The majority of people in the world are unable to digest dairy products and can suffer a range of uncomfortable symptoms as a result.

A recent study of ancient DNA shows that the ability to digest milk may have become common only relatively recently in Europe, and the majority of people in the world have still not developed this trait and are unable to digest dairy products.

What is lactose and why are some people intolerant to it?

Lactose is the main sugar found in milk and other dairy products. Those who have lactose intolerance find it hard to digest the sugar, usually because their small intestine doesn't make enough lactase, the enzyme which digests lactose.

Who is affected?

Around 60% of adult humans are lactose intolerant or have reduced lactose tolerance after infancy. But tolerance varies dramatically depending on geography, affecting some ethnic groups more than others.

In Britain, the condition is most prevalent among the Asian and African-Caribbean communities, according to the NHS. Only one person in 50 of northern European descent has some degree of lactose intolerance, while around 90% of people from China have the condition.

Scientists believe this because places in Africa and Asia historically had little access to milk, so people may not have evolved the ability to digest lactose because there was no benefit in being able to do so.

How have humans tolerance evolved?

Humans are able to digest milk past childhood only because of a genetic adaptation. Research suggests the gene for breaking down lactose in milk was non-existent 20,000 years ago and was still very rare during the Bronze Age – a minuscule amount of time in evolutionary terms.

Brian Handwerk for Smithsonian.com says: “That lightning-fast evolutionary change suggests that direct milk consumption must have provided a serious survival advantage over peoples who had to ferment dairy into yogurt or cheese”.

While still affecting a majority of the world’s population, these days, many humans have access to plentiful alternative foods as well as lactose-free milk or lactase pills that help them digest regular dairy.

“In other words,” says Handwerk, “we can circumvent some impacts of natural selection. That means traits like lactose tolerance might not have the same direct impacts on survival or reproduction that they once did—at least in some parts of the world”.

What are the symptoms of lactose intolerance?

Between 30 minutes and two hours after consuming food or drink containing lactose, the sufferer may have one of the following symptoms: flatulence, diarrhoea, bloated stomach or stomach cramps. In some cases these symptoms can be severe.

Is it the same as a dairy allergy?

No. "Intolerances are different to food allergies; they are not caused by the immune system and are not life threatening," Amena Warner, the head of clinical services at AllergyUK, told The Guardian.

Milk allergy is the second most common food allergy after peanuts. The immune system mistakes the proteins in cow's milk for a threat and releases chemicals that trigger the signs and symptoms of an allergic reaction in response. These include wheezing or vomiting, but can lead to anaphylaxis, a life-threatening reaction. Milk allergy is more common in young children and infants, while lactose intolerance tends to affect teenagers and adults.

Is there a cure?

No, but limiting or eliminating food that contain lactose will manage the symptoms. People can also take lactase substitutes – drops or tablets that help improve lactose digestion.

Most lactose-intolerant people are able to consume small amounts without any problem so may not need to cut dairy products out of their diet all together. In such cases, the Food and Drug Administration recommends consuming them with other foods. "This helps slow down digestion, making it easier for your body to absorb lactose," it says. There is also a wide range of "lactose-reduced" products available.

Eliminating all dairy products means getting vitamin D and calcium from other sources. Foods rich in vitamin D include fatty fish, beef liver and eggs, while green leafy vegetables, soya beans, tofu and nuts all contain high levels of calcium.

While most nutritionists advise against giving up entire food groups unless absolutely necessary, if you do give up dairy completely, you might notice some changes, both good and bad, to your body, says Cosmopolitan.