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Attention milk-lovers: If you have had breast cancer, you may be better off steering clear of dairy fat.
A new study conducted by scientists at Kaiser Permanente, the large U.S. health insurer, and published in the Journal of the National Cancer Institute found that breast cancer survivors consuming high-fat cows’ milk foods had a higher risk of dying from breast cancer than women eating little to no high-fat dairy.
The researchers followed 1893 women diagnosed with early-stage invasive breast cancer for nearly 12 years. When they began the study, and at a follow-up survey six years later, the participants completed food frequency questionnaires indicating how much dairy they were consuming. At the end of the study the women who had consumed the most high-fat dairy foods (more than 1 serving/day) were found to have a 49% higher risk of dying from breast cancer than those who consumed the least high-fat dairy (less than half a serving per day).
It’s not the fat, it’s what's in the fat
The study has thrown much-needed light onto the long-running debate over the role of dairy in cancer. In the past 20 years more than a dozen studies have examined whether consuming dairy products is related to breast cancer, but the results have been confusing: Some studies found that women who ate a lot of dairy had a higher risk of breast cancer, others found a lower risk and still others found no effect at all.
The difference with the Kaiser study was that it didn’t set out to investigate the connection of dairy foods and cancer; instead, it was the first to look specifically at dairy fats. (Incidentally, the study found that overall dietary fats – including saturated fats – had little bearing on breast cancer mortality risk; only dairy fat seems to be the culprit here.)
What links dairy fat to breast cancer is likely to be the hormone estrogen. Much of the cow’s milk we drink today is produced from pregnant cows, whose estrogen and progesterone levels are markedly elevated. When humans consume milk from these cows, this can increase their own estrogen levels, creating an environment that is conducive to breast cancer but has also been linked to prostate, endometrial and ovarian cancers.
The Kaiser study, although eye-opening, has a few small limitations.
For one, it didn’t differentiate between cultured and fresh dairy. Some studies have shown that kefir – a type of fermented milk that tastes a bit like buttermilk – may actually reduce the risk of breast cancer (see this study and this one) and bladder cancer. The short-chain fatty acids produced by propionic bacteria in fermented dairy foods have also been shown to induce apoptosis (cell-death) in colorectal carcinoma cells and gastric cancer cells.
The cancer-protective mechanisms of cultured dairy foods aren’t clearly understood: are they brought about by the colonization of the gut by beneficial bacteria and yeasts in the kefir or yogurt? Do these bacteria somehow modify the estrogens in the milk during fermen-tation? Do they boost the immune system’s ability to fight cancer cells? Or do cultured dairy foods contain outright anti-tumor compounds? More research is needed.
Another limitation is that the study is based on food-frequency questionnaires (FFQ) completed only twice over nearly 12 years. FFQ’s – detailed questionnaires that ask you to recall and write down what you have eaten – are notoriously unreliable (as hilariously described by Michael Pollan in “Unhappy Meals” – scroll slightly more than half-way down the article).
For one, respondents may misunderstand a question and answer it wrongly. They may also struggle to recall what they have eaten (do you remember what you ate yesterday, let alone last week or last month? I don’t!). And – consciously or not – respondents often underestimate the amount of fats or sugars they have consumed, thus distorting outcomes.
The Kaiser study also did not differentiate between the milks of organically and conventionally reared cows. On the one hand, this is a pity, as organic milk has been shown to contain healthier fat composition and higher antioxidant levels.
On the other hand, it is unlikely that organic milk would have fared much better in this study: A Finnish research team recently found that the skimmed (!) organic milk it tested contained nearly seven times more equol (a type of estrogen) than non-organic milk, probably because the cows had been fed legumes that were rich in plant estrogens. While the role of equol in cancer isn't clear, this does give pause for thought.
Another study also found that there was little difference between organic and non-organic milks in terms of the levels of estrone and 17beta-estradiol, two other types of estrogen, and that pasteurization also had little effect on these.
So what’s a dairy eater to do?
In my work as a nutrition coach, my number-one rule is never to remove a food – especially as popular a food as cow’s milk products – without offering up alternatives to my clients.
In this case, there are various options. First, as the authors of the dairy-fat study suggest, there’s low-fat dairy; since most of the cow-pregnancy hormones are found in the fatty portion of the milk, it stands to reason that eating low-fat dairy foods should reduce our exposure to them.
One low-fat dairy food I am happy to recommend is plain kefir (for the reasons outlined above). It makes a refreshing drink and you can jazz it up by blending it with fresh fruit (raspberries are my favorite) and a smidgen of honey.
Unfortunately, most other low-fat dairy foods are best avoided. When I first moved to the U.S. from France, I was amazed to discover "milk" that had fat and lactose removed and calcium and vitamins A and D added to it, resulting in a watery, blueish liquid unlike anything that would naturally emerge from a cow's udder. It didn't taste so great either.
To cover up the less-than-satisfying tastes and textures of low-fat dairy products, food manufacturers add thickeners and sweeteners, fruit flavors and tiny bits of fruit to them. Just say "no" to these -- they're heavily processed foods with very few redeeming nutritional qualities.
Next, there are plant “milks,” such as soy, almond, hemp, rice or hazelnut milks. In small quantities, these are fine, but here are some caveats. Soy milk, for one, is itself a source of plant estrogens – albeit other types than the ones found in cow’s milk – and despite recent reassuring reports I remain cautious about recommending these. Moreover, non-organic soy milk is made from genetically modified soy beans and there is no data available on how safe these are.
I’m slightly happier with the other plant milks, though again there are caveats: they generally contain few nutrients and lots of sugar and/or carbohydrates, which can push our blood-sugar levels sharply higher. Rice milk, for instance, has a glycemic index (GI) reading of between 79 and 92 and a glycemic load (GL) between 17 and 29, according to the University of Sydney’s GI database -- much too high to be consumed by the glass.
Moreover, plant milks are almost always sold in cartons lined with plastic containing estrogen-like compounds that are thought to leach into the milk we drink. Lastly, they often contain an additive called carageenan, a seaweed extract that is thought to cause gastrointestinal inflammation and higher rates of intestinal lesions, ulcerations, and even malignant tumors. So unless you make your own plant milk from scratch, you’re probably best advised to minimize their intake.
Does this mean we should swear off milk altogether? Not necessarily. This brings me – like a broken record – back to the traditional Mediterranean Diet.
For millennia, dairy around the Mediterranean basin was consumed sparingly and almost always in the form of fermented milk curds – such as yogurt, kefir, labneh – and fresh goat’s and sheeps’ cheeses such as feta cheese, halloumi, ricotta and fresh chèvre (log-shaped goat’s cheese). Cow’s milk was rare as the terrain generally did not lend itself to such large, thirsty animals; instead, goats and sheep were better adapted to the rocky, arid landscapes that line the Mediterranean shores and prized not only for their milk but also their meat and – in the sheeps’ case – wool.
Interestingly, in a recent study the levels of estrone and 17β-estradiol in goat's milk were found to be significantly lower than in any of the cow's milk products tested – including organic cow's milk. (I have not been able to find similar data on sheep's milk.) Thus, the occasional goat's milk yogurt or small piece of goat’s cheese are acceptable, but until there is more research, don’t go overboard on these either.
Finally, if you choose not to eat any dairy at all, this should pose no health risk at all. After all, we are the only mammals that continue to consume milk long after we’re weaned – and another species’ milk at that! Indeed, in many of us, the ability to digest lactose (the sugar in milk) begins to decline from the age of two, the age at which, in nature, we might begin transitioning from mothers' milk to solid foods. Might this be Nature’s way of telling us to lay off milk after infancy?
Many people eat dairy foods because they think they need them to maintain bone strength; but rest assured: in many parts of the world – much of Africa and Asia, for instance – milk is not consumed at all and people’s bones do not crumble! Green leafy vegetables, legumes and seeds, bone-in fish (e.g. canned salmon or sardines) and chicken, beef or fish broth are excellent sources of calcium.
So once again, Mediterranean wisdom prevails: if you eat dairy foods, do so sparingly, choose minimally processed ones made from the milk of pasture-fed, organically-reared animals that did not receive hormone treatments, opt for fermented milk wherever possible, and vary your milk source, rotating goat's, sheep's and occasional cow's milk products.
(c) Copyright Conner Middelmann-Whitney. Conner is a nutrition coach, health writer and cooking instructor based in Boulder, Colorado. She is the author of Zest for Life: The Mediterranean Anti-Cancer Diet and publishes Everyday Mediterranean, a weekly meal-plannign and recipe service for people who want to cook more meals from scratch. Conner's recipes are anchored in the traditional Mediterranean diet but adapted to modern lifestyles, making this way of eating accessible even to those who have little cooking experience or time. For more information about Conner's services, visit her website: www.nutrelan.com.
