Conventional fats and oils are composed of glycerides of 12- to 18-carbon long-chain fatty acids (LCFA). These compounds are known as long-chain triglycerides (LCT) and are the predominant form of lipids in the diet. Lipids are an essential source of energy and essential fatty acids, and a vital component of body cells. Therefore, it would be beneficial to have a dietary fat with the added benefit of anti-obesity properties. Medium-chain triglycerides (MCT) have a number of unique characteristics relating to energy density, absorption and metabolism, which give them advantages over the more common LCT. Upon hydrolysis, MCT yield medium-chain fatty acids (MCFA) [caproic (C6), caprylic (C8), capric (C10), lauric (C12)] (Papamandjaris et al., 1997). Naturally occurring sources of MCT are rare, but include milk fat, palm kernel oil, and coconut oil. Human consumption of MCT is currently low but intake should perhaps be greater due to the distinctive properties of MCT, which cause an increase in energy expenditure (EE) and increased satiety that may contribute to weight loss.
First, the energy density of MCFA is less than that of LCFA due to their shorter chain length. MCT provide about 10% fewer calories than LCT -8.3 cal/g for MCT versus 9 cal/g for LCT (Bach et al., 1972). Therefore, the use of MCT can decrease caloric intake and potentially decrease body weight and body fat in the long term.
Secondly, MCT demonstrate additional characteristics of evident advantage. Specifically, MCFA are readily hydrolysed from triglycerides (TGs) by lingual and gastric lipases, compared with LCFA that require intestinal lipase to cleave TGs (Babayan, 1987). MCT oils also require less bile salts and pancreatic enzymes for digestion than LCT. The intramucosal metabolism of MCFA differs from that of LCFA. MCFA have a lower affinity for esterifying and activating enzymes, and there is a minimal re-sterification of MCFA to MCT, as compared with LCFA (Babayan, 1987). In addition, the intraluminal enzymatic hydrolysis of MCT is more rapid and complete than that of LCT since the smaller molecular size of MCT allows slight amounts to enter the intestinal cell without prior hydrolysis (Babayan, 1987). These properties enable MCT to be absorbed rapidly and more completely than LCT.
Thirdly, MCT are metabolized differently from LCT as demonstrated in Fig. 14.1. MCT are disassembled and enter the bloodstream as medium-chain free fatty acids (MCFFAs). These MCFFAs are transported to the liver directly via the hepatic portal circulation, which does not require chy-lomicron formation. In contrast, LCT are absorbed via intestinal lymphatic ducts and transported in chylomicrons through the thoracic duct to reach the systemic circulation. MCFA are transported into hepatocytes and converted to medium-chain fatty acyl CoA esters, which are further transported into mitochondria for conversion to acetoacetate and beta-hydroxybutyrate. These substrates may be further metabolized in the liver to carbon dioxide, water, and energy. Finally, MCFA tend to be preferentially oxidized in comparison with LCFA; therefore, less ingested MCT are deposited in the body as fat.
In summary, these unique characteristics of MCT, including their caloric content, and digestive and metabolic pathways, may give them the potential to induce weight loss as well as to maintain a healthy body weight.
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