Coeliac disease is an immune disorder where the body assumes gliadin, a substance found in gluten, is harmful and reacts accordingly; this can have harmful effects on the body. The small intestine, being tasked with absorbing nutrients, comes under attack from the immune system; while the intestine is covered in villi, protrusions that increase the surface area, the damage and inflammation that occurs in this scenario results in them being flattened thereby impairing the digestive process.
Gluten intolerance, or sensitivity, is much more broad, and can refer to a wide array of difficulties with the digestive system that are set off by gluten. However the issue can be confused, with a lot of sources referring to the two interchangeably, when this is not the case. While it could be said that coeliac disease is a form of gluten intolerance (although the NHS disputes this assertion), gluten intolerance is not necessarily coeliac disease.
Given its broader nature, gluten intolerance is more common than coeliac disease. The latter affects ≤1.2% of the population, less in Africa and Asia, while the former affects are far greater and far more varied percentage; however exact figures are hard to come by, not helped by the aforementioned interchangeable usage between the two terms, as well as the increased popularity of gluten free diets for other reasons. Things are complicated further by the controversial nature of the subject, with assertions about non-coeliac gluten intolerance being called into question by some; other theories on the matter suggest that the reason some people (although it must be stressed not all) whose digestive problems are seemingly alleviated by cutting gluten from their diet might instead be due to other factors involved, such as the processed nature of many of these foods, or simply from having consumed too much gluten as a matter of course (similar to other foods and substances that cause health problems when over consumed, but are agreed to be fine in moderation, such as fruit), or even the placebo effect on account of the condition’s recent prevalence on the media.
As such, unlike, for example, lactose intolerance, gluten intolerance is not terribly well understood. It is presumed to be linked to the relatively recent introduction of grains into the human diet, coinciding with the rise of agriculture and farming approximately 10,000 years ago. It is assumed therefore that some have adapted to its introduction more smoothly than others.
With non-coeliac gluten sensitivity being only recently posited as a unique condition, accurate incidence statistics are, as stated earlier, not common, and should probably be considered under the knowledge that this is early research. Reported figures vary depending on location, but put the number of people who suffer from gluten intolerance at between roughly 5-10% (between about one in ten and one in twenty) of the population at average, although some places, such as New Zealand, report far higher at about 20% (one in five).