milk & health: there aren’t always two (equal) sides to a story

 I had another learning experience down at the gym this afternoon. There I was, happily pedalling away on the exercycle (I believe in varying my cardio, otherwise it gets boring!) & reading a fitness magazine (what else?) when I came across an article on whether or not drinking/eating dairy products is bad for you.

It started out with comments from dieticians to the effect that ‘lactose intolerance’ tends to be self-diagnosed, which probably over-inflates estimates of the actual prevalence of this problem. (From a biological perspective, it should be less common among those of European & perhaps African descent, something that’s related to the repeated ‘discovery’ of dairy farming around 9,000 years ago.)

The article then gave gave another point of view, with a nutritionist commenting that milk today is quite different from what it would have been like 100 years ago, in the sense that animals are farmed more intensively & with greater use of various pharmaceuticals, which are likely to come through into the human diet. She also noted, in a rather shocked tone, that much of the milk comes from pregnant cows, so it likely has higher levels of oestrogens & other pregnancy-related hormones. The implication was that this could be linked to various cancers in humans.

What was the evidence for this? The article tried to be even-handed, looking at information from both sides (milk causes cancer/doesn’t cause cancer). For the ‘no cancer link’ side it cited a study of around 9,000 women, published in a research journal, which found no correlation, let alone causal relationship, between women’s dairy intake & the incidence of breast cancer. Because it was the gym, I didn’t have pen & paper handy to take down the details, but I’m fairly sure it was a 2002 paper by M-H Shin & colleagues, which concludes: We found no association between intake of dairy products and breast cancer in postmenopausal women. Among premenopausal women, high intake of low-fact dairy foods… was associated with reduced risk of breast cancer. 

Searching some more I found this paper (Knektl et al. 1996), & this one (Parodi, 2005), both of which present data supporting the conclusions of Shin et al.. Parodi (2005) also points out that the amount of hormones taken in via dairy products is extremely small compared to a woman’s own endogenous hormone production: about 0.05µg/day from dairy intake against up to 1mg/day in pre-menopausal women & between 40 & 200µg/day in post-menopausal women.

On the ‘milk is implicated in cancer’ side we got a paper in the journal Medical Hypotheses. The paper looks at apparent correlations between diet & the incidence of various cancers, including breast cancer, & suggests that hormones in milk may be implicated in cancer . However, correlation is not the same as causation, & while the suggestion that cows’ milk contributes to some cancers due to its high hormone titre is an interesting hypothesis, again there is no direct evidence presented in support of this. To counter this argument, as noted by Parodi the hormone contribution from dairy products is insubstantial compared to that produced within the body.

The problem I have with the oriiginal magazine article is that it presented both sources as of equal importance & validity. And they’re not. The first three papers I’ve linked to (including the one cited by the article) are from peer-reviewed journals & they’re evidence-based ie they contain data from fairly large cohorts of patients. Medical Hypotheses, as I’ve commented before, isn’t peer-reviewed and the papers it contains are often published because they offer provocative hypotheses. In this case the hypothesis – based on data on cancer rates & diets, but not examining particular cohorts of patients – is an interesting one but the apparent correlations need to be examined in a lot more depth.Sometimes there really aren’t two equal sides to a story.

D.Ganmaa & A.Sato (2005) The possible role of female sex hormones in milk from pregnant cows in the development of breast, ovarian and corpus uteri cancers. Medical Hypotheses 65(6): 1028-1037. doi: 10/1016/j.mehy.2005.06.026

P.Knektl, R.Jarvinen, R.Seppinen, E.Pukkala & A.Aromaal (1996) Intake of dairy products and the risk of breast cancer. British Journal of Cancer 73:687-691

P.Parodi (2005) Dairy product consumption and the risk of breast cancer. Journal of the American College of Nutrition 24: 556S-568S

M-H.Shin, M.D.Holmes, S.E.Hankinson, K.Wu, G.A.Colditz & W.C.Willett (2002) Intake of dairy products, calcium and vitamin D and risk of breast cancer. Journal of the National Cancer Institute 94(17): 1301-1310. doi: 10.1093/jcni/94.17.1301

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