional aspects of free glutamine, before adding it to infant formulas, even if observations from recent clinical trials in humans suggest a critical role in protecting the integrity of the intestinal mucosa. It is true that glutamic acid is present in large amounts in protein-bound form in cows’ milk and milk-based formulas, but the free form could have still unexplored functional effects. Quite interestingly, we have observed that, while the content of free taurine remains almost stable, glutamic acid and glutamine increase approximately 2.5 and 20 times with progressing lactation respectively, representing more than 50% of total FAA at three months [1]. So, the question remains, why should human milk selectively present with increasing concentrations of these free amino acids (particularly glutamine) while progressing towards the weaning period?
REFERENCES
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