The results of the Women's Health Initiative (WHI) dietary modification trial do not support dietary intervention alone to cut cardiovascular or cancer risk.
The study included 48,835 postmenopausal women aged between 50 and 79.
They were randomly assigned to a dietary modification or control group.
Women in the dietary modification group took part in a programme designed to help them cut down their total fat intake so that it accounted for only 20 per cent of calories. They increased their intake of fruit and vegetables to five servings a day, and grains to at least six servings a day. Women in the control group continued to eat normally.
After eight years, the researchers found that women in the dietary intervention group had a mean fat intake 8.2 per cent lower than the control group, and ate at least one portion of fruit or vegetable more per day and half a serving more of grains per day.
However, there were no significant differences in rates of breast cancer, colorectal cancer, CHD, stroke and CVD between the two groups.
Researchers concluded that more focused diet and lifestyle interventions might be needed to improve risk factors and reduce CVD risk, and that dietary intervention might reduce breast cancer risk in a sub-group of women.
Professor Robert Eckel, president of the American Heart Association, said that many questions about the impact of diet and lifestyle on cardiovascular and general health had not been addressed.
'One question that remains unanswered is whether the amounts of fruit and vegetables consumed were adequate to make a difference in risk reduction, as well as whether or not the amount of saturated and trans fatty acid reduction was adequate to see a lower risk,' he said.
Professor Eckel added that physical activity was not taken into account.
'This study underscores the importance of combining strategies to reduce risk,' he concluded.
JAMA 2006; 295: 629-66