What is malabsorption?

Malabsorption is the condition in which the substances we take in with our food are not sufficiently digested in the digestive system, or are not absorbed from the bowel into the body in sufficient quantity. It is also called impaired absorption.

For the food we eat to be usable by the body it must first be broken into smaller parts, and then absorbed from the bowel into the blood or the lymphatic circulation. Most of this takes place in the small bowel.

In malabsorption any stage of this process may be disturbed. The result may be that:

Carbohydrates,
Fats,
Proteins,
Minerals such as iron, calcium and magnesium,
Vitamins such as B12 and folate,
Fat-soluble vitamins such as A, D, E and K

are not sufficiently absorbed.

The absorption of every nutrient is not disturbed in every patient with malabsorption. In some diseases only the absorption of one particular nutrient is affected, while in others the absorption of many nutrients is disturbed together.

How does normal absorption of food happen?

For food to be absorbed there is a well-ordered process taking place in several stages.

First, the enzymes secreted by the stomach, the pancreas and the small bowel help to break the food down. Bile, produced in the liver, is important particularly for the digestion and absorption of fats.

The broken-down nutrients are then absorbed from the inner surface of the small bowel. The many small projections on the surface of the small bowel, called villi, greatly increase the surface area available for absorption.

Most of the absorbed nutrients pass into the bloodstream, while some fats pass first into the lymphatic system.

So for healthy absorption the stomach, the pancreas, the liver and biliary system, the small bowel and the circulation of the bowel all have to work in harmony.

Why does malabsorption develop?

Malabsorption is not a single disease. It is a clinical state that can arise as a result of many different diseases.

Its causes can be considered in several main groups.

1. Food not being sufficiently digested

Even when the absorptive capacity of the bowel is normal, absorption may be disturbed if the food cannot be broken down enough.

In exocrine pancreatic insufficiency, for instance, where the pancreas cannot produce enough digestive enzymes, the digestion and absorption of fats in particular may be disturbed.

Similarly, in some diseases where the production of bile or the flow of bile is seriously disturbed, the absorption of fats may be affected.

2. Disease of the surface of the small bowel

The inner surface of the small bowel is the principal region where food is absorbed. Damage to this surface can reduce the capacity to absorb.

One of the best known examples of this is coeliac disease. In coeliac disease the gluten-related immune response disturbs the structure of the small bowel and can reduce the absorption of food.

Crohn’s disease, some infections and certain rarer diseases of the small bowel can also cause impaired absorption.

3. Disorders of the absorption or digestion of particular nutrients

Malabsorption does not always affect all foods.

In lactose malabsorption, for instance, the activity of the enzyme lactase, which digests the lactose found in milk and dairy products, is insufficient.

The unabsorbed lactose reaches the large bowel and is broken down there by bacteria. As a result, wind, bloating, abdominal pain and diarrhoea can develop, particularly after milk or dairy products are taken.

Problems with the absorption of fructose and certain other carbohydrates can likewise lead to digestive complaints.

4. Small intestinal bacterial overgrowth (SIBO)

Normally there are only limited numbers of bacteria in the small bowel. In some people the quantity of bacteria in the small bowel may increase as a result of slowed bowel movements, changes in the structure of the bowel, or previous surgery.

This is called SIBO (small intestinal bacterial overgrowth).

The excess bacteria may consume some nutrients, disturb the function of the bile acids, and affect the absorption of fats and of vitamin B12 in particular.

Wind, bloating, abdominal discomfort and diarrhoea may be seen.

5. Surgical removal of part of the small bowel

Enough bowel surface has to be present for absorption.

Where a significant part of the small bowel has been removed surgically because of Crohn’s disease, disease of the blood vessels of the bowel, tumours or other reasons, the surface available for absorption may be reduced.

How much of the bowel, and particularly which part, has been removed matters. After very extensive bowel resections, short bowel syndrome may develop.

6. Diseases of the ileum

The last part of the small bowel is called the ileum. The ileum is important particularly for the absorption of vitamin B12 and of bile acids.

Crohn’s disease involving the terminal ileum, or surgical removal of this region, can lead to vitamin B12 deficiency and to impaired absorption of bile acids.

Large amounts of bile acid reaching the large bowel can cause watery diarrhoea in some patients.

What are the symptoms of malabsorption?

The symptoms vary with the cause of the impaired absorption, how long it has been going on, and which nutrients cannot be absorbed.

The commonest complaints are:

Long-standing or recurrent diarrhoea,
Bloating of the abdomen,
Excessive wind,
Abdominal pain or discomfort,
Weight loss,
Being unable to gain weight although the appetite is normal,
Weakness and tiring easily,
Anaemia,
Deficiencies of vitamins and minerals.

But diarrhoea is not necessarily present in every patient with malabsorption. Sometimes the condition is first noticed through a problem outside the bowel, such as iron deficiency, B12 deficiency or a fall in bone density.

What are fatty stools (steatorrhoea)?

Where fats cannot be sufficiently digested or absorbed, more fat than normal may be passed in the stool. This is called steatorrhoea.

The stool may be:

More bulky than normal,
Pale in colour,
Greasy or shiny,
Foul-smelling,
Liable to stick to the lavatory or hard to flush away.

Marked steatorrhoea is seen particularly in pancreatic enzyme insufficiency, in certain diseases of the small bowel, and in problems related to bile.

Why do deficiencies of vitamins and minerals matter?

Malabsorption is not just a matter of diarrhoea and wind. When it goes on for a long time it can lead to deficiencies of the vitamins and minerals the body needs.

Iron deficiency can cause anaemia and weakness.

B12 and folate deficiency can cause anaemia; and when B12 deficiency in particular is long-standing, complaints involving the nervous system may also develop.

Vitamin D and calcium deficiency can contribute to weakening of the bones and to an increased risk of osteoporosis.

Vitamins A, D, E and K are fat-soluble. Deficiencies of these vitamins may therefore be seen in long-standing, severe fat malabsorption.

In some patients whose absorption of protein is seriously disturbed, the protein levels in the blood may fall and swelling may develop, particularly in the legs.

How is malabsorption diagnosed?

It is not right to do the same tests in every patient in whom malabsorption is suspected.

The most important point in making the diagnosis is to establish which nutrient cannot be absorbed, and why.

The patient’s complaints, changes in weight, eating habits, previous operations, the medicines they take and their existing illnesses are all assessed.

In patients where it is thought necessary, the following may be used:

A full blood count,
Iron and ferritin,
Vitamin B12 and folate,
Vitamin D,
Calcium and other minerals,
Albumin and other blood tests,
Tests for coeliac disease,
Stool examinations,
Assessment of fat in the stool,
Faecal elastase where pancreatic insufficiency is suspected,
Breath tests where lactose malabsorption or SIBO is suspected,
Endoscopy and, where needed, small bowel biopsy,
Ultrasound, computed tomography, MRI or other imaging methods.

Which tests are needed is decided according to the patient’s clinical features.

Is an endoscopy needed?

An endoscopy is not needed in every patient with malabsorption.

But if coeliac disease or another disease affecting the lining of the small bowel is suspected, biopsies may need to be taken from the duodenum during an upper gastrointestinal endoscopy.

Where Crohn’s disease is suspected, methods such as colonoscopy, assessment of the terminal ileum, MR or CT enterography, or capsule endoscopy may be used according to the patient’s circumstances.

How is malabsorption treated?

There is no single treatment for malabsorption that can be applied to everyone.

The chief aim of treatment is to find the cause of the impaired absorption and to treat that cause.

For instance, what may be needed is:

An appropriate gluten-free diet in coeliac disease,
Pancreatic enzyme replacement in those patients who need it, in pancreatic enzyme insufficiency,
Treatment directed at the cause in suitable patients with SIBO,
Adjustment of the diet according to the person’s tolerance in lactose malabsorption,
Bringing the disease under control in Crohn’s disease,
Replacement of what is lacking, in deficiencies of vitamins and minerals.

Because the treatments of these diseases are quite different from one another, saying only that “there is malabsorption” is not enough.

What should the diet be in malabsorption?

There is no single diet that applies to everyone with malabsorption.

It is not right for patients to remove a large number of foods from their diet on their own initiative. Unnecessary and excessively restrictive diets can make matters worse in people who are already at risk of nutritional deficiency.

The eating pattern should be decided according to the cause of the malabsorption and which nutrients cannot be absorbed.

When should a doctor be consulted?

Assessment for malabsorption may be needed particularly where there is:

Long-standing or recurrent diarrhoea,
Weight loss of unexplained cause,
Fatty, foul-smelling stools,
Constant wind and bloating,
Recurrent iron deficiency or anaemia,
Deficiencies of B12, folate or vitamin D,
Unexplained thinning of the bones,
An inability to gain weight although the diet is adequate.

Where there are findings such as weight loss, serious anaemia, blood in the stool, diarrhoea that wakes the person at night, or marked nutritional deficiency, assessment should not be delayed.

Remember

Malabsorption is not one disease but an impairment of absorption that can develop as a result of many different diseases.

Sometimes only the digestion and absorption of one particular nutrient, such as lactose, is affected; sometimes the absorption of many vitamins, minerals, fats, proteins and carbohydrates is disturbed together.

For this reason the first step in treatment is not to make unnecessary restrictions of food, but to establish what is causing the malabsorption.