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Diet and Breast Cancer Information
Part 2: Polyunsaturated fats and breast cancer
In 1991, two studies, from USA[i] and Canada,[ii] found that linoleic acid increased the risk of breast cancers. Experiments with a variety of fats showed that saturated fats did not cause breast cancer but, when small amounts of polyunsaturated vegetable oil or linoleic acid itself was added, this greatly increased the promotion of breast cancer.
In 1996 a case control study of over 5,000 Italian women looking at diet and breast cancer was published in Italy.[iii] Dr Sylvia Franceschi and her team were assessing the influence of high intakes of fat and other macronutrients on breast cancer risk. In this study, women with the highest intake of fat had a significantly less breast cancer. The authors conclude: 'The risk of breast cancer decreased with increasing total fat intake . . . the intakes of saturated fatty acids . . . were not significantly associated with breast cancer risk'. In the same year, Hunter also 'found no evidence of a positive association between total dietary fat intake and the risk of breast cancer. There was no reduction in risk even among women whose energy intake from fat was less than 20 percent of total energy intake. In the context of the Western lifestyle, lowering the total intake of fat in midlife is unlikely to reduce the risk of breast cancer substantially.'[iv]
So eating a lot of fat protected against breast cancer ? but, it seems, only if those fats were the right sort. A study of 61,471 women aged 40 to 76, conducted in Sweden, looked into the relation of different types of fat and breast cancer. The results were published in 1998. Like those before it, this study found 'no positive association between intake of total fat and risk of invasive breast cancer'. What it did find, however, was that polyunsaturated fats found in vegetable oils and margarines increased the risk of breast cancer and monounsaturated fats found in animal fats and olive oil protected against breast cancer. Saturated fats were neutral.[v]
In 2003, two studies which purported to show that an increasing intake of animal fat increased the risk of breast cancer were published just two days apart. The first was an American study published on 16 July in the Journal of the National Cancer Institute; the second out on 18 July in the British medical journal, Lancet. I must confess, when I read them both, I was decidedly underwhelmed by the arguments.
The American study of 90,655 women aged 26 to 46, conducted by the Harvard Medical School was reported in the American press with the headline: 'Animal Fats Linked to Increased Breast Cancer Risk, Study Finds.' It reported that a diet high in animal fat raised the risk of breast cancer by as much as 54%. But that's not what the figures showed.[vi]
The women were divided into five 'quintiles' depending on their fat intakes: the first quintile eating the least fat and the fifth quintile the most. But there are no absolute risk figures. Without these, the findings are misleading. However we can work out the numbers of women who did not get breast cancer from the study's data for each quintile: 99.3, 99.2, 99.2, 99.1, 99.3. Thus the number of women who did not get breast cancer was 99.3% in both the first and the fifth quintiles. If eating animal fat increased the risk of breast cancer, you would expect that the more animal fat is eaten, the more breast cancer there would be. But there was no trend of increasing cancer as fat intake increased in this study, and it's not the case in world populations.
Another thing that is strange about this study is that this group published another paper only 4 months earlier in which they were unable to find a breast cancer risk for red meat intake.[vii] In that study, they 'found no evidence that intake of meat or fish during mid-life and later was associated with risk of breast cancer.' It refutes the above study. Interestingly this study isn't even referred to in the study above.
The second 'animal fat causes breast cancer' study wasn't really looking at that at all. It was a study about the way data are collected, as its title: Are imprecise methods obscuring a relation between fat and breast cancer?, suggests.[viii] What it really said, in essence, is 'We "know" that saturated animal fat causes breast cancer, but we can't prove it in trials. So let's see if we can manipulate the figures to get a result that is acceptable'. This is, I imagine, because all trials published so far ? and there have been a lot ? have found that animal fats do not cause breast cancer, only vegetable fats do, but that finding isn't politically correct. So, now, this team looked at the way data are gathered to see if they could spin their findings to show what they want to see.
They compared a food frequency questionnaire, the usual way to gather information, with a food diary which, they say, is very much more reliable. Using this method they studied 25,630 men and women to look for effects of fat intake and breast cancer.
They, too, split fat intakes into quintiles. There were 168 cases of breast cancer between January 1993 and September 2002 in participants who had completed both dietary methods. Unfortunately, this study doesn't break them down into how many ate how much fat, as the Harvard study did. However, 168 cases out of over 25,000 people in 10 years is not very many on which to base reliable findings. And, again the data show clearly that the ones who ate the most fat had less breast cancer than those who ate less. The numbers with breast cancer in the two highest intakes (fourth and fifth quintiles) was less than the numbers of breast cancers in third. So they failed in their attempt to manipulate the data.
These two studies tried to contradict not only all other studies of fats and breast cancer produced to date but our entire evolutionary history: breast cancer has really only 'taken off' in the last century, yet we have been eating animal fats for millions of years.
References
[i]. Carroll K K. Dietary fats and cancer. Am J Clin Nutr 1991; 53: 1064S.
[ii]. France T, Brown P. Test-tube cancers raise doubts over fats. New Scientist, 7 December 1991, p 12.
[iii]. Franceschi S, et al. Intake of macronutrients and risk of breast cancer. Lancet 1996; 347: 1351-6
[iv]. Hunter DJ, et al. Cohort studies of fat intake and the risk of breast cancer ? a pooled analysis. New Engl J Med 1996; 334:356-61
[v]. Wolk A, et al. A Prospective Study of Association of Monounsaturated Fat and Other Types of Fat With Risk of Breast Cancer. Arch Intern Med 1998; 158: 41-45.
[vi]. Cho E, Spiegelman D, Hunter DJ, et al. Premenopausal fat intake and risk of breast cancer. J Natl Cancer Inst 2003; 95: 1079?1085.
[vii]. Holmes MD, et al. Meat, fish and egg intake and risk of breast cancer. Int J Cancer 2003; 104: 221-7.
[viii]. Bingham SA, et al. Are imprecise methods obscuring a relation between fat and breast cancer? Lancet 2003; 362: 212-14
