為何人類會變得能夠喝鮮奶呢？(二)－bubble50 DH Freelance 中翻英英翻中

Dairy benefits

乳製品的好處

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.

她推測，除了營養價值之外，喝鮮奶可能還有其他的優點。飼養牲畜的人會暴露在牠們疾病的影響之下，包括炭疽和隱孢子蟲病。可能正是因為喝了牛奶，所以才產生了能夠對抗這些感染的抗體。確實，鮮奶所產生的保護作用就被認為是給小孩喂母乳的好處之一。

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.

但是，不知道為什麼，牧民當中還是會出現缺乏乳糖酶持續性的情形，可能它就是純屬偶然：就看一群牧民當中是否有人碰巧得到正確的突變。直到最近，地球上的人數少了很多，各地方的人口規模也縮小，使得有些群體就因為運氣不好而錯失機會。

“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.

Swallow說：「最能夠說明這種情形的就是它與生活方式、與放牧有關，但是你得先發生這種突變才行。」只有這樣，天擇才能發揮作用。

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.

Swallow指出，就蒙古的放牧人而言，他們通常會喝含有較低乳糖含量的發酵乳。這樣說的話，既然我們可以更方便的食用加工過後的鮮奶，那麼會發展出乳糖酶續存性這件事就更令人感到不解了。Swallow的博士生Catherine Walker說：「因為各個文化中我們都能看到人類非常善於適應，甚至將鮮奶拿去加工和發酵，所以我才會百思不得其解，為什麼我們得從基因上去適應它。」

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.

不過，提升乳糖酶續存性的因素也不只有一個，而是有許多個。Swallow猜想關鍵可能是鮮奶的營養價值，例如它富含脂肪、蛋白質、糖和像是鈣和維生素D等的微量營養素。

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.

它也是淨水的來源。根據族群所在的居住地，你可能會出於某種原因而進化成能夠忍受它的體質。

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.

Swallow說，目前還不清楚演化是否仍持續活躍地向乳糖酶持續性變異，以及它是否會因此變得更加普遍。她與人在2018年共同撰寫了一份研究報告，這份報告是關於一群在智利科金博地區的牧民，他們的祖先在500年前與新來的歐洲人混種，之後他們就取得了乳糖酶持續性的突變基因。這種特性目前在該族群中廣為傳播：它受到了進化的青睞，就像5000年前的北歐人一樣。

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.”

但這是一個特例，因為科金博人重度依賴鮮奶。從整個世界上來看，情況卻非常不同。Swallow說：「我認為突變的情況已經穩定下來了，除非是在那些依賴鮮奶且有[其他食物]短缺的國家，才會發生。西方的飲食已經很好了，所以選擇壓力就不太可能會存在了。」

Dairy decline?

乳製品減少？

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.

其實，過去幾年的新聞卻提供了相反的感想：大家開始不喝鮮奶了。2018年11月，衛報發表了一篇題為「我們是如何失去對鮮奶的熱愛」的文章，描述銷售燕麥和堅果奶的公司正在迅速崛起，並暗示傳統鮮奶正面臨一場大戰。

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. (Read more about how milk became a staple food in industrialised societies).

但統計數據卻呈現出另一個不同的故事。根據IFCN乳製品研究網站2018年的報告，自1998年以來，為了滿足不斷增長的需求，全球鮮奶的產量每年都在增加。2017年，全球生產了8.64億噸的鮮奶。沒有跡象顯示它正在減慢： IFCN預計到了2030年鮮奶的需求將增長35％，提升至11.68億噸。（了解更多關於鮮奶是如何成為工業化社會下的主食。）

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.

不過，這也掩蓋了一些更為在地化的趨勢。2010年的食品消費研究發現雖然取代它的是碳酸飲料，而非杏仁奶，但是美國的鮮奶消費量在過去幾十年中有所下降。今年秋天，由於發展中國家，尤其是亞洲國家的需求不斷增長，才使得產量與銷量平衡，IFCN也發現了這一點。同時，2015年對187個國家飲料習慣的研究發現老年人比較常喝鮮奶，雖然這沒有說明年輕人對優格等乳製品的消費情況為何，但這也表示鮮奶不太受到年輕人的歡迎。

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.

儘管如此，至少在未來十年內，牛奶替代品似乎不太可能對世界日益增長的鮮奶需求產生重大的影響。

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.

Walker補充說，牛奶替代品對於動物奶來說並不是「相似的替代相似的」情況。特別是，許多替代品都沒有相同的微量營養素。她說，它們對素食主義者和對鮮奶過敏的人最有用，後者是對乳蛋白質產生反應，而與乳糖無關。

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.

特殊的是，大部分鮮奶需求的增長都是來自於那些大多數人無法耐受乳糖酶的亞洲。無論這些人是看中了鮮奶的什麼優點，這個量都超過了他們能夠消化的量或是加工鮮奶所需要的量。

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.

事實上，聯合國糧食及農業組織一直在推動發展中國家的人飼養更多非傳統的產奶動物，像是駱馬，這樣的話，即使無法取得牛奶，或是牛奶太貴了，他們也可以得到喝鮮奶的好處。

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.

更有意思的是，1月發表的一項大型研究，提出了「行星健康飲食」，就是盡可能地將健康水平提到最高並將對環境的影響降到最低。雖然它需要大幅減少紅肉和其他動物產品的食用，但在它之中所包含的每天喝一杯牛奶，已經可以補足這些減少的量。

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.

這樣看來，鮮奶並沒有被淘汰。即使我們的身體大多停止演變出能夠應對它的基因，但若真要說的話，它其實還是很夯的。