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How many children among us did they learn to drink our milk because it would give us strong bones?
The idea makes sense. The milk contains calcium. Calcium is known to improve bone mineral density.
But demonstrating that there is a definite link between milk consumption and possession of solid bones is more difficult than it appears. The ideal study would take two large groups of people and randomly assign each member of a group to drink a lot of milk daily for several decades, while the other group would instead drink some kind of placebo. Obviously, it's too hard to do in practice.
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What we can do instead is take several thousand people, ask them how much milk they have drunk over the years, and then track them for at least a decade to see if people who regularly drink milk are less likely to suffer fractures later in life.
That's what happened in research published in 1997 by Harvard University. An impressive 77,000 nurses were followed for 10 years. The researchers found no significant difference in the number of arm or hip fractures between those who drank a glass of milk a week or less and those who drank two or more.
When the team conducted a similar study with 330,000 health professionals, the milk did not appear to have affected the fracture rate.
Randomized controlled trials have been conducted in which the diet is deliberately enriched with calcium, sometimes through the consumption of milk. In 2015, a New Zealand team reviewed, combined and reanalyzed 15 of these studies. They found that the bone mineral density had increased for two years, but that increase had stopped after this period.
Drinking a glass of milk a day seemed to be associated with both more fractures and premature death
An alternative is to take calcium supplements. Following fears of the risk of long-term side effects from taking supplements, the same team in New Zealand combined data from 51 randomized controlled trials to determine if the benefits outweighed the risks. . Again, they found that the increase in bone strength stopped after a year or two and that calcium supplements could only slow, rather than stop, the loss of bone mineral density in people. elderly. They concluded that this would probably only result in a slight reduction in fracture rates.
When different countries looked at the same data, they came to very different conclusions about their recommended daily calcium intake. The United States, for example, almost recommend double the United Kingdom or India. In the United States, people are advised to drink three 8-ounce (227 ml) glasses a day.
To make things even more confusing, in 2014, the results of two large Swedish studies revealed that drinking more than three glasses of milk a day – a larger amount than most people drink – was of no use to your bones and could even hurt you.
For this study, researchers from the University of Uppsala and the Karolinska Institute gave people questionnaires about their milk consumption in 1987 and again in 1997. Mortality rates were were examined in 2010. People were alarmed to learn that it was associated with drinking a glass of milk a day. both with more broken bones, and with an early death.
Milk probably has beneficial effects on bone health even though they have a shorter shelf life than you might have hoped for.
But before throwing away the milk, there are big warnings.
In Swedish studies, participants had to estimate milk consumption in previous years, which is not an easy task. It's hard to know how much you eat with cereals, tea or cooking.
The study also raises the recurring problem of correlation versus causality. Women who knew they were suffering from osteoporosis may have deliberately drank more milk in the hope of strengthening their bones. The study did not show that milk consumption was definitely the cause of the fractures. And to further complicate the situation, the Swedish team discovered that cheese and yoghurt consumption was associated with lower fracture rates.
The researchers themselves have made it clear that their study should be replicated before being used for dietary advice. Others said that the public should be cautious in changing their consumption based on these results.
So until we know more, the current weight of evidence suggests that it's always ok to continue drinking milk if it suits you. It probably has bone health benefits, even if they have a shorter life span than you might have hoped for.
It's also worth keeping your bones in shape with other methods, such as exercise and adequate vitamin D, sun or (depending on the region where you live) as supplements in winter . (Learn more about whether everyone should take vitamin D supplements).
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Warning
All content in this column is provided as general information only and should not be considered a substitute for the medical advice of your own physician or other health professional. The BBC is not responsible for the diagnoses made by a user based on the content of this site. The BBC is not responsible for the content of the external websites listed, nor does it recommend any commercial product or service mentioned or advised on these sites. Always consult your doctor if you are concerned about your health.
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