Definition
Carbohydrates are a large group of organic compounds ranging from simple sugars to complex starches and fibres. They contain carbon, and hydrogen and oxygen in the same ratio as water (2:1) and can usually be broken down to release energy.
Function
Carbohydrates are the major energy source in human nutrition, providing approximately 45 to 65% of daily energy requirements. Most consumable carbohydrates are broken down into glucose before entering the bloodstream.
Carbohydrates are the primary source of energy for the central nervous system, brain and red blood cells: the brain has no ability to store energy and therefore requires a constant supply of glucose to function properly.1
Carbohydrate deficiency or overload
When energy supply is limited, due to inadequate intake, or inadequate digestion or absorption, dietary protein is used as an energy source rather than carbohydrate. Inadequate protein intake results in the breakdown of lean body mass to provide an adequate supply of amino acids within the bloodstream to support the synthesis of acute phase proteins, production of glucose, healing of wounds, etc.2
Excess glucose and fructose may exacerbate metabolic complications in skeletal muscle, adipose tissue, and liver,3 and can increase complications such as severe infections, multiple organ failure, polyneuropathy and mortality.4,5
Role of carbohydrate during illness
Currently, there are no specific guidelines for carbohydrate intake in the acute or chronically ill population, therefore the DRI recommendations for healthy adults are commonly referred to. For adults, the recommended carbohydrate intake is at least 130g/day based on the average minimum amount of glucose utilised by the brain.1
The metabolic response to illness and injury is characterised by hypermetabolism, (catabolic response) including negative nitrogen balance, insulin resistance, hyperglycaemia, and fatty acid oxidation.6 These changes lead to an increase in hormones such as epinephrine, cortisol, catecholamines, and glucagon that in turn cause significant metabolic alterations,7 which continue to promote catabolism and oxidative stress, ultimately leading to malnutrition and organ damage.
Carbohydrate in Enteral Nutrition
Providing energy as carbohydrates or fat is essential to minimise protein degradation in all illness and injury.10 The clearance rate of circulating lipid is decreased during illness, which may lead to hypertriglyceridemia and hepatic fat accumulation. Thus, carbohydrate should be the primary energy source for patients experiencing changes in metabolism due to illness or injury.6
In most enteral formulas, carbohydrate is the primary macronutrient and principal energy source. However, poorly controlled glucose levels in critically ill patients can lead to glucose variability with hyper- and hypoglycaemia, conditions that can impede recovery and can be fatal.3
Low-glycaemic index enteral nutrition may provide metabolic and clinical benefits in critical illness. Lowering glycaemic index (GI) and glycaemic load (GL) can improve metabolic control.11 Furthermore, increasing the protein-to-carbohydrate ratio can reduce glycaemia.12