Food intolerances are a response of a digestive system unable to handle quantities of certain foods, but are not usually fatal. Symptoms of both maladies are similar, but in allergic reactions, you are more likely to see atopic dermatitis and other rashes or skin disorders like hives or urticaria, shortness of breath or airways swelling up, even anaphylaxis, which is a severe, multi-symptom response to an allergen, often accompanied by cardiac arrest. Symptoms of food intolerance are similar, but do not include rashes or breathing problems and are easily managed.

Modern methods of food preparation can cause food intolerance. The modern prevalence of food allergies and intolerances can be traced to the prevalence of fast foods in our diet and the chemical preservation methods that are often used. While they may taste good, modern foods present a minefield of potential intolerances. 

The sufferer from food intolerance usually lacks a sufficient quantity of a particular enzyme to manage certain foods. With insufficient enzyme action to break down a certain food such malabsorption remains in the body, causing nausea, gas, bloating, diarrhea or chronic constipation. Irritable Bowel Syndrome can often be traced to a persistent use of a food that the body cannot digest. 

Lactose intolerance is a common result of a lactase deficiency, resulting in an inability to digest the main protein in milk. This lack may be hereditary or nationality-related; people from northern climates suffer less from lactose intolerance that those from southern ones. A simple solution to this problem in infants can be to simply substitute soy formula for cows milk.

Adverse reactions to certain foods cannot be predicted, and often it is difficult to tell which foods are causing the problem. In that case, an elimination diet is followed, where potentially problematic foods are eliminated one by one, until there are no more symptoms. Then foods are added back to the diet and responses noted in a food diary. Challenge testing calls for strictly supervised presentation of suspected foods in a double blind test. Another supervised test for food intolerances is the hydrogen breath test, requiring a short period of fasting and analysis of the breath blown into a balloon.

Numerous types of food can produce sensitivity, intolerance, or allergy. Some experience salicylate sensitivity to certain fruits, vegetables, cheeses, cosmetics or simple aspirin. Such a sensitivity or allergy can produce symptoms similar to asthma like wheezing or trouble breathing. Others may experience fructose or alcohol intolerance. A condition known as Celiac disease or gluten intolerance causes sufferers to become unable to absorb nutrients into the intestinal wall and is caused by sensitivity to wheat products. Numerous protein products, such as nuts, shellfish and eggs, can also trigger allergies and intolerances.