When Linda Kerr’s son’s growth flatlined, a doctor suggested the teen might benefit from a gluten-free diet. In support, she tried the diet with him. Her son eventually decided he wasn’t going to follow it, but it did have an unexpected effect: after about a month, Kerr herself found her health improved. “I’d been struggling all of my life with irritable bowel syndrome [a chronic condition that causes cramping, pain and gas],” she says. “I felt so much better following a gluten-free diet.” The retired dietitian, who lives in Westerose, Alberta, has been on a gluten-restricted diet ever since. “I find I can have one serving of gluten a day,” she says. She considers herself gluten sensitive.

In the 15 years since she’s been on this diet, gluten-free foods have moved from the specialty aisles of health-food stores to mainstream grocery stores like Loblaws, Costco and Walmart. Restaurants now highlight gluten-free menu options, and gluten-free bakeries have popped up to offer cakes, cookies and bread.

These companies are targeting a trend: Gluten-free foods are the fastest growing food intolerance category in North America. Market-research firm Packaged Facts estimated the Canadian gluten-free market was worth over $450 million in 2012, rising 27% a year between 2008 and 2012. And a recent survey of U.S. dietitians predicted anti-wheat sentiment would be the largest consumer trend of 2014.

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Before the recent interest in gluten sensitivity, two different types of people avoided gluten, a protein found in wheat, barley and rye, as well as many processed foods. The first, and by far the most common, were people with celiac disease. The disorder, which is part autoimmune and part food allergy, revolves around intestinal inflammation in response to gluten. It causes a broad range of symptoms, including diarrhea, weight loss, anemia and osteoporosis, which can also be very mild and non-specific. Avoiding gluten is recognized as an effective treatment. The second group who avoided gluten had an allergy to wheat itself, which causes a typical antibody response and familiar allergic reaction, such as hives, rash or even anaphylaxis.

But now many more people are choosing not to eat gluten, believing they have non-celiac gluten sensitivity. Although they don’t have celiac disease, they feel better when not eating wheat, reporting less abdominal pain, gas, headaches and fatigue.

Researchers have been studying this phenomenon of non-celiac gluten sensitivity, but their results have been mixed, causing some to question if it even exists. What does the science say about this new disorder – and the diet that comes with it?

Going against the grains

The wheat-bucking trend is being fuelled by celebrities like Gwyneth Paltrow and Miley Cyrus, who Tweeted, “It’s not about weight it’s about health. Gluten is cr-pppp anyway!” The diet launched off the low-carb craze of the late-’90s, with new books like Wheat Belly and Grain Brain promising that giving up gluten was the path to more energy, clearer thinking and, of course, weight loss.

The science doesn’t support that idea – a 2006 study found that 81% of people with celiac disease on gluten-free diets gained weight over two years, even those who were overweight to start with. In addition, adherents may not get enough iron and B vitamins, which are found in fortified flours but not most gluten-free ones. And people who follow the diet are often not eating enough calcium and fibre.

“I think that perhaps people are embracing this diet a little bit too enthusiastically, and they are basing that choice not on scientifically sound data, but on recommendations that they find on the internet, or on what a celebrity is doing,” says Elena Verdu, an associate professor at McMaster University and Canada Research Chair in inflammation, microbiota and nutrition.

The popularity can also lead to a health halo effect for even casual shoppers. While marketers can’t make explicit health claims, merely putting “gluten-free” on a package can mislead consumers into thinking that a box of muffins or sugary cereal is healthy.

But while there’s nothing inherently wrong with gluten, there is some support for the idea of gluten intolerance. The phenomenon was defined in a 2012 BMJ paper as “cases of gluten reactions in which neither allergic nor autoimmune mechanisms can be identified” – when the person doesn’t have celiac disease or a wheat allergy, but seems to have an adverse reaction to gluten. To know gluten is causing their symptoms, people often do a trial period where they go off gluten, then resume eating it. Many feel better when they’re off of it.

Beyond those personal stories, the science is muddy. A 2011 study by Jessica Biesiekierski and colleagues offered hope that non-celiac gluten sensitivity existed. It was inspired by those cases, says Biesiekiersk. “There were a lot of people [who had tested negative for celiac disease] coming to the gastroenterology clinic believing that gluten was a trigger for their IBS-type symptoms,” she says.

The researchers ran a randomized, double-blind study with 34 patients who had IBS and said they felt worse after eating gluten. They found that within a week, those eating gluten had significantly more symptoms. “We were quite surprised at those results – we didn’t actually expect it to be real,” Biesiekierski says.

But a follow up study by Biesiekierski’s team, published in Gastroenterology in 2013, found just the opposite. This time, they gave every participant a high-gluten, low-gluten and placebo phase. They also put everyone on a background diet, called low-FODMAPs, to minimize IBS symptoms.

The results were also surprising: they found that there was no difference between the three phases. “In fact, people got more symptoms during the placebo week,” says Biesiekierski, though a parallel study did find the participants had lower levels of depressive feelings when they weren’t eating gluten. In an accompanying piece in Gastroenterology, Rohini Vagna and Daniel Leffler, from the Beth Israel Deaconess Medical Center’s Celiac Centre in Boston, wrote: “Unfortunately, the [study] raises more questions than it answers. Indeed, the study calls into question the very existence of [non-celiac gluten sensitivity].”

Biesiekierski herself has doubts. “There’s a lot of mixed evidence: it might exist, but we don’t know for sure yet,” she says. A 2013 review of the research, co-authored by Biesiekierski, concluded “the evidence base for gluten as a trigger of symptoms in patients without celiac disease (so-called ‘non-celiac gluten sensitivity’ or NCGS) is limited.”

So why do people feel better when they stop eating gluten? Some may be have undiagnosed celiac disease – it’s underdiagnosed, though there has been a growth in awareness about it. In addition, increasing numbers of people have the disease – it’s currently around 1 in 133. For people with celiac disease, a strict-gluten free diet results in dramatic health improvements, which often begin within days.

It’s crucial for patients to rule out a celiac diagnosis before trying a gluten-free diet, since they need to be eating gluten at the time for the celiac test to work. And it can be difficult to rule out celiac completely. “We know that there is celiac disease that exists without the strict definition of the biopsy, there are people with celiac disease that have some mild inflammation. Depending on the number of biopsies, it can be missed,” says Alaa Rostom, chief of gastroenterology at The Ottawa Hospital. It’s also important to test for a wheat allergy.

The sensitive types

But that still leaves a group of people who don’t have celiac disease and report they feel better when not eating gluten. “It’s not just GI symptoms: often they’ll talk also about headache, a foggy brain, dizziness,” says Rostom.

These people may be experiencing the nocebo effect, where you believe something will make you feel sick, and then it does as a result. “There is so much bad press about the wheat containing diet that many patients think or believe that their symptoms are really caused by gluten, because they read it all day in the newspapers,” says Verdu.

Or perhaps they’re reacting to something else in the wheat. Biesiekierski’s team put all their participants on a low-FODMAPs diet, which doesn’t restrict gluten. The diet, which was developed at Monash University in Australia, improves IBS symptoms in about 75% of patients. And all of the participants in Biesiekierski’s latest gluten study had fewer symptoms on the baseline low-FODMAP diet. Fructan, one of the FODMAPs to avoid, is also found in wheat. By going gluten-free, people might be lowering their FODMAP intake without knowing it.

“The problem with the evidence is really what you can attribute the effects to,” says Peter Gibson, a co-author on both Biesiekierski studies. He adds that while the latter study didn’t prove gluten sensitivity isn’t real, “what we can say is that it’s very, very uncommon.”

Complicating all of these studies is the fact that, unlike for celiac disease, there’s no biomarker or test for gluten sensitivity, so finding the right groups to research is difficult. “It’s very important to try to find a marker, to say that this subgroup of people are going to benefit from a gluten-free diet,” says Verdu. The mechanism behind it is also still unknown.

“The bottom line is that I don’t think that the studies are of sufficient quality to actually say that this condition absolutely does or doesn’t exist,” says Rostom. In the absence of evidence, his answer, like many gastroenterologists’, is that if his patients feel better off gluten, and other issues have been ruled out, he’s fine with them being on the diet. “It’s pragmatic,” he explains.

Part of that is driven out of empathy. IBS is a common complaint, and there are limited treatments for it.

“There is a group of people [with IBS] whose symptoms are a little bit more dramatic, and their response is more intense. They are very frustrated, so they often go out and try a whole bunch of things. You can sympathize with them when they do try these various diets,” says Rostom. “But I also hear from people that say every time I eat tomatoes, or cucumbers, I have problems. If it was clear cut, it would be a lot easier.”