The absorption of tristearin and stearic acid and tripalmitin and palmitic acid: Studies on the rate-limiting steps in rats

Abstract

1. 1. In free feeding experiments in rats the absorption of labelled tristearin was poor. It remained constant over a wide dose range, but fell with a high dose. The absorption in bile fistula rats was diminished. 2. 2. The addition of triolein increased the absorption of labelled tristearin in control rats but not in bile fistula rats, suggesting that poor solubility in bile salt solution is an important step hindering the absorption of tristearin and that triolein stimulates absorption by increasing this solubility. 3. 3. Jejunal segments incubated in a micellar solution of equal concentration of labelled stearic acid and oleic acid in 10 mM sodium taurocholate took up stearic acid at a slightly faster rate than oleic in a ratio of 0.84, but incorporation into triglyceride was greater for oleic acid in the ratio of 1.3. 4. 4. When a micellar solution of equal concentration of labelled oleic and stearic acids were infused into the duodenum the content of oleic acid in lymph triglyceride was greater than that of stearic acid in a ratio of 1.35. 5. 5. When equal weights of labelled oleic and stearic acids were fed, mixed with bran, the content of oleic acid in lymph triglyceride was much greater than that for stearic acid, in a ratio of 8.9 in the first hour, falling to 4.5 by the fourth hour. These ratios are much greater than those during lymphatic absorption from equimolar micellar solution. 6. 6. We conclude that micellar solubilisation is a major rate-limiting step in the absorption of tristearin and stearic acid, that mucosal cell uptake is not rate-limiting and that esterification is only a minor rate-determining step. 7. 7. The absorption of palmitic acid by control rats was slightly increased by a large dose of triolein added to the feed, but that of tripelmitin was unaltered. Reasons for the difference in behaviour between tripalmitin and tristearin are discussed.