Fluoride absorption from the gastrointestinal tract of rats

The contribution of the stomach and the small intestine to absorption of fluoride from the gastrointestinal tract was examined in rats. Fasted adult male rats weighing ~350 g were given 50 μg of fluoride in 1 ml of water by stomach intubation, with ¹⁴C-labeled polyethylene glycol as a marker of water movement through the gastrointestinal tract. Rats were killed at intervals up to 120 min, and the stomach, duodenum, jejunum, ileum, distal ileum and cecum were rapidly clamped and removed for fluoride analysis and ¹⁴C counting. Approximately 90% of the fluoride dose was absorbed in 120 min. Peak plasma fluoride concentration occurred 10 min after intubation and began to decline after 40 min as the rate of fluoride absorption slowed. Absorption from the stomach was derived from rates of gastric emptying and remaining fluoride. Even at 10 min after intubation, when the bulk of the fluoride remained in the stomach, only approximately 25% of fluoride absorption had occurred from the stomach and 75% from the small intestine. After 120 min, 19.8% of total fluoride absorption had occurred from the stomach. Although the stomach is unquestionably a significant site for fluoride absorption, its contribution is much smaller than that of the small intestine.