The reasons why celiac disease and diabetes so often occur together are the focus of a lot of research. Celiac disease and diabetes are both autoimmune diseases. This means they both involve tissue damage from autoimmune attacks: in celiac disease, the body's immune system attacks the small intestine, whereas in diabetes, the body attacks the pancreas. Also, both diseases involve food intolerances that require special diets: no gluten for celiacs, and little or no sugar for diabetics.
It turns out that the two diseases share some genes. In fact, researchers believe that celiac disease and diabetes probably have at least 7 genes in common, and there may be more.
Because the genetic links between the diseases are becoming clearer, many doctors now recommend that anyone who's been diagnosed with type 1 diabetes should also have tests for celiac disease. (Some experts also advise that patients who have type 1 diabetes or celiac disease, or both, should also be tested for autoimmune thyroid disease.)
It's important to remember that just being tested once for celiac disease is not enough, because the disease can develop later in life. Therefore, people with diabetes need to be periodically retested for celiac disease, especially if growth failure, failure to gain weight, weight loss, or gastrointestinal symptoms develop. In general, however, research has shown that children who are genetically predisposed to both diabetes and celiac disease will have celiac disease-associated autoantibodies in their blood either before or at the same time that they have diabetes-associated autoantibodies.
According to the Kovler Diabetes Center at the University of Chicago, people are usually diagnosed with diabetes before they're diagnosed with celiac disease - mostly because doctors and the public are more familiar with diabetes. If you have diabetes and you're wondering whether you might have celiac disease, see Symptoms of Celiac Disease. In addition to the symptoms on that list, there are certain features of undiagnosed celiac disease that are specific to diabetics, including unpredictable or unexplainable swings in blood sugar levels; hypoglycemia a couple hours after a meal, or hypoglycemia that is hard to treat; and reduced insulin needs. All of these are a result of malabsorption related to celiac disease. (In the simplest terms: because of the damage to your small intestine, the food you're eating is not being absorbed into your body.)
Studies have shown that once diabetics with celiac disease are on a gluten-free diet, the episodes of hypoglycemia are reduced, but it takes several months of being on the diet for the effect to be obvious. But while hypoglycemia will become less of a problem, diabetics with celiac disease who start a gluten-free diet will have other challenges to adjust to, as described in The Gluten-Free Diet: Special Issues for Diabetics. But because celiac disease is associated with serious complications, the advantages of staying gluten-free far outweigh the disadvantages.
One large study from Denmark showed that patients with type 1 diabetes and untreated celiac disease had, on average, significantly lower height and weight compared to diabetic patients without celiac disease, and they were significantly younger when they developed diabetes. Two years after the celiac patients started on gluten-free diets, they had gained weight, and those who were younger than 14 years old had also caught up in height. Everyone also had more iron (hemoglobin and ferritin) in their blood.
On the other hand, if you have celiac disease and don't stay gluten-free, there can be very serious consequences.
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Juvenile Diabetes Research Foundation International: Double Diagnosis: Living with Type 1 Diabetes and Celiac Disease