But now a group at University College London has shown that the mutation actually appeared about 7, 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, to 6, 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. Today, drinking milk is common practice in northern Europe, North America, and a patchwork of other places. Milk contains a type of sugar called lactose, which is distinct from the sugars found in fruit and other sweet foods.
But after we are weaned in early childhood, for many people this stops. Without lactase, we cannot properly digest the lactose in milk. As a result, if an adult drinks a lot of milk they may experience flatulence, painful cramps and even diarrhoea.
So the first Europeans who drank milk probably farted a lot as a result. But then evolution kicked in: some people began to keep their lactase enzymes active into adulthood. It is the result of mutations in a section of DNA that controls the activity of the lactase gene. The lactase persistence trait was favoured by evolution and today it is extremely common in some populations. The same is true in a few populations in Africa and the Middle East.
But there are also many populations where lactase persistence is much rarer: many Africans do not have the trait and it is uncommon in Asia and South America. A woman purchases soy milk in Hong Kong. Dairy can make many people feel sick in Asia, where the lactase persistence trait is uncommon Credit: Getty. The obvious answer is that drinking milk gave people a new source of nutrients, reducing the risk of starvation. People who are lactase-non-persistent can still eat a certain amount of lactose without ill effects, so drinking a small amount of milk is fine.
There is also the option of processing milk into butter, yoghurt, cream or cheese — all of which reduce the amount of lactose. Read more about parmigiano, a cheese with so little lactose it can be eaten by the lactose-intolerant. Hard cheeses like parmigiano-reggiano can have little to no lactose Credit: Getty. Accordingly, people seem to have invented cheese rather quickly. Many foods, drinks, and digestive aids are available to help manage lactose intolerance.
Normally, when we eat something containing lactose, an enzyme in the small intestine called lactase breaks it down into simpler sugar forms called glucose and galactose. These simple sugars are then absorbed into the bloodstream and turned into energy. In lactose intolerance, the body doesn't make enough lactase to break down lactose. Instead, undigested lactose sits in the gut and gets broken down by bacteria, causing gas, bloating, stomach cramps, and diarrhea.
Lactose intolerance is fairly common. Kids and teens are less likely to have it, but many people eventually become lactose intolerant in adulthood. Some health care providers view lactose intolerance as a normal human condition and not a disease or serious health problem. Lactose intolerance can cause a variety of symptoms.
It all depends on how much dairy or milk-containing foods people consume and how little lactase their body makes. If you might have lactose intolerance, the docto will ask your symptoms and diet. They might test the breath for hydrogen levels before and after you drink lactose.
Normally very little hydrogen gas is detectable in the breath. But undigested lactose in the colon breaks down and makes various gases, including hydrogen. If you have a hydrogen breath test, you'll blow into a tube for a beginning sample. Then you'll swallow a drink with lactose in it, wait a while, and breathe into the tube again. You'll blow into the tube every half hour for 2 hours to measure hydrogen levels. Glucose is the common sugar that circulates in the bloodstream after digestion of food, used to generate energy throughout the body.
Galactose, which is remarkably similar to glucose in its chemical structure, can be transformed into glucose for energy, but it is also an important component of cell membranes. Except for the milk of mammals, lactose is rarely found in other whole, unprocessed foods.
Infants use it as an important energy source during their first year of life Silanikove et al. It also supports the development of probiotic bacteria in an infant's gastrointestinal tract, which helps protect them from infections Fassio et al. It makes up an estimated 4. Figure 1 — Milk from cows is full of nutrients. Making up the bulk of the solid components are a unique mixture of proteins, a variety of fatty acids, and lactose, the predominant carbohydrate. Though not common in unprocessed foods, lactose is commonly found in processed food products e.
Interestingly, lactose is about six times less sweet than sucrose Ugidos-Rodriguez et al. Lactose is digested by an enzyme commonly referred to as lactase notice the slight but significant spelling difference. Lactase generally resides in the small intestine. Once lactose is broken down by lactase into its glucose and galactose components, they are readily absorbed and used throughout the body.
When lactose is not fully digested in the small intestine and proceeds to the large intestine, some of the resident bacteria cause it to ferment and produce lactate, small-chain fatty acids, and gases e. Humans experience a more favorable outcome in terms of comfort if digestion of lactose occurs in the small intestine, not in the large intestine.
Figure 2 — A simplified diagram of lactose breakdown in the small and large intestine of humans. Lactase activity in the small intestine is preferable compared to breakdown of lactose by bacteria in the large intestine, which can lead to uncomfortable symptoms. As might be expected, lactase activity is typically highest at birth, and most infants can digest lactose. The big issue with lactose digestion is that almost three quarters of the human population experience a gradual drop in lactase production, especially between 3 and 5 years of age Ugidos-Rodriguez et al.
The key determinant for persistent production of lactase in an individual is not the frequency of lactose consumption, but rather the individual's genetic makeup. There is tremendous variability between ethnic groups and regions of the world. Given the ethnic composition of the U. There are known genetic variants in human DNA that regulate lactase expression.
One homozygous genotype results in reduced lactase activity, whereas the alternative homozygous genotype is associated with normal, sustained lactase activity. Lactose intolerance is specifically defined as a situation where physical symptoms arise due to an inability to digest lactose.
Common symptoms of lactose intolerance include diarrhea, nausea, bloating, and abdominal pain. It is possible to be deficient in lactase production without being truly lactose intolerant. Some lactose-sensitive individuals, whether they realize it or not, can tolerate lactose consumption.
They may be deficient in lactase and unable to digest lactose efficiently, but they can ingest limited amounts of lactose without significant symptoms. Not everyone who has lactose digestion issues experiences symptoms every time. The Dietary Guidelines for Americans , updated in , still recommend regular consumption of milk, yogurt, and cheese for most people because of the easily digestible nutrients that are important to include in the diet, including calcium.
The guidelines acknowledge lactose digestion as a common health challenge, which is why the recommendations clarify that lactose-free products are a good option for those who are lactose intolerant.
Dairy products, as well as some soy-based alternatives, are a recommended part of a healthy eating pattern, supplying important nutrients, regardless of the consumer's genetic makeup. Most people lacking persistence in lactase production have few, if any, symptoms after consuming lactose if the quantity consumed during a meal or snack event is less than 12 grams Dekker et al.
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