There is a large variability in the data, but in general, units administered prior to retiring for the night have a slower colonic transit than those dosed in the morning, which is in keeping with the suppressed electrical and contractile activity70-73, and decreased tone in the colon overnight74. There are conflicting values in the literature for the average transit time from caecum to splenic flexure; in one study it was reported to be 14 hours31; however another study reported that 50% of large units reach the splenic flexure within 7 hours of entering the colon, and the size and density of such units had little effect on the transit times75. Steady state experiments, conducted by repeated daily administrations of technetium-99m labelled resin, demonstrate that the transverse colon remains relatively empty during the day.
Studies examining the transit of different sized particles through the colon have suggested that large objects move more rapidly than smaller ones. In a study on a limited number of healthy volunteers (n=6), a pressure sensitive radiotelemetry device (25x9 mm) was seen to move ahead of dispersed pellets (0.5-1.8 mm) in the ascending and transverse colon (Figure 7.6)76. Similarly a tendency for transit rates to increase with increasing unit volume (0.3 to 0.8 to 1.8 cm3) has also been demonstrated75. Differential transit rates have also been noted between 0.5-1.8 mm pellets and 6 mm plastic markers77. In contrast, others have found no difference in the passage of 0.2mm and 5mm particles78 or between 0.2mm, 5mm, and 8.4mm particles79 through the ascending colon. Interestingly, in this latter study, there was a trend towards a shorter residence time within the ascending colon for 0.2mm
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