Food allergies have an immune basis, so you experience symptoms every time you consume a food. Typical symptoms include skin rash, swelling and hives; severe reactions can be life-threatening. (Gas and bloating without any other symptoms don't necessarily constitute an allergic reaction.) Unlike intolerances, a delayed response is rare—allergic reactions usually present within minutes to a few hours after eating the food.

Diseases like celiac are different. For those with the autoimmune disease, the immune system attacks one's own body when gluten is consumed, damaging the villi that line the intestines, which can lead to serious nutritional deficiencies. If a screening blood test for celiac comes back positive, the diagnosis is usually confirmed with a biopsy of the small intestine. The Canadian Digestive Health Foundation suspects more than 330,000 Canadians have celiac disease, but only a third of cases are diagnosed. It's a lifelong disorder, and those with celiac are at an increased risk of other diseases, such as thyroid problems, Type 1 diabetes and small intestinal cancer.

Food sensitivities, meanwhile, can mean different things. Some experts consider sensitivity to be synonymous with intolerance, but there is no consensus in the scientific community. "Sensitivity is a rather loose term," says Dr. Mohsin Rashid, pediatric gastroenterologist and professor of pediatrics and medicine at Dalhousie University in Halifax as well as professional advisory board member of the Canadian Celiac Association. "It means you get symptoms when you consume a food, but it's probably not an intolerance—you're not missing something in the body to digest it—and it's not a typical allergy, either." Symptoms vary from person to person, as does the amount of the food required to provoke the reaction.

Non-celiac gluten sensitivity—when people get gluten-triggered symptoms but don't have celiac disease or the accompanying damage to the intestines—is an example of a food sensitivity. With no test for this sensitivity, the diagnosis is made by ruling out celiac disease. "It is hard to study how many people have non-celiac gluten sensitivity," says Dr. Rashid, "because patients may not be going to their physicians—they are trying a gluten-free diet on their own first." Self-diagnosing is potentially dangerous, as some people who think they have a gluten sensitivity could actually have celiac disease or a wheat allergy, which, if left untreated, could have serious long-term health consequences.

Check out what you need to know about food intolerances before you change your diet.

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This story was originally part of "Can't Stomach It" in the March 2015 issue.             Subscribe to Canadian Living today and never miss an issue!