Food intolerance or food sensitivity is a negative reaction to a food that may or may not be related to the immune system or to food poisoning. It can be caused by the absence of specific chemicals or enzymes needed to digest a food substance, or to the body's responses to certain food constituents (chemicals) both natural or artificial.Symptoms of food intolerance vary greatly, and can be mistaken for the symptoms of an allergy. While true allergies are associated with fast-acting immunoglobulin IgE responses, it can be difficult to determine the offending food causing an intolerance because if the immune system is involved, the response is likely to be IgG mediated and takes place over a prolonged period of time. Thus the causative agent and the response are separated in time, and may not be obviously related. A deficiency in digestive enzymes can also cause some types of food intolerances. Lactose intolerance is a result of the body not producing enough lactase used to break down the lactose in milk. Gluten intolerance results in damage to villi in the small intestine, which makes it difficult for the body to absorb water and nutrients from foods. Another type of food intolerance is an intolerance to food chemicals such as salicylates or salicylate sensitivity. Salicylates are chemicals that can occur naturally in many foods. Salicylate sensitivity causes many symptoms the most common of which are: hives, stomach pain, head aches, mouth ulcers, and it has even been linked to ADD and ADHD.Food intolerance can exist as a separate condition or contribute to the symptoms of complex syndromes such as Chronic Fatigue Syndrome CFS/CFIDS, Myalgic Encephalomyelitis ME, Post-Viral Fatigue Syndrome PVFS and may involve causes such as Leaky Gut Syndrome. For these reasons diagnosis is best carried out by experienced practitioners.Symptoms of a food intolerance include gas, intermittent diarrhea, constipation, irritable bowel syndrome, skin rashes, migraine headaches, and an unproductive cough.