Sydney: A type of milk that is said to be easier to digest threatens to usurp regular milk after the world's biggest dairy companies decided to market it.

A2 milk, produced by Jersey, Guernsey and most Asian and African cow breeds, lacks the A1 protein found in other milk which, A2 advocates claim, causes stomach upsets.

A Chinese study last year suggested that adults sensitive to lactose had less trouble when they switched to A2 milk. The debate about what causes stomach upsets linked to drinking milk has been raging for years.

Now New Zealand's Fonterra, the world's largest dairy exporter, and Nestle, the world's biggest dairy company, will market A2 milk around the globe.

A2 milk was pioneered in the 1990s by a New Zealand scientist, Corran McLachlan, who spent years studying the link between the A1 protein and heart disease. Although that link was rejected by some scientists, Dr McLachlan's later work appeared to show that many people who assumed they suffered from lactose intolerance were actually sensitive to the A1 protein.

Using a genetic test, he was able to identify cows producing only the A2 protein and in 2000 founded the a2 Milk Company. It now has almost 10 per cent of the Australian market for fresh milk, has expanded into China, the US and the UK and has become one of New Zealand and Australia's most successful companies.

Following Fonterra's move into A2, Nestle last month said it would produce A2 baby formula for the vast Chinese market. The A2 Milk Company reported revenues in China and Asian markets up 200 per cent in the six months to December.

In Britain, A2 milk is already available in 2,000 shops and supermarkets. However, the world's largest companies fought it for years. In 2014 Parmalat, the Italian dairy giant engaged a consultancy led by Lynton Crosby, who engineered conservative election campaigns in Britain, Australia and New Zealand, to try to discredit A2 milk.

The Times, London