Fats and other lipids
Of the dietary lipids, 90% to 95% are triglycerides, generally known as fats. Small amounts of di- and monoglycerides, cholesterol, and phospholipids are also ingested. Digestion, absorption, and transport of fat in the body present special problems because of the insolubility of fat in the intestinal contents and other body fluids. Depending on the length of the carbon chain of the constituent fatty acids, there are also differences in the solubility of triglyceride molecules and, therefore, in the ways that the body handles them.
The bulk of fat in a normal diet consists of so-called long chain triglycerides (LCT) with fatty acid length from carbons up (including some 12 carbon fatty acids). Medium chain triglycerides (MCT, predominantly 8-10 carbon fatty acids) are of little significance except in therapeutic diets consumed by patients with certain mal- absorption syndromes. Short chain triglycerides (SCT, less than 8 carbons) are used in a manner similar to MCT and are found mainly in milk fat.
Al though differences exist in the ra te of absorption of triglycerides \vith different fatty acid composition (chain length and saturation), it is rapid enough in healthy people to allow 95% to 99% absorption \vith intakes up to 250 g per day.
Because of the completeness of fat absorption in healthy adults, comparative studies of the absorption ra tes of dietary fa ts have been conducted mainly in disease states with reduced overall fat absorption or in premature infants whose capacity to handle dietary fat is also limited. It is evident from such studies that the melting point of fat influences the rate at which it is absorbed. For this reason, very hard fats, those with a large proportion of completely saturated long chain fatty acids, are absorbed less readily than liquid oils with a high content ofunsaturated or short chain fatty acids. The effect of triglyceride composition on the overall rate of fat absorption can probably be attributed to differences both in the rate of hydrolysis and in the rate of absorption of the end products, although the mechanisms involved are not known. As was mentioned earlier, these differences are of little significance in healthy people but may be very important in determining the extent of steatorrhea present in certain malabsorption syndromes.
