What's the truth about the risk of drinking milk?
Women have long been encouraged to drink milk, but a study now links milk drinking to premature death. Norman Swan gets to the bottom of the data.
Those who saw the headlines late last year while sipping their latte must have choked when they read that male and female milk drinkers had an increased risk of dying prematurely. After decades of women being exhorted to drink more milk to protect their bones, the study found that milk drinking was associated with an increased risk of bone fractures.
They are the kind of findings that make you think, ‘why bother changing my lifestyle if one day the so-called experts are going to recant and decide that what they thought was good for me is now bad and vice versa?’
So should you and I throw out the lattes in favour of espressos and long blacks and change to soy milk on our morning Weeties?
Well, let’s start with a declaration of interest. I am heavily invested – by dint of addiction – to a morning kick-start of a double shot flat white and it’s going to take a lot to prise that away from me.
The study, published in the British Medical Journal, was what’s called a cohort design. That’s where (usually) large numbers of people taken from the general population are interviewed, surveyed, tested and followed for years to see what behaviours or factors are linked to increased or decreased risks of various diseases. As far as evidence goes, cohort studies are OK, but they can only observe statistical associations and don’t nail cause and effect. For that you need randomised controlled trials. And there aren’t randomised trials of milk drinking. The other challenge with cohort studies is that if you want to see what effect foods such as milk or dairy products have, you need to eliminate the influence of factors such as education, physical activity and other dietary intakes. The statistics to do that are imperfect. It’s impossible to control for these so-called confounders.
There were two Swedish cohorts in this study, one of 61,000 women and the other of 45,000 men, mostly recruited in middle age and followed for 20 years in the case of the women and 11 years for the men. The researchers gathered huge amounts of detailed data from both groups and found that for the women, the risk of dying prematurely almost doubled when they compared those who drank three or more glasses of milk per day to those who consumed less than a glass. Each added glass of milk raised the risk of early death by 15%. That’s important because in observational studies such as this, a dose effect strengthens the plausibility of the finding. In other words, the more you consume, the higher the risk. Men also increased their risk of premature death but less so than women. It wasn’t just that the big drinkers were fatter and that counteracted the benefits of milk: those with similar levels of obesity fared worse if they were milk drinkers.
There was no bone fracture reduction, but a small increase among women at the high end of the milk intake. They also found evidence of a link between milk intake and the type of activation of the immune system that causes inflammation. The good news was that processed dairy products such as fermented milk, yoghurt and cheese were linked to reduced death and fracture rates.
Because these are only observations, you need to have a biological explanation that supports the findings. The one given by the authors relates to a sugar called D-galactose, much of which comes from lactose, a molecule made from glucose and D-galactose. In animal studies of ageing – and brain ageing in particular – D-galactose has long been used to cause premature ageing through increased inflammation and oxidative stress. D-galactose, say the authors, is at far lower levels in fermented dairy products, which might explain their protective effects.
The authors say the findings are not conclusive because they can’t be sure they controlled for all the other factors in these people’s lives. And other studies don’t necessarily agree with them. Recent results from Asian cohorts show benefits from drinking milk. On the other hand, the evidence on cheese and yoghurt reducing fracture risks is pretty solid.
The take away – so to speak – for me is that I’ll still have my double shot flat white in the morning and my bone health will continue to be helped by that nice unsweetened Greek-style yoghurt and the odd bit of cheese.
What I won’t be doing is converting to soy lattes.
I’d rather die sooner.