Page 38 - To keep a balance in disease specific intestinal insufficiency. Diagnostics and practical nutritional aspects - Nicolette Wierdsma

Chapter 3
and specificity, when using a small fat load (20 g). It can also be performed with the stable 13C carbon isotope (12-15).
Assessment of protein (mal)absorption
Fecal nitrogen excretion test
Unabsorbed protein can be estimated by quantifying fecal nitrogen content (Kjehdahl technique), multiplied by 6.25 for conversion into proteins. In this method, all the excreted nitrogen is interpreted as residues of endogenous ingested protein, while this is partly derived from intestinal leakage, shredded cells, bacteria and others (16). No link is made with ingested protein from nutritional intake, so, in its current form, it does not reflect intestinal protein absorption.
Radio actively labeled protein test
In research setting, labeled isotope techniques are used to study protein absorption and metabolism. By combining the use of specifically produced intrinsically L-[1-13C]phenylalanine-labeled dairy protein with continuous intravenous L-[ring-2H5]phenylalanine infusion, in vivo dietary protein digestion and absorption kinetics can be assessed (17,18). Rarely, radio actively albumin is used to assess protein malabsorption, since protein malabsorption induces a decrease in albumin synthesis rate (19-20).
Fecal alpha-1 antitrypsine (clearance)
The protein fecal alpha-1 antitrypsine is a non-digestable protein and can be measured in feces and serum. Subsequently, clearance can be calculated as a marker for the amount of intestinal protein loss. It is mostly used as a diagnostic test for intestinal protein leakage or diagnosing protein losing enteropathy rather than it is a malabsorption test (14).
Assessment of fecal energy content
Bomb calorimetry
The energy content of feces may be accurately assessed by using bomb calorimetry, which is a technique which measures the heat of combustion of a standardized fecal sample in a ballistic bomb calorimeter. It is considered as
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