The new data comes from research carried out by the RAND Corporation in South Africa, and provides evidence to back up previously suggested policy ideas based around lowering the price of healthy foods, rather than unhealthy ones.
"These findings offer good evidence that lowering the cost of nutritionally preferable foods can motivate people to significantly improve their diet," said Roland Sturm, study co-author and a senior economist at RAND - a US-based non-profit research organisation.
"But behaviour changes are proportional to price changes. When there is a large gap between people's actual eating behaviours and what nutritionists recommend, even a 25% price change closes just a small fraction of that gap."
Writing in the American Journal of Preventive Medicine, Sturm and his team found that a rebate of 25% increased the ratio of healthy to total food purchased by 9.3%. In addition, the rebate increased the ratio of fruit and vegetables to total food purchases by 8.5%, and decreased the ratio of less-desirable food to total food purchases by 7.2%.
Sturm and his colleagues examined data from a South African private healthcare program that provides a rebate of between 10% and 25% on purchases of healthy food. The program, which begun in 2009, now has more than 800 supermarkets and 260,000 households participating.
In the South African program, shoppers can get the rebate on a list of over 6,000 foods selected by a panel of nutritionists, physicians and behavioural scientists. The list is estimated to account for around 20% of food spending at supermarkets, with eligible items including fruits and vegetables, whole grains, non-fat dairy, but excludes most items with added sugars or salt.
The team collected supermarket scanner data linked to 170,000 households and survey data on dietary patterns from 350,000 individuals - including both those who participated in the rebate program and those who did not.
Regardless of how the information was analysed, Sturm and his colleagues found that lower prices for healthy foods were significantly associated with better self-reported diet. The authors also noted that the effects of the price subsidies appeared stable over time, while the 25% rebate had consistently greater impact than the smaller rebate.
Based on self-report, individuals who received a 25 percent rebate consumed an additional half-serving of fruits and vegetables each day, and were also less likely to consume fast food, foods high in sugar and salt, fried foods and processed meats.
However, there was no evidence that the rebate program reduced rates of obesity or that fewer participants were overweight, said the research team.