Increasing evidence may indicate, however, that this concept is obsolete, and energy loss through mal-absorption may be an overlooked problem in ICU patients. Using screening libraries bomb calorimetry, a method to quantify the energetic losses in faecal material, investigators demonstrated substantial loss of calories in the faeces of ICU patients, most of them fed postpylorically [7]; in 13 patients with a faeces collector because of loose stools, the caloric value of energy loss was a mean of 301 kcal/day, and 3 patients had a loss of more than 500 kcal/day in the stools [7].Decreased intestinal absorption in ICU patients may conceivably be multifactorial; gut mucosal atrophy and decreased splanchnic perfusion have been described extensively during critical illness.

Also, digestive secretory function (essential for degradation of more complex nutrients) may be qualitatively or quantitatively affected, as neurohumoral control is frequently impaired. Furthermore, intestinal motility is possibly inappropriate for optimal digestion and absorption of nutrients due to the persistence of migrating motor complexes during feeding [2,8].Conducting studies assessing intestinal absorption in ICU patients is a difficult task. In particular, caution should be taken when interpreting the kinetics of 3-O MG (to represent glucose) absorption in the ICU setting. ICU patients do indeed show pharmacokinetic differences compared with normal individuals, including increased volume of distribution and variable clearance of substances [9,10].

In septic patients, the volume of distribution of hydrophilic substances is often greater due to an increased capillary permeability resulting in fluid shifts from the intravascular compartment to the interstitial space. Distribution volume may also be increased in ICU patients by the presence of mechanical ventilation, hypoalbuminaemia (increased capillary leakage), extracorporeal circuits, postsurgical drains, or burn injury [10]. The resulting effect would be a decreased plasma concentration of the molecule with a risk of misinterpretation of some important kinetic parameters (area under concentration-time curve, maximal concentration, time to peak concentration). This may be of particular relevance if the sampling period is relatively short. To circumvent this problem, quantification of urinary excretion of the test Dacomitinib substance during prolonged urinary collection may be preferable. Alternatively, the so called ‘dual probe’ method has been proposed; in this method, simultaneous administration of probe substances that respond in a similar way to variables such as extra-cellular fluid volume or renal clearance enables calculation of urinary excretion ratios, thereby eliminating the effect of these factors [4].