30th July 2023

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.

• 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.