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为什么人类要演化出忍受牛奶的能力

更新时间:2019/5/4 9:23:30 来源:纽约时报中文网 作者:佚名

Why humans have evolved to drink milk
为什么人类要演化出忍受牛奶的能力

Dairy milk has competition. Alternative “milks” made from plants like soya or almonds are increasingly popular. These alternatives are often vegan-friendly and can be suitable for people who are allergic to milk, or intolerant of it. The runner-up in the 2018 series of The Apprentice (UK) ran a flavoured nut milk business.

乳制品有了竞争对手,由大豆、杏仁等植物制成的替代“牛奶”越来越受欢迎。这些替代品通常适合素食主义者,以及对牛奶过敏或者乳糖不耐受的人。2018年,英国版《学徒》(The Apprentice)节目的亚军,就经营着调味坚果奶生意。

But the rise of alternative milks is just the latest twist in the saga of humanity’s relationship with animal milk. This relationship dates back thousands of years, and it has had a lot of ups and downs.

但替代奶的兴起,只是人类与动物奶关系的一个最新发展。这种关系可以追溯到几千年前,历经起伏。

When you think about it, milk is a weird thing to drink. It’s a liquid made by a cow or other animal to feed its young; we have to squirt it out of the cow’s udders to obtain it.

想想看,奶水真是一种奇怪的饮品。它是牛或者其他动物喂养幼崽的液体食物,为了获得牛奶,我们还得把它从母牛的乳房里给挤出来。

In many cultures it is almost unheard of. Back in 2000, China launched a nationwide campaign to encourage people to consume more milk and dairy products for health reasons – a campaign that had to overcome the deep suspicions of many older Chinese people. Cheese, which is essentially milk that has been allowed to go off, can still make many Chinese people feel sick.

在许多文化中,这几乎是闻所未闻的事情。2000年时,全中国范围内发起一场运动,鼓励人们为了健康要多消费牛奶和各种奶制品。这项运动主要针对老年人,帮助他们克服对奶制品的极大疑虑。牛奶变质形成的奶酪,仍然让很多中国人感到恶心。

Set against the 300,000-year history of our species, drinking milk is quite a new habit. Before about 10,000 years ago or so, hardly anybody drank milk, and then only on rare occasions. The first people to drink milk regularly were early farmers and pastoralists in western Europe – some of the first humans to live with domesticated animals, including cows. Today, drinking milk is common practice in northern Europe, North America, and a patchwork of other places.

与人类30万年的历史相比,喝牛奶算是相当新的习惯。大约1万年前,人类几乎不喝牛奶,然后是偶尔喝一喝。第一批经常喝牛奶的是西欧早期的农民和牧民,他们是第一批与驯养动物生活在一起的人,其中就包括奶牛。今天,喝牛奶在北欧、北美和其他一些地方,已经很普遍。

Baby food

婴儿食品

There is a biological reason why drinking animal milk is odd.

喝动物奶为什么奇怪,在生物学上是有原因的。

Milk contains a type of sugar called lactose, which is distinct from the sugars found in fruit and other sweet foods. When we are babies, our bodies make a special enzyme called lactase that allows us to digest the lactose in our mother’s milk. But after we are weaned in early childhood, for many people this stops. Without lactase, we cannot properly digest the lactose in milk. As a result, if an adult drinks a lot of milk they may experience flatulence, painful cramps and even diarrhoea. (It’s worth noting that in other mammals, there aren’t any lactase-persistent adults – adult cows don’t have active lactase, and neither do cats or dogs, for example).

牛奶所含的乳糖,不同于水果和其他甜食中的糖。当我们还是婴儿的时候,我们身体会产生一种特殊的酶,这种叫做乳糖酶的东西能帮助我们消化母乳中的乳糖。但在婴儿期断奶后,许多人身上就没有这种酶了,就不能很好地消化牛奶中的乳糖。因此,一个成年人喝了很多牛奶后,可能会感到肠胃胀气、疼痛痉挛,甚至腹泻。值得注意的是,其他成年哺乳动物体内也没有活跃的乳糖酶,成年奶牛没有,猫狗也没有;但成年动物并不喝奶,所以也没有乳糖不耐受的问题。

So the first Europeans who drank milk probably farted a lot as a result. But then evolution kicked in: some people began to keep their lactase enzymes active into adulthood. This “lactase persistence” allowed them to drink milk without side effects. It is the result of mutations in a section of DNA that controls the activity of the lactase gene.

最早一批喝牛奶的欧洲人可能因此胀气、多屁。然后演化开始了,一些人身上的乳糖酶开始保持至成年,这种“乳糖耐受能力”使他们喝牛奶不会产生副作用。这是控制乳糖酶基因活性的DNA片段突变的结果。

“The first time that we see the lactase persistence allele in Europe arising is around 5,000 years BP [before present] in southern Europe, and then it starts to kick in in central Europe around 3,000 years ago,” says assistant professor Laure Ségurel at the Museum of Humankind in Paris, who co-authored a 2017 review of the science of lactase persistence.

“我们所知的欧洲最早的乳糖耐受能力等位基因,出现在大约5000年前的欧洲南部,然后在大约3000年前,出现在中欧,”巴黎人类博物馆(Museum of Humankind)的助理教授赛格瑞尔(Laure Ségurel)说。2017年,她与人合著了一篇关于乳糖耐受科学的评论文章。

The lactase persistence trait was favoured by evolution and today it is extremely common in some populations. In northern Europe, more than 90% of people are lactase persistent. The same is true in a few populations in Africa and the Middle East.

随着演化,人类的乳糖耐受能力明显提高,今天在一些人群中,乳糖酶极为常见。在北欧,超过90%的人具有乳糖耐受能力。在非洲和中东的部分人口中,也是如此。

But there are also many populations where lactase persistence is much rarer: many Africans do not have the trait and it is uncommon in Asia and South America.

但在许多地区,乳糖耐受能力要罕见得多:许多非洲人不具备这种特性,在亚洲和南美洲也很少见。

It is hard to make sense of this pattern because we don’t know precisely why drinking milk, and therefore lactase persistence, was a good thing, says Ségurel: “Why was it so strongly advantageous in itself?”

这种状况让人难以理解,我们并不确切知道喝牛奶有什么好处,因此对乳糖耐受的重要性也不太了解,赛格瑞尔说:“为什么有乳糖耐受能力的人有如此大的优势?”

The obvious answer is that drinking milk gave people a new source of nutrients, reducing the risk of starvation. But on closer inspection this doesn’t hold up.

答案显而易见,牛奶为人们提供了一个新的营养来源,减少了饥饿的风险。但仔细观察,这种说法似乎站不住脚。

“There’s a lot of different sources of food, so it’s surprising that one source of food is so important, so different from other sorts of food,” says Ségurel.

“食物来源多种多样,所以某一种食物如此重要,是其他种类食物不能代替的,这是让人惊讶的事情,”赛格瑞尔说。

People who are lactase-non-persistent can still eat a certain amount of lactose without ill effects, so drinking a small amount of milk is fine. There is also the option of processing milk into butter, yoghurt, cream or cheese – all of which reduce the amount of lactose. Hard cheeses like cheddar have less than 10% as much lactose as milk, and butter is similarly low. “Heavy cream and butter have the lowest lactose,” says Ségurel.

乳糖不耐受的人仍然可以吃一定量的乳糖,不会产生不良后果,所以喝少量的牛奶是没问题的。还有一个办法,就是将牛奶加工成黄油、酸奶、奶油或者奶酪,这些食品中的乳糖含量都较少。像切达干酪等硬奶酪,乳糖含量不到牛奶的10%;黄油的乳糖含量也很低。帕尔马干酪(Parmigiano)是一种硬质的奶酪,乳糖含量极低,乳糖不耐受的人完全可以吃。“高脂肪的奶油和黄油的乳糖含量最低,”赛格瑞尔说。

Accordingly, people seem to have invented cheese rather quickly. In September 2018, archaeologists reporting finding fragments of pottery in what is now Croatia. They carried fatty acids, suggesting that the pottery had been used to separate curds from whey: a crucial step in making cheese. If that is correct (and the interpretation has been questioned), people were making cheese in southern Europe 7,200 years ago. Similar evidence from slightly more recent times, but still more than 6,000 years ago, has been found elsewhere in Europe. This is well before lactase persistence became common in Europeans.

所以,古人们很快就发明了奶酪。 2018年9月,考古学家在克罗地亚地区发现了一些陶器碎片。这些碎片上带有脂肪酸,表明这些陶器曾经用于分离凝乳和乳清,这是制作奶酪的关键步骤。如果确实无误的话(这点受到质疑),南欧人在7200年前就开始制作奶酪了。在欧洲其他地方,也发现了类似的证据,虽年代稍晚,但也在大约6000多年前。所以,当乳糖耐受尚未在欧洲普遍的程度,人们就懂得制作奶酪了。

That said, there is clearly a pattern behind which populations evolved high levels of lactase persistence and which didn’t, says genetics professor Dallas Swallow of University College London. Those with the trait are pastoralists: people who raise livestock. Hunter-gatherers, who do not keep animals, did not acquire the mutations. Neither did “forest gardeners” who cultivated plants, but not livestock.

伦敦大学学院(University College London)的遗传学教授斯沃洛(Dallas Swallow)说,即便如此,能否进化出高水平的乳糖耐受能力,显然受特定因素的影响。最受影响的是牧民,也就是饲养牲畜的人。狩猎者、食物采集者都不养动物,就没有获得这种基因突变的条件。所谓“森林园丁”,就是种植农作物的人,身体也没有出现这种突变。

It makes sense that people who did not have access to animal milk were not under great evolutionary pressure to adapt to drinking it.

那些不会接触到动物奶水的人,没有巨大的演化压力来消化动物奶,这是可以理解的。

The question is, why did some pastoralist people acquire the trait and not others?

问题是,为什么有些牧民获得了这种特性,有些人则没有?

Ségurel points to east Asian herding peoples, such as those in Mongolia, who have some of the lowest rates of lactase persistence even though they rely heavily on milk from their animals for food. The mutations were common in nearby populations in Europe and western Asia, so it would have been possible for them to spread into these east Asian groups, but they didn’t. “That’s the big puzzle,” says Ségurel.

赛格瑞尔指出,东亚的游牧民族,比如蒙古人,虽然严重依赖动物奶水为食,乳糖耐受能力却最低。基因突变在欧洲和西亚人群中很常见,所以应该有可能传播到临近的东亚人群中,但事实上并没有。“这是个很大的谜团,”赛格瑞尔说。

Dairy benefits

乳制品的好处

She speculates that drinking milk might have other advantages besides its nutritional value. People who keep livestock are exposed to their diseases, which can include anthrax and cryptosporidiosis. It may be that drinking cow’s milk provides antibodies against some of these infections. Indeed, milk's protective effect is thought to be one of the benefits of breastfeeding children.

她推测,牛奶除了营养价值外,还有其他好处。饲养牲畜的人容易感染疾病,包括炭疽热和隐孢子虫病。喝牛奶可能提供抵抗这些病菌感染的抗体。的确,奶水的保护作用同母乳喂养的好处差不多。

But some of the mysterious absences of lactase-persistence could be down to sheer chance: whether anyone in a group of pastoralists happened to get the right mutation. Until fairly recently there were a lot fewer people on Earth and local populations were smaller, so some groups would miss out by plain bad luck.

有些人群乳糖耐受能力不足,有些不可思议,但可能是偶然因素决定,即一群牧民中是否有人碰巧发生了这样的基因突变。地球人口爆炸是近代才发生的事,此前,地区族群人口非常少,所以有些群体会因为运气不好而错过机会。

“I think the most coherent part of the picture is that there’s a correlation with the way of life, with pastoralism,” says Swallow. “But you have to have the mutation first.” Only then could natural selection go to work.

“我认为,演化过程中最有逻辑的部分,是人群乳糖耐受能力与生活方式及畜牧生产方式有关,”斯沃洛说。“但你必须得有突变。”只有这样,自然选择才能发挥作用。

In the case of Mongolian herders, Swallow points out that they typically drink fermented milk, which again has a lower lactose content. Arguably, the ease with which milk can be processed to be more edible makes the rise of lactase persistence even more puzzling. “Because we were so good at adapting culturally to processing and fermenting the milk, I’m struggling with why we ever adapted genetically,” says Swallow’s PhD student Catherine Walker.

斯沃洛指出,蒙古牧民通常喝的是发酵奶,这种奶的乳糖含量很低。可以说,为了更易食用而对牛奶进行加工,这并不费力,这也使乳糖耐受的进化变得很难解释。“我们非常善于从文化上适应牛奶的加工和发酵,我也在试图搞清楚为什么我们会在基因上适应它,”斯沃洛指导的博士生沃克(Catherine Walker)说。

There may have been several factors promoting lactase persistence, not just one. Swallow suspects that the key may have been milk’s nutritional benefits, such as that it is rich in fat, protein, sugar and micronutrients like calcium and vitamin D.

促成乳糖耐受的因素可能有好多。斯沃洛认为,最关键的可能是牛奶的营养价值,比如它富含脂肪、蛋白质、糖,以及钙、维生素D等微量元素。

It is also a source of clean water. Depending on where your community lived, you may have evolved to tolerate it for one reason over another.

牛奶也是洁净的水源。特定人群可能由于某种原因,不断演化,直到可以容忍它,具体哪个原因要看生活的地区。

It’s unclear whether lactase persistence is still being actively favoured by evolution, and thus whether it will become more widespread, says Swallow. In 2018 she co-authored a study of a group of pastoralists in the Coquimbo region of Chile, who acquired the lactase-persistence mutation when their ancestors interbred with newly-arrived Europeans 500 years ago. The trait is now spreading through the population: it is being favoured by evolution, as it was in northern Europeans 5,000 years ago.

斯沃洛说,目前还不清楚,乳糖耐受能力是否仍然在不断进化提升过程中,是否更多族群会拥有这样的能力。2018年,她与人合写了一份研究报告,对象是智利科金博地区(Coquimbo)的牧民。500年前,这个族群与新来的欧洲人出现跨种族生育,混血儿获得了乳糖酶耐受能力。这种特性现正在族群间传播,在演化中变得显著,就像5000年前北欧人一样。

But this is a special case because the Coquimbo people are heavily reliant on milk. Globally, the picture is very different. “I would think it’s stabilised myself, except in countries where they have milk dependence and there is a shortage [of other food],” says Swallow. “In the West, where we have such good diets, the selective pressures are not really likely to be there.”

但这是一个特例,因为科金博人严重依赖牛奶。在全球其他地区,情况则非常不同。“除了那些依赖牛奶、其他食物短缺的国家可能还有一些这类进化,除此以外,我认为已经基本停滞了。”斯沃洛说。“在西方,我们有这么好的饮食来源,基本不存在选择压力。”

Dairy decline?

奶制品的衰落

If anything, the news over the last few years offers the opposite impression: that people are abandoning milk. In November 2018, the Guardian published a story headlined “How we fell out of love with milk”, describing the meteoric rise of the companies selling oat and nut milks, and suggesting that traditional milk is facing a major battle.

如果有什么不同的话,那就是过去几年里,新闻给人们留下了正好相反的印象。2018年11月,《卫报》(Guardian)发表了一篇题为《我们如何失去了对牛奶的爱》的文章,描述了销售燕麦和坚果牛奶的公司迅速崛起,并暗示传统牛奶将面临严峻挑战。

But the statistics tell a different story. According to the 2018 report of the IFCN Dairy Research Network, global milk production has increased every year since 1998 in response to growing demand. In 2017, 864 million tonnes of milk were produced worldwide. This shows no sign of slowing down: the IFCN expects milk demand to rise 35% by 2030 to 1,168 million tonnes.

但统计数据却提供了不同的说法。根据IFCN乳制品研究网(IFCN Dairy Research Network)2018年的报告,自1998年以来,全球牛奶产量每年都在增长,以满足不断增长的需求。2017年,全球牛奶产量为8.64亿吨。这一趋势毫无放缓的迹象,据IFCN预计,到2030年,牛奶产量将增长35%,达到11.68亿吨。

Still, this masks some more localised trends. A 2010 study of food consumption found that in the US milk consumption has fallen over the last few decades – although it was replaced with fizzy drinks, not almond milk. This fall was balanced by growing demand in developing countries, especially in Asia – something the IFCN has also noted. Meanwhile, a 2015 study of people’s drinking habits in 187 countries found that milk drinking was more common in older people, which does suggest that it is less popular with the young – although this says nothing about young people’s consumption of milk products like yoghurt.

不过,一些局部的趋势并非如此。2010年一项有关食品消费的研究发现,在过去几十年里,美国的牛奶消费量有所下降,取而代之的是碳酸饮料,而不是杏仁奶。全球而言,这种下降被发展中国家(特别是亚洲国家)增长的牛奶消费量所抵消。IFCN也注意到了这一点。2015年,一项研究调查了187个国家的饮用习惯,显示出老年人喝牛奶的情况更为普遍,年轻人不太喜欢牛奶。这份报告并没有说明年轻人对酸奶等奶制品的消费情况。

Still, it seems unlikely that alternative milks will make much of a dent in the world’s growing appetite for milk, at least over the next decade.

不过,至少在未来10年内,全球牛奶需求会不断增长,替代奶对牛奶的影响都不大。

Walker adds that alternative milks are “not a like-for-like substitution” for animal milk. In particular, many don’t have the same micronutrients. She says they are most useful for vegans and for people allergic to milk – the latter being a reaction to milk protein, and nothing to do with lactose.

沃克补充道,替代奶并非是动物奶水的“对等替代品”。尤其是,许多替代奶缺乏牛奶中的微量元素。她说,替代奶对素食者以及对牛奶过敏的人最有用,牛奶过敏是指对牛奶蛋白质过敏,与乳糖无关。

It’s particularly striking that so much of the growth in milk demand is in Asia, where most people are non-lactase-persistent. Whatever advantages the people there see in milk, they outweigh the potential digestive issues or the need to process the milk.

尤其引人注目的是,对牛奶的需求有相当大一部分来自亚洲,那里的大多数人都没有乳糖耐受能力。无论如何,那里的人们认识到牛奶的好处,因此可能产生的消化不良,或是需对牛奶加工,就都不成问题。

In fact, the United Nations Food and Agriculture Organisation has pushed for people in developing countries to keep more non-traditional dairy animals, such as llamas, so that they can obtain the benefits of milk even if cow’s milk is unavailable or too expensive.

事实上,联合国粮农组织(United Nations Food and Agriculture Organisation)一直敦促发展中国家饲养更多的非传统乳畜,比如羊驼,这样他们就能从这些动物产的奶水中获益,哪怕牛奶供应不足或价格过高。

What’s more, a major study published in January described a “planetary health diet” that is designed to both maximise health and minimise our impact on the environment. While it entails drastically cutting down on red meat and other animal products, it nevertheless includes the equivalent of one glass of milk a day.

此外,今年1月的一项重要研究报告提出了“全球健康饮食”计划,希望最大限度增进健康,最大限度减少对环境的影响。该计划提议大幅减少红肉和其他动物产品的消费,但它仍然提倡每天一杯奶。

Milk, it seems, is not down and out. If anything it’s still on the up – even if our bodies have mostly stopped evolving in response to it.

人类提高饮奶适应力的进化已经停止。但是,牛奶的生产和消费并没有走下坡路。它还在上升阶段。

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