US: Women who ate less fat and more fruits, vegetables and grains did not reduce their risk of two types of cancer or heart disease, although they did show signs of being healthier, researchers said on Tuesday.
The broad conclusion of the study - that a healthier diet doesn't ward off disease - drew criticisms from some experts who said it could be misinterpreted and taken as an excuse by some people to eat as much of anything they want.
Editorials accompanying the three related studies published in the Journal of the American Medical Association pointed to the relatively healthy population of women in the study, and noted the women who modified their diets might have cut out fats found in fish and nuts now known to be healthy."
"I encourage people to interpret this with caution because we have many unanswered questions," Dr Jeanette Keith said.
Other outside experts described the multimillion-dollar, government-funded study as the most definitive to date, upending widely-held beliefs that people can avoid disease by eating healthier foods.
The same study previously drew the surprising conclusion that hormone-replacement therapy carries heightened risks of health ailments such as stroke.
"This large randomised clinical trial provides the most definitive evidence to date of the impact of a low-fat diet," said Dr Michael Thun of the American Cancer Society.
The women on healthier diets did suffer 9 per cent fewer cases of invasive breast cancer and had 15 per cent less estradiol in their blood, a form of oestrogen that raises the risk of breast cancer. They also had 9 per cent fewer self-reported colon polyps, which are precursors of colon cancer.
Nearly 20,000 of the 49,000 women participants, aged 50 to 79, were instructed to reduce dietary fat and to eat at least five servings of fruits and vegetables daily and six or more servings of grains.
The impact of a better diet on indicators for heart disease was mixed, with only slightly lower levels of cholesterol.