How many of us were told as children to drink our milk because it would give us strong bones?

The idea does make some sense. Milk contains calcium. Calcium is known to improve bone mineral density.

But demonstrating a definitive link between the consumption of milk and the possession of strong bones is harder than it sounds. The ideal study would take two large groups of people and randomly assign every member of one group to drink plenty of milk daily for several decades, while the other group would drink some kind of milk placebo instead. Obviously, this is too difficult to do in practice.  

What we can do instead is to take many thousands of people, ask them how much milk they’ve been drinking over the years, and then follow them for at least a decade to see whether the people who regularly drink milk are any less likely to suffer from broken bones later in life.

This is what happened in research published in 1997 conducted by Harvard University. An impressive 77,000 female nurses were followed for 10 years. The researchers found no significant difference in the numbers of arm or hip fractures between those who drank one glass of milk a week or less and those who drank two or more.

When the team did a similar study with 330,000 male health professionals, again milk didn’t seem to make a difference to fracture rates.

Randomised control trials have been conducted in which the diet is deliberately fortified with calcium, sometimes through drinking milk. In 2015 a team in New Zealand reviewed, combined and reanalysed 15 of these studies. They found that for two years there was an increase in the bone mineral density, but that after that time the increase stopped.  

An alternative is to take calcium supplements. Following fears of the risk of long-term side effects from taking the supplements, the same team in New Zealand combined the data from 51 randomised controlled trials to assess whether the benefits outweighed any risks. Again, they found that the increase in bone strength stops after a year or two, and that calcium supplements could only slow down –­­­­rather than halt – the loss of bone mineral density in old age. They concluded that this was likely to translate only to a small reduction in terms of fracture rates.  

When different countries have examined the same data, they have come to very different conclusions about their recommended daily intake of calcium. The US, for example, recommends almost twice as much the UK or India. In the US, the guideance has been for people to drink three 8oz (227ml) glasses a day.

To confuse things further, in 2014 came the results of two large Swedish studies which led to headlines that drinking more than three glasses of milk a day – a larger amount that most people drink – was no help to your bones and might even harm you.

For that study, researchers at Uppsala University and the Karolinska Institute gave people questionnaires about their milk consumption in 1987 and again in 1997. Mortality rates were examined in 2010. People were alarmed to hear that drinking a glass of milk a day appeared to be associated both with more broken bones, and with early deaths.  

But before we chuck away the milk, there are some big caveats.

In the Swedish studies, participants were required to estimate their milk consumption during the previous years, which is no easy task. It’s hard to know how much you eat with cereal, or in tea or in cooking.

The study also throws up the perennial problem of correlation versus causation. Perhaps women who knew they had osteoporosis deliberately drank more milk in the hope of strengthening their bones. The study didn’t show that drinking milk was definitely causing the fractures. And to complicate the picture further, the Swedish team found that cheese and yoghurt consumption was associated with lower fracture rates.

Swedish studies have shown that yoghurt consumption is linked to lower rates of bone breaks

The researchers themselves made it clear that their study would need to be replicated before it was used to give dietary advice. Others said the public should be cautious about changing their consumption based on these results.  

So until we know more, the current weight of evidence suggests that it is still OK to continue to drink milk if you like it. It probably does have benefits for bone health, albeit benefits which are shorter lived than you might have hoped.

It’s also worth keeping your bones strong through other methods such as exercise and getting enough vitamin D from your diet, from sunshine or (depending on where in the world you live) from supplements in winter.  (Read more about whether everyone should be taking vitamin D supplements).