New research has stoked the health and nutrition controversy over "A1" cows' milk, producing findings said to link its consumption to inflammation and other possible health harm.
Depending on cows' genetic makeup, the beta-casein - a protein - in their milk can be of the A1 type, which some research has suggested may be linked to the development of type 1 diabetes, heart disease and other health conditions.
The other main type is A2, which has not been linked to these conditions.
It is estimated about a third of New Zealand's cows produce purely A2 beta-casein, 15 per cent purely A1, and the rest a mix of both, although most dairy factories process all the milk types together.
The study, by Crown-owned AgResearch and A2 Corporation - which sells A2 milk - compared rats fed A1 milk with those given A2.
Reporting their results in the International Journal of Food Sciences and Nutrition, the scientists, including Dr Keith Woodford, of Lincoln University, wrote that food took longer to pass through the stomach and intestines of the A1 group.
Dr Woodford, an independent adviser to A2 Corporation, said this was consistent with the observation that A1 was associated with digestive discomfort, when compared with A2.
He said the experiment showed previously observed differences in human digestive functioning between A1 and A2 beta-casein were proven with rats in trial conditions.
Fonterra chief technology officer Jeremy Hill said the claims needed to be validated in human trials.
The Ministry for Primary Industries declined to comment until it had evaluated the accuracy of the research.