Not only are food star ratings helping us know what's better to buy, it's starting to spur manufacturers to make healthy products.

A trans-Tasman review of the ratings found the biggest decline in sugar and salt content among the products that were viewed as least healthy.

Cliona Ni Mhurchu, professor in population health at University of Auckland, told Kate Hawkesby that generally nutrition labels have been 

"Our work that they are encouraging manufacturers to make healthier products, so that's a win-win."

She says that the changes shows that there is less reliance on shoppers to make the decisions themselves. 

"Our work is really positive in that, if these changes are happening, what it means is that we are relying less on shoppers to use the labels."

She says that, as the system is voluntary, there isn't much incentive for manufacturers to put the label on the least healthy options.