Most people are lactose intolerant . In fact this is the default condition for the human species . Moreover , most mammals cannot digest milk after weaning . This is because the gene that controls the production of the enzyme lactase is slowly turned off as the individual human or animal ages . Without the enzyme lactase , the individual human or animal cannot breakdown the sugar lactose found in milk . As a result , the lactose in the milk needs to broken down by bacteria in the gut . A byproduct of this bacterial breakdown is bloating and diarrhea . However , about 10 , 000 years ago a couple of human cultures that kept cattle and goats began eat and drink more milk products . Those who had a mutation which keep the lactase enzyme on were able to naturally digest lactose safety into adulthood . These individuals are known as lactase persistent . So humans have two natural states those people who can naturally digest milk into adulthood because they continue to produce the enzyme lactase and those people who cannot naturally digest milk into adulthood because they cannot produce the enzyme lactase . With the enzyme you can digest milk just fine , without it you can ' t . A simple mutation 10 , 000 years ago changed many of our lives and diets forever .