My blog last Friday questioned the wisdom of recent advice dished out to us on how to prevent cancer. The crux of this issue is that such recommendations are based on so-called ‘epidemiological’ studies which may show associations between things (e.g. obesity and cancer) but cannot be used to prove that one is causing the other.
The only way to know for sure whether advice to, say, eat less red meat and fat and eat more fruits and vegetables, is beneficial is to get individuals to make relevant changes to their diet, and compare their health outcomes to those who did not make such changes. Such ‘intervention’ studies are expensive and difficult to do, and are therefore a rare breed compared to the much more plentiful epidemiological study. However, intervention studies do exist, and when they do, the provide the best opportunity we have for assessing the relevance and likely effectiveness of conventional nutritional advice.
One of the biggest dietary intervention trial is known as the Women’s Health Initiative Dietary Modification Randomized Controlled Trial (WHI trial). The trial was initiated in 1993, and recruited a total of almost 50,000 post-menopausal women and followed them for an average of about 8 years. About 60 per cent of these women were allowed to continue on their normal diet (the control group). The remaining 40 per cent of women were instructed to make their diet ‘healthier’ but reducing total fat to 20 per cent of calories, to eat at least 5 portions of fruits and vegetables a day, and to eat 6 or more portions of wholegrains a day. This group ” the intervention group ” received frequent group sessions designed to educate these women about healthy eating and support them in their quest.
It’s one thing telling people to do things, it’s another thing them doing it. In studies of this nature there is always going to be a bit of ‘slippage’. However, over the course of the study, there appeared to be very real differences in the average diet between the two groups.
Notably, compared to the control group, the intervention group ended up eating:
22 per cent less fat
23 per cent less saturated fat
20 per cent less cholesterol
25 per cent less trans fats
17 per cent more fibre
an additional serving of fruit or vegetables each day
an additional ½ a serving of grains each day
Taken as a whole, most health professionals, dieticians and nutritionists would view such differences as an indication that the intervention group ate a significantly healthier diet than the control group.
The results of this mammoth and expensive ($ 415 million) study are now in the public domain. The most recent appraisal of the data appeared last month in the Journal of the National Cancer Institute and focused specifically on cancer outcomes.
Previous analyses, published last year, had shown not reduced risk of either breast cancer or colon cancer in the intervention group [2,3]. In the latest study, the scientists involved in this project extended their analysis to more than 20 other cancers. And what the results showed is this:
The ‘healthy’ eating group were not found to be at a reduced risk of any of these cancers (NONE of them)
The logical interpretation from this is that this study singularly failed to demonstrate that ‘healthy eating’ reduces the risk of cancer.
Now, have a read through the ‘results’ and ‘conclusions’ from the abstract (summary) of the study.
RESULTS: Ovarian cancer risk was lower in the intervention than in the comparison group (P = .03). Although the overall ovarian cancer hazard ratio (HR) was not statistically significantly less than 1.0, the hazard ratio decreased with increasing intervention duration (P(trend) = .01). For the first 4 years, the risk for ovarian cancer was similar in the intervention and control groups (0.52 cases per 1000 person-years in the intervention group versus 0.45 per 1000 person-years in the comparison group; HR = 1.16, 95% confidence interval [CI] = 0.73 to 1.84); over the next 4.1 years, the risk was lower in the intervention group (0.38 cases per 1000 person-years in the intervention group versus 0.64 per 1000 person-years in the comparison group; HR = 0.60, 95% CI = 0.38 to 0.96). Risk of cancer of the endometrium did not differ between the groups (P = .18). The estimated risk of total invasive cancer was slightly lower in the intervention group than in the control group (HR = 0.95, 95% CI = 0.89 to 1.01; P = .10).
CONCLUSIONS: A low-fat dietary pattern may reduce the incidence of ovarian cancer among postmenopausal women.
What these results highlight is the fact that in the second half the study, the risk of ovarian cancer was statistically lower than in the intervention group (compared to the control group).
Actually, though, over the whole course of the study, the risk for ovarian cancer was not lower in the intervention group. So it seems what has gone on here is some ‘data dredging’ ” the practice of looking at the data long and hard enough and enough different ways to finally turn up something ‘positive’.
Two other things are noteworthy about how this study has been ‘spun’. Firstly, the authors of the study appear to have swept under the carpet the stunning failure of ‘healthy’ eating to reduce the risk of cancer. And, secondly, it looks like have actually attempted to present these dismal results in a positive light – .not just with regard to ovarian cancer, but cancer in general. Note again the part of the last line of the ‘results’ section of the abstract reads: The estimated risk of total invasive cancer was slightly lower in the intervention group than in the control group.
Note also, though, the rest of this line reads: (HR = 0.95, 95% CI = 0.89 to 1.01; P = .10). What this means is that the ‘slightly lower’ incidence of cancer in the intervention group was not statistically significant, and therefore is most likely due to chance. I’m wondering if such positive spin and misrepresentation of the facts doesn’t border on the unethical.
And here’s another thing, one major goal of healthy eating advice has to do with weight. The idea here is that cutting back on fat (particularly heart-stopping saturated fat and cholesterol) will promote weight loss or aid maintenance of a healthy weight. Other data from the WHI trial has showed that the average weight difference between the intervention and control groups was 0.4 kg (about a pound) after 7½ years . Imagine all that potential sacrifice and deprivation for years and years to find that, in terms of weight, it’s got you precisely nowhere.
How was this other failing of ‘healthy eating’ summarised? The ‘conclusion’ from this study reads: A low-fat eating pattern does not result in weight gain in post-menopausal women. That’s a funny way of putting it.
1. Prentice RL, et al. Low-fat dietary pattern and cancer incidence in the Women’s Health Initiative Dietary Modification Randomized Controlled Trial.
J Natl Cancer Inst. 2007;99(20):1534-43 [Epub Oct 9 2007]
2. Beresford SA, et al. Low-fat dietary pattern and risk of colorectal cancer: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial.
3. Prentice RL, et al. Low-fat dietary pattern and risk of invasive breast cancer: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial.
4. Howard BV, et al. Low-fat dietary pattern and weight change over 7 years: the Women’s Health Initiative Dietary Modification Trial.