Absorption of urea through the oral mucosa and estimation of the percentage of secreted whole saliva inadvertently swallowed during saliva collection

SummaryObjectiveTo determine whether some of the urea added to certain chewing gums may be absorbed through the oral mucosa and whether some saliva is inadvertently swallowed during the collection of saliva elicited by the chewing of gum.DesignOn two occasions, 10 experienced saliva collectors made a 5 min collection of unstimulated whole saliva and then chewed gum for 10 min and during this time collected their saliva. On one occasion, they chewed one tablet of gum containing 0.5 mg of Phenol Red, a non-absorbable substance, and one tablet of a gum containing 27.3 mg of urea. On another occasion, they chewed two tablets of the Phenol Red gum. Their saliva and the chewed gum were assayed for their Phenol Red and urea contents and the totals calculated. Since saliva normally contains urea, the recovery of urea was calculated as the difference between the amounts recovered in the two collection sessions.ResultsThe mean recovery of Phenol Red was 96.7%, but in three participants the amount recovered was less than the 95% confidence limits for assay error. The mean recovery of urea was 85.7% and in nine of the 10 participants, the amount recovered was less than the confidence limits for assay error. In all participants, the percentage urea recovery was less than that of Phenol Red.ConclusionThe results showed: (1) that Phenol Red appears to be a useful, non-absorbed marker for studies of drug absorption through the oral mucosa, (2) that when the salivary urea concentration is higher than that in plasma, urea may be absorbed through the oral mucosa, (3) that even experienced saliva collectors may inadvertently swallow some of the saliva they produce. This latter finding has implications for all clinical studies of saliva.