MIAMI - It's hard to eat right all the time, but making small improvements by choosing healthier foods now and then may significantly boost one's chances of living longer, says a US study.

The report in the New England Journal of Medicine is the first to show that improving diet quality over at least a dozen years is associated with lower total and cardiovascular mortality.

Researchers at Harvard University tracked dietary changes in a population of nearly 74,000 health professionals who logged their eating habits every four years.

Researchers used a system of diet-quality scores to assess how much diets had improved.

For instance, a 20-percentile increase in scores could "be achieved by swapping out just one serving of red or processed meat for one daily serving of nuts or legumes," says a summary of the research.

Over the 12-year span, those who ate a little better than they did at the start - primarily by consuming more whole grains, fruits, vegetables, and fatty fish - saw an eight to 17 percent lower risk of dying prematurely in the next 12 years.

Those whose diets got worse over time saw a higher risk of dying in the next 12 years of follow-up, on the order of a six to 12 percent increase.

"Our results highlight the long-term health benefits of improving diet quality with an emphasis on overall dietary patterns rather than on individual foods or nutrients," says senior author Frank Hu, professor and chair of the Harvard Chan School Department of Nutrition.

"A healthy eating pattern can be adopted according to individuals' food and cultural preferences and health conditions," he adds.

"There is no one-size-fits-all diet."

Agence France-Presse