A nutritionist is defending health star ratings, despite the little impact they have. 

Otago University has led a probe of the decade-old voluntary system for rating food products, finding they only slightly improved diet. 

It calculates it'd save the health system about 70 times more if it were made compulsory. 

Nutritionist Nikki Hart told Mike Hosking it is a helpful system for consumers. 

She says people aren't looking at the back of the packet, so the rating gives people a good idea. 

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