• Your DNA is to blame for your lactose intolerance (Getty Images)Source: Getty Images
Why can some people indulge in the deliciousness of fresh milk, while a cup full of the white liquid will send others running into the toilet? According to the experts, the answer is in your DNA.
By
Yasmin Noone

6 Jun 2016 - 12:51 PM  UPDATED 6 Jun 2016 - 3:30 PM

Adults who can drink a glass of milk without any bowel-related side effects are the result of an evolutionary fluke. 

Despite the widespread availability of fresh cow’s milk products in supermarket aisles, lactose intolerance is the human norm. 

“The fact that some people are able to have lactose in their diet past the age of five is a fluke,” says Professor of Anthropology at Sydney's Macquarie University, Greg Downey.

“It’s an evolutionary fluke that’s arisen because of human technology and culture.” 

Prof Downey reasons that, like most mammalian animals, humans were only ever meant to drink the milk from their mother as a baby.

Born with the lactase enzyme to break down the sugar in milk called lactose, this enzyme usually switches off after we are weaned off breast milk.

Lactose intolerance is not an illness but a quirk of our inheritance.

But, thanks to a genetic mutation that occurred in humans 3000-5000 years ago, this enzyme remains active in a proportion of the globe’s teenage and adult population.

“Lactose intolerance is not an illness but a quirk of our inheritance,” he says. 

“It’s a condition just like having blue eyes is a condition. It’s a fact of our variation as humans.” 

Prof Downey believes that between 65 and 75 per cent of all people, post-adolescents, across the globe are lactose intolerant and can’t drink milk without feeling bloated, gassy, getting diarrhoea or vomiting. 

However, he adds, “the decreasing prevalence of drinking milk makes it hard to determine who is lactose tolerant and who is not”.

“Lactose intolerance can set in at different times in your life but most people won’t know they are lactose intolerant if they don’t drink milk often.

“Many kids also transition straight from milk to soft drinks…And since milk is not a part of people’s diets in some cultures [like in various Asian countries], it’s hard to know the true figures of lactose intolerance today.”

Why some people drink milk and others can’t

Prof Downey says there is a strong correlation between people who have the gene and those who hail from dairy farming ancestors. 

“We evolved a capacity to drink milk because we changed our environment and put dairy in it.”

It’s believed that, for some reason, a number of communities around the globe decided to start drinking animal milk thousands of years ago. 

Prof Downey explains that milk consumption must have conferred some sort of survival benefit to the first peoples who drank it, whether it helped them guard against infectious diseases or strengthened their bones. 

Human DNA then mutated to keep the lactase gene ‘switched on’ in specific groups of surviving dairy farmers who drank animal milk, after they were weaned off mother’s breast milk. 

“Think of it like this: if you are a kid who is given milk growing up and you keep on drinking milk because your parents have cows, then drinking milk could increase your chance of survival three-to-four per cent.

“And if kids keep drinking milk, one generation after another for around 30 generations, then you could wind up with more tolerance in a population.” 

If you are a kid who is given milk growing up and you keep on drinking milk because your parents have cows, then drinking milk could increase your chance of survival three-to-four per cent.

Human geneticist from La Trobe University, Associate Professor John Mitchell, estimates that around 70 to 80 per cent of all northern Europeans currently have the lactase gene.

That includes people who have descended from dairy farming ancestors living in communities throughout the British Isles, Denmark, Norway, Sweden, Poland, Germany and Lithuania (but not Iceland, as it was settled later). 

“So being northern European, I am lactose tolerant and can drink fresh milk,” says Dr Mitchell.

“My DNA differs from the DNA of those who are lactose intolerant because there is a gene mutation that I keep producing.

“The fact that I am lactose tolerant makes me the mutant form and the lactose intolerant person is the [‘normal’] ancestral form.” 

It is thought that northern Europeans descended from pastoralists. Although there is no conclusive evidence, Dr Mitchell believes that the Yamnaya people from southern Russia first brought cattle and horses to Europe and spread the mutation, as they expanded across the continent. 

Between 30-60 per cent of people living in the Arabian Peninsula in the Middle East can digest lactose, as can 50 to 70 per cent of people living in the Mediterranean. 

The lactase gene is also common in the Sudan and Somalia, and in Kenya’s Maasai population.

Between 30-60 per cent of people living in the Arabian Peninsula in the Middle East can digest lactose, as can 50 to 70 per cent of people living in the Mediterranean. 

However, says Dr Mitchell, approximately 90 per cent of adults in East Asia are lactose intolerant. There are also very low frequencies of Indigenous Australians, Native Americans and Pacific Islanders who produce the lactase enzyme after childhood. 

Dietitian Natasha Murray says the majority of Aboriginal and Torres Strait Islander people she works with in the Northern Territory exhibit lactose intolerance from a young age. 

“I find that most Indigenous adults can have milk in their tea but can’t drink a glass of milk,” says Murray, spokesperson for the Dietitians Association of Australia.

“This is because their body isn’t making enough of the enzyme required to break down lactose so any more than a little bit of milk in their tea and their body can’t handle it.” 

Dr Mitchell adds that although race and culture can explain whether or not a person can drink milk, most people have such a mixed bloodline that it might be hard to pinpoint the presence of a lactase gene on the basis of nationality alone. 

“Even if your family has been living in Australia for 200 years, few people would know who their true ancestors were as we have so many,” says Dr Mitchell. 

“And all it takes is one ancestor to have ‘switched on’ the gene.”

Did you miss Dr Mitchell on DNA Nation last night? Catch up on SBS On Demand.

 
 
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