The milk sugar lactose is broken down by the enzyme lactase .

奶中的乳糖由乳糖酶分解。

A competing hypothesis argues that ancient europeans domesticated milk-bearing livestock because lactase persistence was already quite common in certain populations .

另一个假说争论说，远古欧洲人之所以畜养产奶牲畜是因为乳糖酶持续性基因已经在人群中有了一定的比例。

In most cases mammals stop producing lactase after weaning but a nucleotide switch in their dna can keep lactase flowing into adulthood a trait called lactase persistence .

在大多数情况下，哺乳动物在断奶之后就产生乳糖酶，但是其dna中的一个核糖开关能使成年人也能产生乳糖酶，该特征称为乳糖酶持续性。

Switching from milk formula to soy formula is helpful for severe diarrhea ( from acquired lactase deficiency ) and for vegetarianism .

从牛奶配方换到大豆配方，对素食主义者和治疗重度腹泻（获得性乳糖酶缺乏症）有帮助。

Lactase persistence is far from a universal condition ( half of modern humans cannot digest lactose ) and there are at least two competing theories as to how it became the norm for european populations .

乳糖酶持续性不是一个普世的状况（一般的现代成年人不能消化乳糖），现在至少有两种相互竞争的理论来解释为甚么乳糖酶持续性变成了欧洲人的标准基因。