Food intolerance, also known as non-IgE mediated food hypersensitivity or non-allergic food hypersensitivity, refers to difficulty in digesting certain foods. It is important to note that food intolerance is different from food allergy.
FOOD INTOLERANCE is not caused by the immune system. The onset of symptoms is usually slower, and may be delayed by many hours after eating the offending food. The symptoms may also last for several hours, even into the next day and sometimes longer. Intolerance to several foods or a group of foods is not uncommon, and it can be much more difficult to decide whether food intolerance is the cause of chronic illness, and which foods or substances may be responsible.
With food intolerance, some people can tolerate a reasonable amount of the food, but if they eat too much (or too often) they get symptoms because their body cannot tolerate unlimited amounts.

Enzyme Defects:

Enzymes are required to help with the breakdown of natural substances found in certain foods. If these enzymes are missing, or in short supply, then eating the food can cause symptoms because part of the content of the food cannot be properly dealt with by the body.
Most foods require some enzyme activity in their digestion, and enzyme deficiencies can be an important factor in food intolerance.
Most common symptoms of food intolerance:

Nausea
Stomach pain
Diarrhea
Vomiting
Gas, cramps, or bloating
Heartburn
Headaches
Irritability or nervousness
Irritable bowel syndrome
Chronic constipation
Eczema
Asthma
Headache
Tiredness and fatigue
Joint pain
ACNE

Once all intolerance causing foods are identified your dietitian can prescribe an appropriate diet to avoid those foods. Lists of suitable foods are available. Your dietitian will ensure adequate nutrition is achieved with safe foods and supplements if needed.
Over a period of time it is possible for individuals avoiding intolerance causing foods to build up a level of resistance by regular exposure to small amounts in a controlled way, but care must be taken, the aim being to build up a varied diet with adequate composition.
Most intolerance causing foods:

Gluten
Corn
Wheat
Oat
Seafood
Alcohol
Yeast
Dairy
Nuts
Peanuts
Walnuts
Pecans
almonds
Fish
Shellfish
Milk
Eggs
Soy

How to discover food intolerance?

Food tolerance tests are available at your dietitian. Those tests measure IgG antibodies which may be linked to inflammatory conditions in the body, manifesting in a range of health issues.
You are advised to keep a diary and write down which foods are eaten, what the symptoms were like, and when they appeared. The data in the diary can help your dietician identify which foods are causing adverse reactions, and what steps to take.
Some people find that if they stay off the specific food for a while, they have no reaction when eating it again.
This is known as tolerance. Maintaining tolerance is often a question of knowing how long to abstain, and how much of it to eat when it is being reintroduced.