The Washington Post published an article that gets to the heart of the
difficulty with Evidence Based Medicine. The URLs for these newspaper sites
are always a bit problematic, because each newspaper has a different policy
on how long they keep these URLs active and who is allowed to see them.
It starts out with a rather provocative statement, but one that is very
hard to argue with:
The public is bombarded with messages about diet and cancer
prevention. Unfortunately, the advice is pretty inconsistent. One day a
diet prevents cancer, the next day it doesn't. In the early '90s, beta
carotene (a vitamin A precursor present in fruits and vegetables) was said
to prevent lung cancer. But several years later, headlines read, "Beta
carotene pills yield no benefit" (The Post, 1996). And while people have
been told for years to eat a high-fiber diet to reduce the risk of colon
cancer, recently we were told "High-fiber diets are not anti-cancer
miracle" (Montreal Gazette, 2005).
The most recent contradictory research studied the whether a low fat diet
could be linked with a decrease in the risk of breast cancer. One study
examined 48,835 post menopausal women without prior breast cancer.
- Low-fat dietary pattern and risk of invasive breast cancer: the
Women's Health Initiative Randomized Controlled Dietary Modification Trial.
Prentice RL et al. Jama 2006: 295(6); 629-42.
[Medline]
[Abstract]
[Full text]
[PDF]
(Medicine, Diet)
The researchers randomly assigned 19,541 women to an intervention group
where women were encouraged to consume on 20% of their calories from fat and
increase servings of vegetables and fruits. The remaining women (29,294) were
a control group and no dietary advice was provided.
The women in the intervention group did follow this advice to some extent.
In the intervention group, the consumption of fat was reduced from 76 grams
per day on average (SD=34) to 41 grams per day (SD=21). The control group saw
little change with a mean consumption of 76 grams (SD=34) at baseline and 63
grams (SD=31) after one year.
The researchers followed these patients for an average of 8.1 years. The
annualized incidence rate was 0.42% in the treatment group and 0.45% in the
control group. This difference was not statistically significant (Hazard
Ratio = 0.91, 95% CI 0.83-1.01) though you might argue that the result was
borderline significant since the confidence interval just barely includes the
value of 1. The p-value is 0.07.
The authors draw the following conclusion
Among postmenopausal women, a low-fat dietary pattern did not result
in a statistically significant reduction in invasive breast cancer risk
over an 8.1-year average follow-up period. However, the nonsignificant
trends observed suggesting reduced risk associated with a low-fat dietary
pattern indicate that longer, planned, nonintervention follow-up may yield
a more definitive comparison.
This contradicts finding of another study, the Women's Intervention
Nutrition Study. These findings were presented at the 2005 meeting of the
American Society of Clinical Oncology. Although I could not find the original
presentation, there are many summaries on the web:
This study recruited 2,437 women who had breast cancer and were treated
either by masectomy or lumpectomy plus radiation. These women were randomly
assigned to receive an intervention (975 women) or a control group (1,462
women). The intervention was a recommendation to eat a low fat diet. The
intervention group had multiple coaching sessions from a professional
dietician.
The intervention did have an impact, since the average women in the
intervention group consumed 33 grams of fat per day (SD=17), considerably
better than the control group (51 grams of fat per day on average, SD=24).
The results were impressive in this study. The recurrence of breast cancer
was 12.4% in the control group and only 9.8% in the treatment group. Most
summaries did not report a confidence interval or p-value, though the
Washington Post article puts the p-value at 0.03.
Is it fair to say that two studies are contradictory when one has a
statistically significant result (p=0.03) and the other has a borderline
result (p=0.07). Is it fair to expect diet to have the same effect on women
who are trying to prevent their cancer from recurring as it would on women
who were trying to avoid their first diagnosis of cancer?
Another problem is the tendency for researchers and the media to overstate
the impact of a positive finding. Why is it that a p-value that is just
barely below the threshold of 0.05 is not also called a borderline finding?
07/08/2008.
