Fat or Fiction

The latest apparent flip-flop is about low-fat diet and breast cancer. Last month, a front-page headline read: 8216;8216;Low-Fat Diet8217;s Benefit Rejected: Study Finds No Drop in Risk for Disease.8217;8217; But less than a year ago, a prominent headline sent a different message: 8216;8216;Study of Breast Cancer Patients Finds Benefit in Low-Fat Diet8217;8217;

The newer study, part of the federally funded Women8217;s Health Initiative WHI, involved about 50,000 women in an eight-year experiment to see if fat reduction in diet might reduce cancer risk. Findings were published last month in the Journal of the American Medical Association.

The earlier trial, called the Women8217;s Intervention Nutrition Study WINS, involved about 2,500 women in a similar dietary experiment. Results were presented in May 2005 at the American Society of Clinical Oncology.

In each, post-menopausal women were randomly assigned either to a low-fat diet group that received intensive dietary counseling or to a comparison group whose members were given standard advice about a 8220;healthy diet8221; but were not asked to change what they ate.

How the studies differed

The studies differed in two major ways. First, they studied different groups of women: one with prior breast cancer WINS and one without WHI.

Women with breast cancer are substantially more likely to develop a new breast cancer than women without breast cancer. In general, people at high risk stand to gain more from interventions than people at low risk. In other words, if dietary fat were going to have an effect, it would be more obvious in women with breast cancer.

And this is exactly what happened. In both studies, the low-fat diet group developed fewer new breast cancers. But the effect of diet was larger in the women who already had the disease: For every 1,000 women in the study with breast cancer, 25 fewer new breast cancers were detected. In comparison, for every 1,000 women without breast cancer, two fewer breast cancers were detected.

The second major difference between the studies was in how statisticians judged the findings. This difference was profound. Based on conventional statistical practices, the effect of the low-fat diet in women with breast cancer was deemed to be real8212;the result of more than chance. But in women without breast cancer, the effect of diet was judged to be statistical noise.

Reality vs. chance

The judgments about what is real and what is statistical noise are based on what researchers call 8220;P values8221;. Based on the size of the study groups and the number of cancers in each, the 8220;P value8221; communicates how often you would expect to see an effect this big simply as a result of chance. By convention, scientists say 8220;P values8221; below 5 per cent are 8220;statistically significant8221;8212;meaning not likely attributable to chance. And 8220;P values8221; of 5 per cent and higher are considered statistical noise that is, likely due to chance.

The 8220;P values8221; for the effect of low-fat diet on breast cancer in the two studies were similar. For women with breast cancer, it was 3 per cent. For women without breast cancer, the 8220;P value8221; was 7 per cent.

What should women do?

To decide whether to act on the results of a study, you need to understand what it will take to make the change8212;here, how much dietary sacrifice you must make to reduce risk of breast cancer8212;and whether you think the sacrifice is worth the benefit.

In these studies, women assigned to the test groups had to limit themselves to 20 to 40 grams of fat a day. To achieve this goal, women in both studies had to eliminate things like butter on bread, cream cheese on bagels, oil in salad dressing, regular ice cream, most cakes, cheeses and so on.

For women with breast cancer, the diet change may well be worth the effort: Their risk of another breast cancer is high and the diet may lower it enough to be worth the sacrifice. For women without breast cancer, whose breast cancer risk is low to begin with, the small possible benefit of the diet may not be worth the sacrifice.

Tough challenge

People who choose to follow different diets may well differ in their cancer risk because of their family history genetic makeup or habits for example, smoking. In general, observational studies tend to overstate the effect of diet because a 8220;healthy diet8221; is often a marker for a healthy lifestyle. People who watch what they eat probably exercise more, avoid smoking and follow other behaviors linked to good health. Any of those behaviors might also help explain why these people have lower rates of cancer.

Many have criticized the study of women without breast cancer, saying the low-fat diet didn8217;t lower fat intake enough, focussed on total fat rather than changing from animal to plant fat, or wasn8217;t a Mediterranean diet8212;based on grains, olive oil, vegetables, fruits and fish. While intriguing, these criticisms are themselves based on observational research8212;no more credible than the low-fat findings that stimulated this study to begin with.

Bottomline

The best data to date suggesting the potential for a diet8212;or any lifestyle alterations8212;to affect cancer risk is limited. The single notable exception is smoking: there is no doubt that not smoking dramatically lowers cancer risk.

The effect of diet on cancer is likely to be small for most people because diet is heterogeneous and the effect of any given food may depend on its interaction with other foods. And the smaller the effect, the harder it is to demonstrate statistically.

So it is really not surprising that results of research about diet and cancer flip-flop. Low-fat diets probably do lower the risk of breast cancer8212;but the effect on risk is small8212;particularly for women with no prior history of the disease. Changing diet to reduce breast cancer8212;or any other cancer8212;is a personal decision, not an imperative.

8211;Lisa M Schwartz, Steven Woloshin 038; H Gilbert Welch