30th July 2023

Nutrition

Nutrition and Malnutrition

Background

Nutrition refers to the biological process by which an organism utilises food to sustain life. Nutrients are the various components of a diet. Macronutrients form the majority of the diet and hence are the main supply of energy to the body, being grouped into fats, carbohydrates, and protein. Micronutrients are not required in as great amounts, but are still essential, and include vitamins and minerals. Greater detail on micronutrients and their importance in human physiology and disease can be found in the Nutrition and Physiology topic.

‘Good nutrition’ refers to dietary intake which enables a person to appropriately meet their energy demands. ‘Malnutrition’ is whenever the nutritional process falls short of this, including inadequate intake by amount or imbalance, impaired absorption, or due excessive demands. Malnutrition can be of all nutrients, individual nutrients, and also include excessive diet, i.e. obesity.

A technical definition of malnutrition can be given as:

• BMI of less than 18.5
• Unintentional weight loss greater than 10% within the last 3–6 months
• BMI of less than 20 and unintentional weight loss greater than 5% within the last 3–6 months

Malnutrition has the potential to affect every organ system. Common manifestations include reduced muscle and fat mass, which in turn contributes to a state of frailty. It can also lead to reduced cardiac output and respiratory function, and a variety of biochemical abnormalities owing to inadequate intake of particular nutrients. Impaired immune function and delayed wound healing are also frequently seen in malnourished people.

The focus of this topic is on malnutrition. Further information can also be found in the Nutrition and Physiology topic.

Causes of malnutrition

The causes can be divided into the following subcategories.

Reduced dietary intake and inadequate diet

This, in turn, can be due to a variety of reasons, including:

• Reduced appetite as a consequence of malignancy, chronic nausea, and other systemic illnesses
• Eating disorders, e.g. anorexia nervosa
• Oropharyngeal and upper GI disorders causing difficulty with intake and swallowing, e.g. oesophageal strictures, chronic candidiasis, severe acid reflux
• Dementia – leads to reduced appetite and unintentional self-neglect (i.e. forgetting or being unable to cook)
• Social factors – poor diet, neglect or self-neglect, alcoholism, substance misuse, food poverty
  • The biggest cause in children of malnutrition

Malabsorption

It can be the primary pathology for a variety of diseases or occur as a consequence. See further sections for more information on key conditions.

• Inflammatory bowel disease – especially Crohn’s disease
• Coeliac disease
• Cystic fibrosis
• Chronic pancreatic insufficiency
• Chronic diarrhoeal illnesses – e.g. tropical sprue, giardiasis
• Short gut syndrome
• Whipple’s disease
• Lactose intolerance (and other food intolerances)
• Biliary disease

Excessive losses

Generally in the context of severe burns, these patients have specific nutrient requirements and losses, which often leads to malnutrition if not appropriately addressed.

Increased energy expenditure

Chronic systemic illnesses may lead to increased energy demands, which, if not met by increased dietary intake, can lead to malnutrition. This is often exacerbated by the co-existence of reduced appetite.

Examples include:

• Malignancy
• Chronic infectious diseases – e.g. tuberculosis
• HIV/AIDS
• Cystic fibrosis

Assessment of malnutrition

Assessment includes investigation to determine the underlying cause, as well as assessment of a patient’s nutritional status and any physiological consequences of malnutrition.

Investigations

• Blood tests – including FBC and haematinics, U&Es, LFTs, TFTs, bone profile, vitamin D levels
• Screening tests for malabsorptive diseases – e.g. cystic fibrosis sweat test, autoantibodies for Coeliac disease, and other autoimmune diseases
• Stool sample for MC&S – assessing for chronic infectious causes
• Faecal elastase – for pancreatic insufficiency
• Focused imaging or endoscopy for any particular suspected disease – e.g. colonoscopy for potential lower GI malignancy, oesophagogastroduodenoscopy (OGD) in oesophageal disease, CT imaging for relevant malignancies
• Social assessments, if appropriate
• Cognitive assessments, if appropriate

Screening and assessment tools

Frameworks such as the MUST (Malnutrition Universal Screening Tool) can be used to assess a patient’s nutritional status, and in inpatient settings is performed as standard screening to identify any patients at risk of malnutrition. This is a five-step process, and involves assessment of BMI, any recent weight loss, and current acute illness. The full tool is provided in the References section.

Management of malnutrition

The most crucial factor in managing malnutrition is identifying and addressing the underlying cause, which can involve medical, surgical or social interventions.

Patients may also require referral to dieticians, who can provide specialist support primarily focusing on optimising intake. They are an especially crucial part of the multidisciplinary team for chronic illnesses like cystic fibrosis.

Diet and nutritional support

NICE provides comprehensive guidelines on nutritional support. It should be considered in any malnourished patient (see definition above) and also people with little or no food intake for >5 days and who are unlikely to have adequate nutrition in the next 5 days.

Nutritional support can encompass oral, enteral or parenteral nutrition, including supplements and artificial feeding. Prescriptions are tailored to an individual’s requirement, but a general guideline is:

• 25–35 kcal/kg/day total energy (including that derived from protein)
• 0.8–1.5 g protein (0.13–0.24 g nitrogen)/kg/day
• 30–35 ml fluid/kg (with allowance for extra losses and inputs)
• Ensure adequate electrolytes, minerals, micronutrients and fibre

Oral supplements include high energy juices and milkshakes, with Fortisips and Fortijuices being commonly prescribed brands.

Enteral nutrition refers to supplementation that can be given when a patient has an adequately functioning GI tract. This includes the use of the oral route, and also ‘tube feeding’ with NG (nasogastric), PEG (percutaneous endoscopic gastronomy), and PEJ (percutaneous endoscopic jejunostomy) tubes, when the oral route is not accessible and is hence bypassed to provide direct nutrition to the GI tract

Parenteral nutrition is where the GI tract cannot be used to absorb nutrients, due to conditions relating to malabsorption, following bowel resections, or bowel obstruction. Nutrition is instead delivered through the IV route, with long-term venous access routes such as central lines used to administer this. Total parenteral nutrition (TPN) refers to patients who receive all of their nutritional requirements via this route. Parental nutrition can be given at regular intervals throughout the day, akin to meal times, or can be given continuously overnight, and is able to be delivered at home.

All of these forms of nutrition need to be prescribed by a qualified medical professional and require specialist input for long-term monitoring and management.

Malabsorptive Diseases

Malabsorption (the impaired absorption of nutrients in the GI tract) is a key cause of malnutrition and hence necessitates further attention. Key conditions are highlighted below, and more information can also be found in the Lower GI Pathology, Upper GI Pathology, Pancreatico-biliary Pathology, and Bowel Infection and Inflammation topics.

Malabsorptive diseases can be broadly subcategorised by the primary site of pathology.

Intestinal causes

Coeliac disease

Coeliac disease is an autoimmune condition in which autoantibodies are produced in response to the protein gliadin found in wheat. It is pathologically characterised by chronic inflammation, crypt hyperplasia and villous atrophy of the small bowel, especially the duodenum. The immune response is mediated by T cells, and hence is a type IV hypersensitivity reaction.

It has 2 peaks of presentation – childhood, when gluten is first introduced into the diet, and early middle age, which is the commonest age of onset in the UK.

The typical presentation is of chronic abdominal pain, postprandial bloating, diarrhoea (often steatorrhea due to the malabsorption), weight loss, fatigue, and in children, failure to thrive. It is often associated with anaemia. Extraintestinal manifestations include dermatitis herpetiformis and arthralgia.

Suspected coeliac disease can be screened for by testing tissue transglutaminase antibodies (anti-tTGA) and IgA anti-endomysial antibodies (anti-EMA IgA). Total IgA levels should also be checked, as IgA deficiency can lead to false negatives. The gold standard diagnostic test is endoscopy, with at least 5 biopsy samples required from different (duodenal) sites. It is essential that a patient continues to eat gluten for the 6 weeks preceding this, or false negatives can occur. The mainstay of treatment is complete elimination of gluten from the diet.

Complications include micronutrient deficiencies, and it is also associated with osteoporosis, hyposplenism, and an increased risk of GI lymphoma.

Inflammatory bowel disease (IBD)

Crohn’s disease and ulcerative colitis can both lead to malabsorption, but this is more common in Crohn’s disease. Malabsorption occurs due to the chronic inflammation of the bowel impairing the ability of the GI tract to absorb nutrients. Malabsorption may also be a consequence of bowel resections, which are sometimes required to treat refractory IBD.

More information on the diagnosis, clinical features and management of IBD can be found in the Bowel Infection and Inflammation topic.

Tropical sprue

Tropical sprue is a condition which can be seen in travellers to tropical regions, and presents with acute or chronic diarrhoea, leading to malabsorption. Folic acid and vitamin B12 are particularly common associated deficiencies. The exact aetiology is unknown, but it is thought to have an infectious cause. It causes pathological features similar to coeliac disease, as bowel mucosa over time will exhibit crypt hyperplasia and villous atrophy.

There is no targeted treatment, and management is instead supportive, including fluid and electrolyte correction, treatment of anaemia, and replacement of any deficient micronutrients.

Whipple’s disease

Caused by infection with Tropheryma whipplei, and has a preponderance to middle aged men. Initially presents with abdominal pain, diarrhoea/steatorrhoea, and weight loss, and is associated with arthralgia, lymphadenopathy and hyperpigmentation of the skin. If untreated, it can progress to affect the CNS and heart, with the most serious consequences including seizures, ataxia, endocarditis and myocarditis.

Diagnosis involves upper GI endoscopy, and a key feature is villous granules that are strongly positive for the Periodic acid-Schiff stain.

Treatment involves 2 weeks of IV ceftriaxone or benzylpenicillin, followed by a 1-year maintenance treatment of oral co-trimoxazole. In cases where the CNS or heart is involved, oral doxycycline and hydroxychloroquine are also given.

Chronic diarrhoeal illnesses

Any chronic diarrhoeal illness can lead to malabsorption, as any intake will not be able to be appropriately absorbed, and over time they can cause chronic inflammation in the gut, which impairs absorption.

As well as other primary bowel diseases already discussed, infectious illnesses such as giardiasis can cause chronic diarrhoea. Giardiasis by the protozoan Giardia lamblia, and is frequently seen in travellers and people living in areas with poor sanitation. Generally self-limiting, it can be treated with metronidazole, if required.

Short gut syndrome

This refers to, quite literally, the occurrence of a shorter than normal small bowel, generally due to bowel resection in the treatment of various diseases, or due to congenital abnormalities. Having a reduced amount of bowel impairs the ability of the GI tract to absorb nutrients. Management is supportive, with nutritional supplements generally required, and even parenteral nutrition in severe cases.

Pancreatic causes

The pancreas produces many enzymes essential for digestion and absorption of nutrients, as its exocrine function. Any disease affecting the pancreas can therefore impair this process and lead to malabsorption; this is globally termed as ‘pancreatic insufficiency’. Symptoms include abdominal pain, postprandial bloating, steatorrhoea and weight loss. It is often associated with diabetes, which is due to impairment of the endocrine function of the gland; as whatever disease process is impairing its exocrine function may also impair its endocrine function.

Pancreatitis and pancreatic cancer may cause pancreatic insufficiency due to the chronic inflammation in the organ. It is also associated with cystic fibrosis due to impairment of mucus production and secretion from the pancreas gland. Obstruction and strictures may also impair the secretions of enzymes, and resection will damage the gland’s function, also.

Diagnosis can be made with the faecal elastase test; samples will be deficient, as this enzyme is produced by the pancreas. Other diagnostic investigations include abdominal imaging such as x-ray, CT and MRI, and ERCP and MRCP for more focused imaging of the pancreaticobiliary tree – with these tests predominantly focused on detecting the cause.

Management involves treatment of the underlying cause and the use of pancreatic enzyme replacements. The most commonly available formulation is Creon, which contains amylase, lipase and protease. Enzymes are typically taken alongside meals and snacks, in order to enable digestion and absorption.

Biliary causes 

The biliary tree is a vital part of the body’s absorption system, through the production and reabsorption of bile acids, which are essential for fat absorption. Bile acids are produced in the liver and then stored and released by the gallbladder. Reabsorption occurs in the terminal ileum.

Any disease affecting the biliary tract can lead to bile acid malabsorption (BAM), as well as those affecting the ileum. This includes primary biliary cholangitis, primary sclerosing cholangitis, gallbladder stones, Crohn’s disease, coeliac disease, pancreatic insufficiency, bacterial overgrowth, or following cholecystectomy or bowel resection.

BAM causes a distinct non-bloody pungent diarrhoea (steatorrhoea), which is generally chronic. The diagnostic test used to be the selenium homocholic acid taurine (SeHCAT) test, but NICE recommendations now advise treatment trials, 48-hour total stool bile acid levels, and fasting C4 levels instead.

Treatment involves a low fat diet and bile acid binders, such as cholestyramine.

Refeeding Syndrome

Overview

Refeeding syndrome is a potentially fatal consequence of artificial nutritional replacement in malnourished patients, and refers to a collection of electrolyte disturbances due to the shifts in homeostasis that occur.

The characteristic electrolyte abnormalities are:

• Hypophosphataemia
• Hypokalaemia
• Hypomagnesaemia

It is also associated with deficiency of key micronutrients, such as thiamine.

Causes

Anyone who is malnourished can develop refeeding syndrome, but it is a particular risk factor for the elderly, people with eating disorders, cancer patients, alcoholics, and the chronically malnourished, with there being an association between the chronicity of the malnourishment and the risk of the disease.

Clinical features

These are generally the consequences of the associated electrolyte deficiencies.

They include muscle weakness, abdominal pain, constipation, and nausea/vomiting. More serious manifestations are of the CNS and cardiovascular system, such as ataxia, seizures, delirium, coma, and life-threatening cardiac arrhythmias. Torsades de pointes and subsequent ventricular fibrillation are a particular risk due to hypokalaemia.

Thiamine deficiency can cause Wernicke’s encephalopathy, and the irreversible Korsakoff’s syndrome, if untreated.

Management 

The risk of life-threatening arrhythmias means patients identified to have refeeding syndrome may require cardiac monitors. Any complications of refeeding syndrome should be appropriately managed.

Prevention is key – patients at risk of refeeding syndrome should be identified prior to nutritional replacement. Preventative measures include early thiamine supplementation, and a carefully monitored slow rate of artificial nutrition and fluid replacement, for which each hospital will have local guidelines. Regular electrolyte monitoring is required.

NICE recommends that refeeding is started at no more than 50% of energy requirements in “patients who have eaten little or nothing for more than 5 days” in order to reduce the risk of developing refeeding syndrome.