Got milk? If you do, take a moment to ponder the true oddness of being able to drink milk after you're a baby.

    No other species but humans can. And most humans can't either.

    The long lists of food allergies some people claim to have can make it seem as if they're just finicky eaters trying to rationalize likes and dislikes. Not so. Eggs, peanuts, tree nuts, fish, shellfish soy and gluten all can wreak havoc on the immune system of allergic individuals, even causing a deadly reaction called anaphylaxis.

    But those allergic reactions are relatively rare, affecting an estimated 4% of adults.

    Milk's different.

    First off, it's not actually an allergy in that there's not an immune response.

    People who are lactose intolerant can't digest the main sugar -lactose- found in milk. In normal humans, the enzyme that does so -lactase- stops being produced when the person is between two and five years old. The undigested sugars end up in the colon, where they begin to ferment, producing gas that can cause cramping, bloating, nausea, flatulence and diarrhea.

    If you're American or European it's hard to realize this, but being able to digest milk as an adult is one weird genetic adaptation.

    It's not normal. Somewhat less than 40% of people in the world retain the ability to digest lactose after childhood. The numbers are often given as close to 0% of Native Americans, 5% of Asians, 25% of African and Caribbean peoples, 50% of Mediterranean peoples and 90% of northern Europeans. Sweden has one of the world's highest percentages of lactase tolerant people.

    Being able to digest milk is so strange that scientists say we shouldn't really call lactose intolerance a disease, because that presumes it's abnormal. Instead, they call it lactase persistence, indicating what's really weird is the ability to continue to drink milk.

    There's been a lot of research over the past decade looking at the genetic mutation that allows this subset of humanity to stay milk drinkers into adulthood.

    A long-held theory was that the mutation showed up first in Northern Europe, where people got less vitamin D from the sun and therefore did better if they could also get the crucial hormone （it's not really a vitamin at all） from milk.

    But now a group at University College London has shown that the mutation actually appeared about 7,500 years ago in dairy farmers who lived in a region between the central Balkans and central Europe, in what was known as the Funnel Beaker culture.

    The researchers used a computer to model the spread of lactase persistence, dairy farming, other food gathering practices and genes in Europe.

    Today, the highest proportion of people with lactase persistence live in Northwest Europe, especially the Netherlands, Ireland and Scandinavia. But the computer model suggests that dairy farmers carrying this gene variant probably originated in central Europe and then spread more widely and rapidly than non-dairying groups.

    The European mutation is different from several lactase persistence genes associated with small populations of African peoples who historically have been cattle herders.

    Researchers at the University of Maryland identified one such mutation among Nilo-Saharan-speaking peoples in Kenya and Tanzania. That mutation seems to have arisen between 2,700 to 6,800 years ago. Two other mutations have been found among the Beja people of northeastern Sudan and tribes of the same language family in northern Kenya.

    喝奶么？喝的话，抽出点儿时间来关注一下你为什么过了婴儿期还能够喝牛奶吧。

    只有人类这一物种能够喝牛奶。同时大多数人又是不能喝的。

    有人说自己对很多食物过敏，就好像他们特别挑剔，给自己喜欢和不喜欢吃的食物找借口。事实并非如此。鸡蛋、花生、坚果、鱼、海鲜酱和麦麸都能给敏感体质人群的免疫系统造成极大损伤，甚至引起致命的过敏反应。

    但这些过敏反应相对并不常见，大概有4%的人有此反应。

    而牛奶就不同了。

    首先，牛奶并不是那种会引起免疫应答的过敏原。有乳糖抗性的人群不能消化牛奶中的主要糖分--乳糖。正常人在2至5岁之间，体内消化乳糖的酶--乳糖酶就停止分泌了。这些不能消化的糖分最终积聚在盲肠中发酵产气，引起痉挛、肿胀、呕吐、肠胃胀气和痢疾。

    美国人和欧洲人很难想象，成人能够消化牛奶其实是一个很奇特的基因适应性现象。

    这不是个普通现象。全世界大约不到40%的成年人能够在童年过后保留消化乳糖的能力。该数字在美洲印第安人中几乎为0,亚洲人为5%,非洲和加勒比人为25%,地中海人群50%,北欧90%.其中瑞典为乳糖耐受性比例最高的国家。

    能消化牛奶这一功能真的太奇特了。科学家们甚至说我们不应该把乳糖抗性当成一种疾病，那样的话首先就把它看做不正常了。相反的，科学家们把乳糖抗性称为乳糖耐受性，以表明成年后能够继续喝牛奶才是奇怪的事情。

    过去十年内，有很多研究围绕这些成年后保留喝牛奶能力的人群基因展开。长期以来的理论是：该突变基因首先出现在北欧，那里的人们从光照中吸收的维生素D少，因此从牛奶中吸收的关键激素就多（实际上乳糖不是维生素）.

    但如今伦敦大学学院的一个研究小组表明该突变基因实际上来源于7500年前，也就是漏斗颈陶文化时期，居住在巴尔干半岛中部和欧洲中部之间的奶农。

    研究人员对欧洲地区的乳糖耐受性的传播、乳品业、其它食品聚集过程和基因进行了计算机建模。

    如今，乳糖耐受性比例最高的人群居住在欧洲西北部，尤其是荷兰、爱尔兰和斯堪的纳维亚。但计算机建模表明，携带该基因变种的奶农来自中欧，之后比非乳品业人群分布得广泛而迅速。

    欧洲的突变基因就与世代游牧的非洲少数人群身上的几种乳糖耐受性基因不同。马里兰大学的研究人员在肯尼亚和坦桑尼亚讲尼罗-撒哈拉语的人群中发现了这种突变基因。此基因大概出现于2700至6800年前。另外两种突变基因也已在苏丹东北部的贝贾人和肯尼亚北部讲贝贾语的部落中发现。