Introduction

Food intolerance is an adverse reaction to certain foods or ingredients that occurs every time the food is eaten, particularly if larger quantities are consumed.

It doesn’t involve the immune system and is generally not life-threatening.  Neither is it the same as food poisoning, which is caused by toxic substances that would cause symptoms in anyone who ate the food. Food intolerance doesn't include psychological reactions to food either. But if someone eats a food they are intolerant to, this could make them feel ill or affect their long-term health.

Causes

Food is broken down by enzymes, proteins produced by our body needed for the complete digestion of food. Lack or inadequate amount of enzymes causes some of the food (if not all) to remain in larger components unable to pass into the blood stream from the small intestine. This undigested food exerts osmotic effects drawing fluids and salts into the gut which moves rapidly towards the large intestines. This increased level of fluid and salts in the colon helps the bacteria that naturally reside in our larger intestine to ferment undigested food into acids, Carbon Dioxide (CO₂), Methane (CH₄), Hydrogen (H₂) and Hydrogen Sulphide (H₂S), the odour associated with flatulence. Hydrogen is one of the gases that cause bloating but due to its small size it escapes through the walls of the intestine into the bloodstream and is expelled through the lungs.

For example, one of the most common types is intolerance of cow's milk, which contains a type of sugar called lactose (lactose intolerance). Many people have a shortage of the enzyme lactase, which is normally made by cells lining the small intestine. Without this enzyme they can't break down milk sugar into simpler forms that can be absorbed into the bloodstream. Another common type but well unknown is Carbohydrate Intolerance which hinders overweight people from loosing weight.

Another common example is a deficiency of an enzyme called aldehyde dehydrogenase, which is needed to break down alcohol. Drinking even small amounts can make affected people feel unwell, very common in Asians (see alcohol intolerance).

Types

There are different types of here's a strong genetic pattern to food intolerances. Lactose intolerance is less common among northern and western Europeans (ten to 15 per cent are affected) than in Asian, African, native American and Mediterranean populations (60 to 90 per cent are affected).

Babies are usually born with higher levels of lactase, so lactose intolerance usually only begins after the age of about two, as the body begins to produce less of the enzyme. However, many people don't experience symptoms until they're much older. A temporary lactase deficiency may follow gastroenteritis, especially in children.

Symptoms

Food intolerances are rarely harmful but may cause unpleasant symptoms, including nausea, bloating, abdominal pain and diarrhoea, which begin about half an hour after eating or drinking the food in question.

The severity of symptoms varies depending on the amount of enzyme the person makes and how much of the food has been consumed. In alcohol intolerance, there may be intense flushing of the skin, nausea, palpitations, headache and feeling faint.

Diagnosis and treatment

The easiest test for a food intolerance is to remove the food from your diet, see if symptoms improve and then try reintroducing the food. If symptoms return, an intolerance is likely.

Lactose intolerance can be tested for more thoroughly using a lactose tolerance test, a hydrogen breath test and a stool acidity test. Your doctor can arrange these and other food intolerance tests if necessary.

Food intolerance can be managed simply by cutting the food out of your diet. Babies or younger children with a lactose intolerance can be given soya milk instead of cow's milk. Adults may be able to tolerate small amounts of troublesome foods, so may need to experiment to work out what they can eat.

Digestion's Mechanism

Digestion is the process by which the body breaks down food into absorbable nutrients. The body absorbs and assimilates everything that we ingest. Digestion includes physical actions such as chewing and peristalsis (involuntary contraction and dilation of muscles to force forward movement), as well as the chemical actions of enzymes, bile and acids.

The Mouth

As soon as we place food in our mouth the digestive system starts to work by biting and chewing. Only one digestive enzyme, amylase (for starch), is present in the mouth. However, food is normally not in the mouth long enough to permit complete digestion and little nutrient absorption takes place in the mouth.

The Stomach

The primary function of the stomach is to break large proteins into smaller peptides and peptones. Pepsin is an enzyme produced in the acidic environment of  the stomach (1.5 to 7.0 pH) which digests proteins into smaller peptides of varying lengths. Other enzymes such as gelatinase (for gelatin) digest specific proteins.

Amylase (the enzyme that breaks down starch) is inactivated, or destroyed, when stomach pH falls below 6.5. Before this happens however, up to 50% of starches may be partially broken down. Some fat is emulsified (broken up into smaller pieces) in the stomach by bile acids, and the enzyme lipase, to aid digestion in the small intestine. At the average stomach pH of 2.0, however, most fat is formed into large globules that pass unchanged into the small intestine. Small amounts of sucrose (table sugar) may be broken apart into glucose and fructose by acid hydrolysis from the bile acids in the stomach.

The Intestines

Virtually all absorption of nutrients (macronutrients such as carbohydrates and fats and micronutrients such as vitamins and minerals) occurs in the intestines. Absorption of nutrients is, in fact, the primary function of the small intestine. Most carbohydrates, for instance, are absorbed in the small intestine. The pancreatic enzymes secreted into the small intestine also contain amylase which breaks down starches into a disaccharide (two sugars joined together) called maltose. In the intestines, enzymes such as maltase and lactase break disaccharides into single sugars (monosaccharides), such as glucose. When adults and older children do not have enough lactase to digest lactose (the sugar in milk) lactose intolerance results and milk cannot be completely digested.

When food leaves the stomach, digestion is completed in the small intestine with the help of the enzymes secreted by the pancreas (lipase, amylase, protease, maltase, trypsin and chymotrypsin). The pH of food mass increases from about 2.0 to 6.5 (still slightly acidic), as it passes from the stomach through the small intestine to the colon. Very few nutrients, except water, is absorbed by the large intestine (colon).

Enzymes

An enzyme is a protein that acts as a catalyst in a biological reaction. It binds itself to a substance and converts it into another substance. Enzymes are very specific in their functions, which is why there are different enzymes for different biological reactions. In the case of digestion, distinct forms of food require specific enzymes. Unless proteins, fats and carbohydrates (sugars) are reduced to smaller absorbable components they will remain in the digestive track. The following list gives a glance for what each enzyme breaks and where it is produced:

• Amylase - starches into maltose (a disaccharide): saliva and pancreas - released into the small intestine
• Lipase - fats (lipids): in stomach and pancreas - released into the small intestine
• Pepsin - proteins into absorbable peptides and peptones: stomach
• Gelatinase - gelatin: stomach
• Maltase - maltose  into monosaccharides: produced by the pancreas - released into the small intestine
• Lactase - lactose into monosaccharides: small intestine
• Trypson - proteins into peptides and amino acids: pancreas - released into small intestine
• Chymotrypson - proteins into peptides and amino acids: pancreases - released into small intestine

Persons who suffer from food intolerance may suffer from weight loss and malnutrition if kept ignored.

Symptoms vary from individuals when they start. They may start within 15 minutes but they may also initiate after an hour. There is a high difference between patients in the perception of symptoms.

The severity of symptoms varies depending on the amount of lactose each individual can tolerate. Some of the symptoms may be confused with those of milk allergy but milk allergies can cause the body to react quicker, more often within a few minutes. If you are not sure compare the symptoms of lactose intolerance with those of milk allergy.