Nutrition in Acute Critical Illness

Nutrition in acute critical illness concerns how patients are nourished during the early, unstable phase of a life-threatening condition, when the body's metabolic response to injury or infection dominates. In this phase the choices that matter most are when to start feeding, by which route, and how much to give, against a backdrop of catabolism, organ dysfunction, and changing tolerance.

Definition

The assessment and delivery of nutrition support to acutely critically ill patients, whose stress metabolism alters energy and protein requirements and constrains the route, timing, and amount of feeding that is safe and beneficial.

Scope

The topic covers nutritional assessment and support of the general critically ill adult: the metabolic stress response, the enteral versus parenteral route, the timing of nutrition, and the dose debate between full-target feeding and permissive underfeeding. It draws on the major intensive-care nutrition trials and guidelines and treats them as evidence to interpret rather than as instructions to apply.

Core questions

• When should nutrition support begin in the acute phase of critical illness?
• Is the enteral or the parenteral route preferable for early nutrition?
• Should early feeding aim for full caloric targets or be deliberately restricted?
• How are energy and protein requirements estimated when measured values are uncertain?

Key concepts

• Metabolic stress response
• Hypercatabolism and muscle wasting
• Early enteral nutrition
• Parenteral nutrition
• Permissive underfeeding
• Caloric and protein targets
• Refeeding syndrome
• Indirect calorimetry

Mechanisms

Acute critical illness triggers a neuroendocrine and inflammatory response that mobilizes endogenous fuel: stress hormones and cytokines drive proteolysis of skeletal muscle, lipolysis, and hepatic gluconeogenesis, producing hyperglycemia and insulin resistance. Because the body is already generating substrate, externally delivered nutrition adds to rather than simply replaces this supply, which is why excessive early feeding can be harmful. Enteral nutrition is generally favored for preserving gut function, while parenteral nutrition is reserved for when the gut cannot be used; the trials by Harvey and colleagues and by Casaer and colleagues explored how route and timing affect outcomes, and Arabi and colleagues tested deliberately lower caloric delivery.

Clinical relevance

Decisions about route, timing, and dose of nutrition are part of everyday intensive-care practice, and the guidelines that describe them (Singer et al., 2019; McClave et al., 2016) are built on the trials summarized here. This entry explains the reasoning and the evidence so the reader can appraise it; it characterizes population-level recommendations and is not a basis for individual feeding orders.

Epidemiology

Loss of muscle mass and malnutrition are common during prolonged critical illness and are linked to weakness, longer ventilation, and worse recovery, which motivates structured nutritional assessment on admission. The acute-phase trials of the 2010s repeatedly found that more aggressive early feeding did not improve, and sometimes worsened, short-term outcomes.

Evidence & guidelines

The ESPEN intensive-care guideline (Singer et al., 2019) and the SCCM/ASPEN guideline (McClave et al., 2016) are the principal syntheses. They draw on randomized trials including early-versus-late parenteral nutrition (Casaer et al., 2011), permissive underfeeding (Arabi et al., 2015), and the enteral-versus-parenteral route question (Harvey et al., 2014), and they frame recommendations cautiously because trial results have often favored restraint in the acute phase.

History

Early critical-care nutrition emphasized supplying as many calories as possible to counter catabolism. From the 1990s onward, concern about overfeeding and its complications grew, and a series of large randomized trials in the 2010s — on parenteral timing, feeding route, and caloric dose — shifted the field toward more measured early feeding, a change reflected in current guidelines.

Debates

Full-target feeding versus permissive underfeeding in the early phase

Trials such as Arabi and colleagues' permissive-underfeeding study and Casaer and colleagues' parenteral-timing study suggest that reaching full caloric targets early may offer no benefit and may carry harm, but the ideal early dose and its dependence on patient subgroup remain debated.

Enteral versus parenteral route for early nutrition

Enteral feeding is generally preferred for maintaining gut integrity, yet a large trial by Harvey and colleagues found no clear mortality difference between routes, keeping the relative weight of route versus dose under discussion.

Seminal works

• singer-2019
• casaer-2011
• arabi-2015
• harvey-2014

Frequently asked questions

Why might feeding a critically ill patient less in the early days not be harmful?

In the acute phase the body mobilizes its own fuel from muscle and fat, so added nutrition supplements rather than replaces this supply; several trials found that deliberately lower early caloric delivery did not worsen, and aggressive parenteral feeding did not improve, outcomes.

Is enteral or parenteral nutrition preferred in critical illness?

Enteral nutrition is generally favored when the gut can be used because it helps preserve gut function, with parenteral nutrition reserved for when enteral feeding is not possible; a large randomized trial found no clear mortality difference between the routes.