Seven years ago, Sam Curtis, 32, began to experience sudden flares of bloating and stomach cramps. She felt these attacks were particularly acute after consuming dairy products, ranging from ice cream to cheese, and even hot chocolate.
"My symptoms seemed to vary with the quantity of milk," she says. "Ice cream would keep me locked in the bathroom, while if a sauce contained dairy and I wasn't aware, I would bloat and have unusual bowel movements for a day or two."
After a bad episode of cramps, Curtis decided to diagnose herself with a test she ordered online based on a "holistic therapy" called bioresonance testing. On receiving their report, which listed proposed sensitivities to different dairy products, she switched to vegan alternatives for butter, milk and cheese.
It is hard to drink coffee with friends these days without hearing about their lactose intolerances – a condition linked to a deficiency in the lactase enzyme – and a look at the science suggests it is on the rise.
According to the British Nutrition Foundation, 5 per cent of people in the UK are lactose intolerant. Private healthcare specialist Bupa places it closer to one in 10. Soy, oat and almond milks are deemed to be healthier, and the UK's plant milk market is thought to be worth in the region of £263 million ($509 million).
But is lactose intolerance really as widespread as it appears to be?
A study published in Nature last week casts doubt on this malaise. Using genetic analysis, scientists from University College London and the University of Bristol have found that our ancestors consumed milk for thousands of years, despite lacking the ability to produce lactase as adults. Humans only evolved this capability in the past 3000 years and, even now, genetic surveillance shows it is present in only a third of the world's population.
But just as humans tolerated milk for millennia without lactase, it seems most of us are still capable of doing so. China is now the world's second largest consumer of dairy products, even though 92 per cent of its adults don't produce lactase.
When Mark Thomas, professor of evolutionary genetics at UCL and a co-author of the Nature study, examined data from the UK Biobank, he found little relationship between the ability to produce lactase and milk-drinking habits. "People who don't produce lactase actually generally don't suffer symptoms at all, without drinking very large quantities of milk, over a litre of milk or something like that," he says.
On closer examination, many lactose intolerances are self-diagnosed, often through dubious methods. The bioresonance-based testing method used by Curtis is scientifically unproven.
Then there's the case of Anne Larchy, a 50-year-old in Finchley, who maintained she had never "liked" milk since childhood, and found that consuming dairy products such as cottage cheese, yogurt or cheesecake often left her with rashes or an upset stomach.
She received a diagnosis of lactose intolerance through a form of alternative medicine called muscle kinesiology testing, in which a practitioner applies force to a muscle group and asks the body about its nutritional status.
While Curtis and Larchy have reported improved health since excluding all forms of dairy from their diet, both bioresonance testing and muscle kinesiology have been described as forms of pseudoscience.
Scientists suspect that many such cases of lactose intolerance are often confused with cow's milk allergy, which is a severe and in some cases dangerous condition, where the body overreacts to the protein casein which is found in many dairy products.
"One good way of telling if somebody has milk allergy rather than lactose intolerance is if they say that they get symptoms when they eat cheese," says Thomas.
"Hard cheese has got virtually no lactose in it at all, butter has virtually none. Yoghurt has about a third of the lactose of milk, and soft cheese has got some but not a massive amount."
Rather than self-testing, Tom Sanders, an emeritus professor of nutrition and dietetics at King's College London, recommends a hydrogen breath test.
"It's not something that GPs do as they regard it as such a minor thing, but it's quite an easy test," he says. "You give someone a standard dose of lactose, wait half an hour, and then get them to breathe, a bit like with an alcohol breath test. If hydrogen is present, it shows that the lactose hasn't been digested fully."
But at the same time, paediatricians are alarmed at the rise in alternative milk products for infants and toddlers. Cow's milk allergy is thought to only affect around 1 per cent of under-twos.
"There is unnecessary concern about lactose intolerance in babies as well as milk allergy," says Robert Boyle, an expert in paediatric allergies at Imperial College London.
"Babies get a lot of symptoms when they drink milk, be it mother's milk or formula milk, because they're drinking such large volumes. Babies will drink up to a fifth of their body weight in milk each day, so it's not surprising they get some tummy ache, runny poos and vomiting with that. But this gets labelled as lactose intolerance or allergies much more than necessary."
Boyle points out that alternative milks tend to be high in sugars that can put children at risk of tooth decay and contribute to childhood obesity. Some baby formula milks contain up to twice as much sugar as a Fanta fizzy drink.
"In general, if you switch to a lactose-free product, you are usually increasing what's called your child's free sugar intake," says Boyle.
In adults, a theory for why someone might appear to be lactose intolerant is not related to the dairy itself, but general diet.
Thomas explains that if someone is deficient in lactase and also consuming a diet high in processed foods, this could lead to imbalances in the gut microbiome and longer transit times. This means that undigested milk will be staying in the colon for longer, and certain bacterial species will be able to break it down into chemical compounds that are not necessarily desirable, leading to symptoms such as stomach cramps, bloating and flatulence.
Instead, if that person is consuming a diet much higher in fibre, their transit times will be far quicker and the bacterial species in their colon are likely to be more beneficial, meaning that milk will pass through their body without causing noticeable problems, despite the lack of lactase.
One possibility for why our ancestors were able to tolerate milk for such a long period of time, despite not producing lactase, is because their diet is likely to have been very high in fibrous foods. Thomas suggests that milk-drinkers who eat lots of fresh vegetables are less likely to suffer the consequences of lactase non-persistence than somebody who's eating processed food.
He feels that an exaggerated fear of lactose could have a negative health impact, with people veering away from traditional milk and opting for less beneficial alternatives.
"An unfounded fear could have a detrimental impact because while alternative milks may have nutritional value, nothing approaches milk in terms of nutritional makeup," he says.
"Milk has an almost perfect combination of nutrients. The protein is of the highest quality. It's also got lots of nice fat soluble vitamins and lots of minerals. It's the only food we consume that's specifically evolved to be nutritious."
Additional reporting by Abigail Buchanan