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医学知识:喝牛奶真能强化骨骼吗?

更新时间:2019/5/15 21:10:10 来源:纽约时报中文网 作者:佚名

Should we drink milk to strengthen bones?
医学知识:喝牛奶真能强化骨骼吗?

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

有多少人小时候被教育说要多喝牛奶,因为牛奶能强化骨骼?

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

这个想法的确有些道理。牛奶含钙,而钙可以提升骨密度。

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.

可是要证明喝牛奶和骨骼强壮之间存在必然联系其实很难。理想状态下,研究要有两组人数众多的参试者,随机要求其中一组连续几十年每天都喝一定分量的牛奶,而另一组则饮用没有任何效果的安慰剂。显然,这样的研究难以实践。

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.

但我们可以找几千个人,问问他们这些年喝了多少牛奶,然后追踪他们10年以上,看看经常喝牛奶的人之后发生骨折的概率是否更低。

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.

哈佛大学1997年发表的研究就使用了这个方法。研究追踪了多达7.7万名女护士,时间长达10年。研究人员发现,每周喝一杯或不到一杯牛奶的人和每周至少喝两杯的人相比,手臂或髋部的骨折次数并无明显差异。

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.

研究小组还对33万名从事健康工作的男性进行了类似调查,依旧发现牛奶对骨折的概率似乎没有任何影响。

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.

对此还进行了不少随机对照试验,当中让其中一组采取高钙饮食——有时通过喝牛奶来摄取钙。2015年,新西兰的一个研究小组回顾、整合并重新分析了其中15项研究,发现头两年参试者的骨密度有所增加,但之后就停止了。

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.

也可以选择服用钙片。还是这个新西兰的研究小组,他们怀疑服用钙片会产生长期的副作用,于是汇总了51项随机对照试验的数据,以评估钙片的益处是否大于它的风险。他们再次发现,骨骼强度的增加在一两年后就停止了,钙片只能减缓年老时骨密度的降低,而无法阻止。他们认为,这很可能只会小幅降低骨折的概率。

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.

不同国家研究过相同的数据后,对每日钙摄入量的建议却差异很大。例如,美国的建议量几乎是英国和印度的两倍,推荐人们每天喝3杯8盎司(227毫升)的牛奶。

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.

让人困惑的还不止这些。2014年,瑞典两次大型调查的结果成了当时的头条新闻:每天喝3杯以上牛奶(大多数人不会喝这么多)对骨骼没有任何帮助,甚至可能对身体有害。

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.

在这项研究中,乌普萨拉大学(Uppsala University)和卡罗林斯卡学院(Karolinska Institute)的研究人员分别于1987年和1997年以问卷形式调查了人们的牛奶摄入量,2010年又对死亡率进行了调查。结果发现每天喝一杯牛奶似乎与骨折多发和早逝都有关,让人惊恐。

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

但在我们摒弃牛奶之前,有些重要的事情应该知道。

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.

瑞典这两次调查都是让参试者去估计前几年牛奶的摄入量,这可不容易。很难知道你吃麦片的时候加了多少牛奶,茶里或者食物里又有多少。

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.

这项研究还引出了那个老生常谈的问题,这之间是相关联系还是因果联系。女性如果知道自己患有骨质疏松症也许会特意多喝牛奶,希望能增强骨骼。研究并没有显示出饮用牛奶一定会导致骨折。瑞典的研究小组还发现,摄入奶酪和酸奶会降低骨折发生的概率,这让情况更复杂了。

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.

研究人员明确表示,他们的研究结果需要在反复证明后才能用来提供饮食建议。有人认为,公众如果因为这些研究结果而去改变喝牛奶的习惯,还需三思而后行。

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.

在掌握更多证据之前,现在想喝牛奶还可以接着喝。喝牛奶可能有益于骨骼健康,但是效果可能没有你所想象的长远。

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.

我们也应该通过其他方法来保持骨骼强壮,譬如进行锻炼和摄取足够的维生素D,可以从饮食和阳光中获得,或是在冬季服用营养品。

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