A2 milk is allergenic

To the Editor: Recent media reports have claimed numerous health benefits for A2 milk1,2 (eg, “new wave milk”, “wonder milk”). It is becoming more widely available, particularly in health food shops, and is advertised on Queensland television. We believe it is important to offer clear information about this product and cows’ milk allergy.

A2 milk is produced by cows homozygous for the A2 polymorphic variant (his→pro) at amino acid 67 of the b-casein gene. A difference in degradation patterns of the A1 and A2 variants is purported to lead to differences in immunological or pharmacological effects,3-5 which we will not comment on here. Regarding cow’s milk allergy, β-casein is one of at least seven proteins in cows’ milk with allergenic significance (α-, β- and κ-casein, α- and β-lactoglobulin, lactoferrin and transferrin). One would not expect a single amino-acid difference in one protein to have a significant effect on milk allergenicity.

We have found in discussion with parents of milk-allergic children, as well as from inquiries from the community to AllergySA, that there is a perception that A2 milk may be less allergenic than “normal” milk (which contains A1 and A2 b-casein). Although most proponents of A2 milk have made no explicit claims about allergenicity — and indeed some have cautioned against the use of A2 in milk-allergic individuals — there have been media reports that may have led to this perception.6 However, these reports are misleading. For example, it is quite likely that children with a previous history of cow’s milk allergy who have been found to tolerate A2 milk have in fact “grown out” of the allergy, which is the usual natural history. Others may never have had true milk allergy.

We obtained a sample of pure A2 milk from A2 Dairy Marketers (Acacia Ridge, QLD) and used it for skin-prick testing of 11 consecutive milk-allergic children (Box). The tests compared A2 milk with “normal” (A1/A2) milk and cow’s milk protein extract. The mean diameter of the wheal raised by normal milk was not significantly different to that raised by A2 milk (8.2 mm for normal milk v 10.7 mm for A2 milk; P = 0.09, paired t test). No patient had a negative reaction to A2 milk when the reaction to normal milk was positive.

We did not perform an oral challenge with A2 milk in these children, as many had experienced severe allergic reactions, and the predictive value of a positive skin-prick test in the presence of a clear recent history of clinical allergy is high.

We therefore caution that A2 milk should not be used by those with IgE-mediated cow’s milk allergy, particularly those who have had recent severe reactions to milk.