舊金山紀事報–喝奶了嗎？ 若你是如此，花點時間想想，為何你早已不是嬰兒了，還在喝奶？而且是喝別種動物的奶？ 除了人類以外沒有任何其它物種能如此，大部分人類也無法消化牛奶。有人列出長長的過敏食物清單，看起來似乎是挑食者想要合理化其好惡的藉口，但並非如此；蛋、花生、堅果、魚、貝類、大豆、麩質…都能對過敏者造成免疫系統混亂，嚴重者甚至會造成死亡。但這些過敏反應都是相對少數，在成人中估計只有4%。
而牛奶不一樣。
首先釐清的是，大部分對牛奶有不良反應的人並非真的過敏–意即這不是免疫系統的反應。而是乳糖不耐症的人不能消化乳糖–也就是牛奶的主要成份之一。正常人在2～5歲間消化乳糖的酵素會停止分泌， 無法消化的乳糖會堆積在結腸(大腸)，在此開始發酵產生氣體，造成腹绞痛、腹脹、噁心、腹瀉。
如果你是美國或歐洲人，大概很難想像，但成年人能夠消化牛奶是一個奇怪的基因適應變化。這並不尋常，世上其實只有不到40%的人在脫離兒童時期後仍然有消化乳糖的能力。在上述40%的人數中，美洲原住民為0%，亞洲裔為5%，非洲裔與加勒比海裔為25%，地中海裔為25%，北歐裔為90%，這當中，瑞典人的比例為最高的。
正因能消化牛奶是十分奇怪的，所以學者說我們不該稱乳糖不耐是一種"症狀"。因為假定其為不正常，相反地，他們就必須稱成年後仍具消化能力者為"乳糖酶持久症"，研究為何消化乳糖的能力會不斷持續。
過去10年間，有很多的研究在找尋可能的基因突變主因，令這些相對少數的人在成年後仍然繼續喝奶而沒事。長期以來理論顯示，突變首先來自北歐，那裡的人來自陽光產生的維生素D較少，而若飲用牛奶則獲取了關鍵的激素，則自陽光產生的能力有所改善。
但現在倫敦大學學院顯示，以白種人為主之基因突變是從7500年前在巴爾幹半島與中歐的酪農經營者而來，這些古代人屬於Funnel Beaker文化。
這個報告本週刊登在PLoS Computational Biology上。研究人員用電腦模擬乳糖酶持久基因的散播，跟食物採集的技術一起傳到全歐洲去。今天，最高比例有乳糖酶持久能力的人屬西北歐區域，尤其是荷蘭、愛爾蘭、北歐的人，但是模擬結果顯示最初帶有這種基因的人發源自中歐，然後迅速散播到其它地區。
隸屬環境與遺傳演化研究所的該報告作者Mark Thomas 說道：在歐洲，一個基因的改變與乳糖酶持久基因大有關係，並且給予這些人較大的生存優勢。這些歐洲的基因變化與一些非洲人因早期牧牛使然而產生乳糖酶持久能力不一樣。
馬里蘭大學的研究人員確認一個屬於非洲人的突變基因來自尼羅河與撒哈拉沙漠區域的人，約在2700～6800年前出現，另兩個突變基因在東北蘇丹被發現。
附註 :
除了乳糖不耐問題外，牛奶裡面的蛋白質(酪蛋白)跟養分都是專門為了使小牛成長而設計，跟人類一點關係都沒有。也因此人體的免疫系統會對這些不適合人體的蛋白質產生過敏的現象，例如：多痰、鼻塞、腹瀉、紅疹等等。
牛奶裡面也含有大量的細菌病毒、荷爾蒙(雌激素)、抗生素（治療乳房炎）、感染源及化學藥劑等，喝多了容易提高婦科問題發生機率，也會增加身體毒素的累積。 牛奶帶有許多蛋白質是酸性的食物，也因此身體為了中和由蛋白質所產生的酸性，必須損耗身體裡面的鈣質，最後導致骨質疏鬆的問題，其所謂高鈣的強壯神話實為騙局（鈣含量高沒錯，但會令人類在年紀增加後骨質疏鬆）。
此外，乳類脂肪含量高，是發胖的主要因素。
強化骨本，只要食用有色蔬果、全穀或豆類與運動即可。
Sixty percent of adults can't digest milk
By Elizabeth Weise, USA TODAY
SAN FRANCISCO — 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, most people who have bad reactions to milk aren't actually allergic to it, in that it's not their immune system that's reponding to the milk.
Instead, 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 paper was published this week in PLoS Computational Biology.
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.
Author Mark Thomas of University College London's dept of Genetics, Evolution and Environment says: "In Europe, a single genetic change...is strongly associated with lactase persistence and appears to have given people with it a big survival advantage."
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.
來源: http://www.usatoday.com/tech/science/2009-08-30-lactose-intolerance_N.htm
