Fats, or lipids, are found in all cells and perform a variety of functions essential for life. These include their roles in energy storage, membrane structure, and incorpora-tion in vitamins, hormones, and prostaglandins (Zubay, 1983). Fats are used to cushion and insulate the body and function as electrical insulation in the nervous system. Triglycerides are the most common form of fat found in foods and stored in body fat depots. Triglycerides are comprised of three fatty acids (figure 1) esterified to a glycerol backbone (figure 2). Most naturally occuring triglycerides contain fatty acids 16-20 carbon atoms in length. Such fatty acids are called “long chain fatty ac-ids” (LCFAs), and their corresponding triglycerides are called “long chain triglycerides” (LCTs).
Medium chain triglycerides (MCTs) are comprised of medium chain fatty acids (MCFAs), which are 6-12 carbons in length. Although the carboxylic acid part of fatty acids is soluble in water, the hydrocarbon chain is not. Thus, LCFAs are not water soluble. Since the hydrocarbon chains of MCFAs are shorter, MCFAs are more water soluble than LCFAs. Likewise, MCTs are also relatively soluble in water, due to ionization of the carboxylic acid groups and the small size of the hydrocarbon chains . Their small molecular size and greater water solu-bility cause MCTs to undergo a differ-ent metabolic path than that experienced by LCTs (Bach and Babayan, 1982).
Occurrence and Purification of MCTsMedium chain triglycerides occur naturally in small quantities in a variety of foods, and are present naturally in the blood of the human fetus and in human milk (Bach and Babayan, 1982; Souci, Fachmann, Kraut, 1989/90). In cow’s milk, C6-C14 fatty acids together account for 20% of the total fatty acid composition (Christensen et al, 1989). Commercially, medium chain fatty acids are prepared by the hydrolysis of coconut oil (an abundant source) and are fractionated by steam distillation. The MCFAs so obtained consist of predominantly C8:0, with lesser amounts of C10:0, and minute amounts of C6:0 and C12:0. The fractionated MCFAs are re-esterified with glycerol to generate MCTs (Bach and Babayan, 1982). MCT oil softens or splits certain plastics such as polyethylene and polystyrene, but not polypropylene. It is recommended that MCT oil be stored in metal, glass, or ceramic containers (Sucher, 1986). MCT oil has a caloric density of 8.3 calories per gram; one tablespoon equals 14 grams and contains 115 calories. MCTs are not drugs and have no pharmacological effects (Bach and Babayan, 1982). Historical Uses of MCTsSince their introduc-tion in 1950 for the treatment of fat malab-sorption problems, me-dium chain triglycer-ides have enjoyed wide application in enteral and parenteral nutri-tion regimens (Bach and Babayan, 1982).
Fat emulsions can be used to provide up to 60% of nonprotein calories. Before the availability of lipid emulsions suitable for intravenous use, glucose was used as the only nonprotein source of calories (Mascioli et al, 1987). Not only did this result in essential fatty acid deficiencies, but it was also undesirable because it increased hepatic lipogenesis and respiratory work. Although inclusion of LCTs in intravenous feedings represented an improvement, problems remained with slow clearance of LCTs from the bloodstream and inter-ference with the RES component of the immune system. When medium chain triglycerides or structured lipids (triglycerides containing both MCFAs and LCFAs) are added to the regimen, calories are provided in a more readily oxidizable form (Schmidl, Massaro, and Labuza; 1988), and less interference with the RES is observed (Mascioli et al, 1987). In one case, MCT was fed as the exclusive source of fat (along with a small amount of LCT to provide essential fatty acids) to a patient with chyluria (a fat malabsorption disease) for over 15 years without producing side effects (Geliebter et al, 1983).
Sports NutritionAlthough MCTs have been used in hospital environments for years, their use by healthy individuals is relatively new. Recently, athletes have begun to use MCTs to II. MetabolismDigestion and Absorption of FatsSince LCTs are not very soluble in water, the body has to go through an elaborate digestive process in order to absorb these nutrients. Bile salts are secreted by the gall bladder to help dissolve the LCTs. Upon ingestion, LCTs interact with bile in the duodenum (upper small intes-tine) and are incorporated into mixed micelles (Record et al, 1986). Enzymes called lipases (pancreatic lipase and phospholipase A2) remove the fatty acid molecule from the glycerol backbone. The mixed micelles are passively absorbed into the intestinal mucosa where the free fatty acids are re-esterified with glycerol. The in-testinal mucosa synthesizes a lipoprotein carrier called a chylomicron to transport the reformed triglyceride. Chy-lomicrons are secreted into the lymph and are released into the venous circulation via the thoracic duct. In the bloodstream, lipoprotein lipase again breaks down the triglycerides into two free fatty acids and a monoglyc-eride.
The monoglycerides go to the liver to be further degraded, while many of the circulating free fatty acids are taken up and stored by adipocytes (fat cells). When carbohydrates are consumed insulin is released, and in-sulin stimulates adipocytes to re-esterify the fatty acids into triglycerides and store them as body fat. In general, body fat stores are not mobilized and used for energy to any significant extent in the presence of insulin.In contrast, since MCFAs are more water soluble they are more easily absorbed and do not require this complicated digestive process. MCTs can be absorbed intact and do not require the action of pancreatic lipase or incorpo-ration into chylomicrons. Instead, a lipase within the intestinal cell degrades the MCT into free MCFAs and glycerol. The MCFAs are bound to albumin, released into the bloodstream, and transported directly to the liver by the portal vein. The vast majority of MCFAs are retained by the liver where they are rapidly and extensively oxidized. Whereas conventional fats are largely deposited in fat cells, MCTs are transported directly to the liver and used for energy. Very little of the MCFAs ever escape the liver to reach the general circulation (Bach and Babayan, 1982). Only 1-2% of MCTs are incorporated into depot fat (Geliebter et al, 1983; Baba, Bracco, and Hashim, 1982). Medium chain triglycerides are digested and absorbed much faster than conventional fats (in fact, as rapidly as glucose) and are immediately available for energy.
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