Biokinetics of¹³C in the human body after oral administration of¹³C-labeled glucose as an index for the biokinetics of¹⁴C

Abstract

The retention of C in the human body after oral administration ofC-labeled glucose was studied in three healthy volunteer subjects to estimate the 50 year cumulative body burden for C as an index of the committed dose of the radioisotopeC. After administration of C-labeled glucose, the volunteers ingested controlled diets with a fixed number of calories for 112 d. Samples of breath and urine were collected up to 112 d after administration. Samples of feces were collected up to 14 d after administration. Hair samples were obtained at 119 d after administration and analyzed as a representative index of the rate of excretion of organicC via pathways such as skin cell exfoliation and mucus secretion. All samples were analyzed for C/C atomic ratio to determine the rate of excretion via each pathway. We then constructed a metabolic model with a total of four pathways (breath, urine, feces, and other) comprising seven compartments. We determined the values of the biokinetic parameters in the model by using the obtained excretion data. From 74% to 94% of the C administered was excreted in breath, whereas<2% was excreted in urine and feces. In the other pathway, the excretion rate constant in the compartment with the longest residence time stretched to hundreds of days but the rate constant for each subject was not statistically significant (P value > 0.1). In addition, the dataset for one of the three subjects was markedly different from those of the other two. When we estimated the 50 year cumulative body burden for C by using our model and we included non-statistically significant parameters, a considerable cumulative body burden was found in the compartments excreting to the other pathway. Although our results on the cumulative body burden ofC from orally administered carbon as glucose were inconclusive, we found that the compartments excreting to the other pathway had a markedly long residence time and therefore should be studied further to clarify the fate of carbon in the human body. In addition to excreta, data for serum and blood cell samples were also collected from the subjects to examine the metabolism of C in human body.

Basis for the ICRP’s updated biokinetic model for carbon inhaled as CO2

“…The assumption of identical biokinetics of carbon entering the body as bicarbonate or carbon dioxide was also made in the development of the model described in this paper. Broadly similar systemic kinetics of the carbon label has been observed for other forms of carbon including 13 C-glucose (Masuda et al 2016), but attention was restricted here to bicarbonate as a reasonably well established biokinetic analogue of inhaled carbon dioxide.…”