A NEW Zealand-backed company is warning consumers regular milk may cause harm, in a bid to promote its own specialist A2 milk.

The company is sourcing its milk from herds it says produce the A2 milk protein casein, in contrast to regular milk containing the A1 and other caseins.

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But the a2 Australia corporation's website says "studies suggest" the A1 casein (milk protein) found in regular milk "may" put children at risk of contracting type-one diabetes and adults at risk of heart disease.

One of the key studies that a2 Australia uses to back its claims found consumption of the A1 casein "was significantly and positively correlated with ischaemic (coronary artery) heart disease in 20 affluent countries".

Yet the same study, published by New Zealand researchers Murray Laugesen and Robert Elliott, found no correlation between national tobacco consumption and heart disease.

In reviewing the A2/A1 research, University of Sydney nutritionist professor Stewart Truswell said the NZ study's finding on smoking showed how unreliable between-country correlations were in searching for causes of disease.

Prof Truswell concluded there was "no convincing evidence or probable evidence that the A1 casein in cow's milk is a factor causing heart disease ... (or) childhood diabetes".

a2 Australia has only a 0.6 per cent share of the drinking-milk market but its sales more than doubled last year.

Dairy Australia nutritionist Malcolm Riley said a2 Australia relied on the argument that regular A1 milk may cause harm.

"We don't accept there's any good evidence to show there's any benefit of A2 over A1," Dr Riley said.

Yet a2 Australia continues to promote the message on the risks of drinking regular milk, even warning digestion of the A1 casein "can produce" an opioid known as beta-casomorphin-7 (BCM-7).

The a2 Australia website states: "It has also been suggested that A1-derived BCM-7 may have other effects on human health.

"Its opioid activity has been linked to gastrointestinal discomfort, histamine release and other symptoms of intolerance reactions. More recently, bovine milk derived BCM-7 has also been detected in the blood stream of formula-fed infants."

Yet a European Food Safety Authority report last year, which referenced more than 500 publications, found no cause-effect relationship between BCM-7 consumption and any non-communicable disease, such as childhood diabetes.

While acknowledging the limitations of between-country correlations, a2 Australia nutritionist Sonja Kukuljan said the community could not afford to be dismissive of the effects of BCM-7.

"We need more studies to be conducted desperately and then to be published," Dr Kukuljan said.