Low fat diet 'has no effect on heart risk'
risk of cardiovascular disease (CVD), claims a long-term dietary
intervention study claimed to be the largest ever conducted - a
conclusion that is inconsistent with earlier research.
Other clinical trials and observational studies have reported strong associations between dietary fat intake and a lowered risk of CVD, some focusing on specific dietary factors like wholegrain, and fish oil and omega-3 fatty acids.
The Women's Health Initiative (WHI) Dietary Modification Trial enrolled 48 835 post-menopausal women with an average age of 62.3 between 1993 and 1998. The average follow-up time was 8.1 years.
Dietary intervention for 19,541 women entailed cutting down fat intake to less than 20 per cent, eating five servings of fruit and veg, and six portions of grains per day.
Since the study was primarily focussed on dietary fat links to breast and colorectal cancer, the intervention diet did not stick to current dietary recommendations to cut CVD risk, such as less salt and more potassium.The intervention trial, reported in the Journal of the American Medical Association (Vol. 295, pp. 655-666), did succeed in reducing fat intake by 8.2 per cent and increasing vegetable intake by 1.1 servings, but these dietary changes did not result in any significant effect on the incidence of CVD, stroke or coronary heart disease (CHD).
"As delivered, the dietary intervention was not expected to have substantial effects on lipoprotein levels, but it is possible that a diet specifically lower in saturated and trans fat combined with increased intakes of vegetables, fruits, and grains might have led to a decrease in CVD risk," wrote the researchers.
The study appears to ask more questions than it answers. The authors, led by Barbara Howard of the Medstar Research Institute, Howard University, acknowledge the limitations, such as "the lack of targeting key nutrients relevant to CVD and the focus on women aged between 50 and 79 years."
"The possibility that the effect might have been greater in men or if the diet had been initiated at younger ages cannot be ruled out," they said.
The bias from self-reporting of the food frequency questionnaire responses at baseline and after one year, asking volunteers to remember food intake over a three-month period, may also have affected the results.
In an accompanying editorial by Cheryl Anderson and Lawrence Appel from John Hopkins University (JAMA, Vol. 295, pp. 693-695) said: "The intervention did not implement dietary modifications that lower blood pressure - eg, reduced salt intake, increased potassium intake, the DASH (Dietary Approach to Stop Hypertension) diet, and weight loss."
The limitations of the study, combined with the contradictory evidence of other trials, calls into question the value of such an intervention.
However, Anderson and Appel argue: "The WHI Dietary Modification Trial can have implications for nutrition policy related to CVD prevention. As noted by the authors, the trial did not test current dietary guidelines for CVD prevention, none of which focus on reducing total fat intake as a means to prevent CVD."
Current policies towards fruit, vegetables and wholegrain would probably be unaffected, said Anderson and Appel. On the other hand, the study may indicate that saturated fat levels should be lowered from 10 per cent to seven per cent of total energy intake.
"Dietary changes can have powerful, beneficial effects on CVD risk factors and outcomes," said Anderson and Appel.
The authors of the study concluded: "To achieve a significant public health impact on CVD events, a greater magnitude of change in multiple macronutrients and micronutrients may be necessary."
CVD causes almost 50 per cent of deaths in Europe, and is reported to cost the EU economy an estimated €169 billion ($202 billion) per year. According to the American Heart Association, 34.2 percent of Americans (70.1 million people) suffered from some form of cardiovascular disease (CVD) in 2002.
