What is anorexia of illness?
During sickness it is very common to lose one’s appetite and reduce energy intake. This anorexia of illness is one of a group of symptoms collectively known as sickness behaviors. Whether anorexia and other sickness behaviors are adaptive is uncertain and the optimal amount of nutrition to provide during illness is an unanswered question in medicine. However, various authors have speculated that anorexia of illness might be beneficial by assisting host defense during infection or by changing energy provisioning for various parts of the body. If true, it is possible that providing fewer than normal nutrients during illness might speed recovery and improve survival. On the other hand, malnutrition or starvation has been associated with mortality in infection.
What are the possible benefits of illness anorexia?
Exton (1997) postulated that anorexia may be beneficial by limiting the availability to pathogens of essential trace metals, notably iron; and he presented evidence that dietary restriction can enhance certain immune functions.
Straub et al. (2010) proposed an “energy appeal reaction” model that sees the catabolic state of the APR as an attempt to redirect nutrient energy toward meeting the high metabolic costs of fighting infection.
Anorexia also promotes cell apoptosis and might aid in pathogen clearance (LeGrand 2000). Anorexia may also be a gamble on the part of the host that the host can better withstand the nutritional stress better than the invading organism.
Because the host has stored energy reserves, anorexia may disproportionately affect gut pathogens and make infected intestinal epithelial cells more susceptible to apoptosis (LeGrand and Alcock 2012).
Anorexia may occur as a means of preventing pathogenic gut microbes from gaining access to growth limiting nutrients (LeGrand and Alcock 2012).
The idea of permissive underfeeding in critical illness was proposed by Zaloga and Roberts (1994). They hypothesized that maximizing nutrition “may adversely affect the host response to injury, especially when given in excess of energy and protein needs.” Recent evidence suggests that calorie restriction during illness might be protective:
Is there evidence from animal experiments?
Adamo et al. (2007), working on the assumption that anorexia during infection is beneficial, found that force-feeding bacterially infected caterpillars with a high lipid diet increased mortality.
Murray et al. (1978) force-fed Listeria-infected mice back to their preinfection food intake levels and found that these mice with “good/normal nutrition” had lower survival than did the infected sham force-fed mice.
Is there evidence from human trials?
More recently, human trials have shown similar results:
Observational trials:
Patients with higher illness severity were found to have longer stays in the ICU when feeding was initiated early, versus late (Huang et al 2012). Arabi and colleagues showed that achieving target nutrition (e.g. higher calories) is associated with worse outcomes in the ICU (2010) in keeping with previously reported work (Krishnan et al. 2003). This idea remains controversial, and other observational studies have reported better outcomes with higher calorie delivery (Heyland Cahill and Day 2011; Elke et al. 2013).
Randomized controlled trials:
Arabi and colleagues (2011) randomized patients to underfeeding (60%) vs. normal (100%) replacement of calorie needs in critical illness. This group found decreased deaths in the underfeeding group.
Writing Assignment
You are called to do a nutrition consultation for a patient in the ICU. The question is should we feed this patient less calories, more calories, or the same calories as the patient needed before they got sick?
Reading 1: Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars (for discussion)
Reading 2 Schetz artificial nutrition (for writing assignment)
Reading 3 (mandatory; emailed to you, contact me if you did not receive it)
Optional (but recommended for the writing assignment) reading 4:
Kenra et al Infection and Anorexia
More references:
Krishnan JA, Parce PB, Martinez A, Diette GB, Brower RG. Caloric intake in medical ICU patients: consistency of care with guidelines and relationship to clinical outcomes. Chest 2003, 124: 297-305.
Categories: Uncategorized
Joe Alcock
Emergency Physician, Educator, Researcher, interested in the microbiome, evolution, and medicine
EvolutionMedicine 
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