Metabolic & Physiologic Effects
Short-chain SFA are uncommon in the human diet, except in dairy products. However, they can be fermented from undigested carbohydrates by gut microbiota in the large intestine. Thus, it may be problematic to separate the effects between dietary fiber and dietary SC SFA (i.e., dairy fat). It is likely that most colonic SC SFA are a result of colonic fermentation. Limited evidence from in vitro and animal studies on cardiometabolic effects is available. Propionic acid (3:0) has been suggested to have beneficial effects on obesity and Type 2 diabetes and have immunosuppressive actions, although mechanisms are not well established. Butyric acid (4:0) is available in bovine milk at 2–5 wt%, which on a molar basis, corresponds to approximately a third of 16:0 levels. It has been demonstrated to exert gene-regulating properties and has been proposed as a protective link between diet and (colon) cancer[32,33] and intestinal inflammation. Proposed mechanisms have included inhibition of NF-κB activation and inhibition of gene expression via histone deacetylase, which has been demonstrated to induce apoptosis in tumor cells. This has been the subject of another review.
Medium-chain TGs have been reported to exert different metabolic effects than LC TGs. Primarily, MC TGs are more prone to β-oxidation in the liver and not stored as fat to the same extent as LC TGs but instead increase resting metabolic rate and thermogenesis. Slight decreases in adiposity have been demonstrated for caprylic (8:0) and capric (10:0) acid intake in humans compared with LC TGs from sunflower or rapeseed oil. MC TGs are also more readily absorbed and transported to the liver via portal circulation, which makes them appropriate in malabsorptive states. Lauric acid is special in that it can be absorbed both by the portal and lymphatic systems and is thus inconsistently described in the literature, sometimes as an MC SFA, and sometimes as an LC SFA. MC TGs may also have a laxative effect and are frequently included in very low calorie diet formulae for weight loss. Monoglycerides from MC SFA, and particularly monolaurin from 12:0, have established antimicrobial effects and have been demonstrated to exert protective effects against Streptococci, Helicobacter pylori, protozoa, caries and topical protection against HIV in primates, as recently reviewed by German and Dillard. Since these MC TGs are present in human milk and colostrum they have been suggested to provide protection to the mucosal surfaces of the infant.
Besides effects on cardiometabolic disease, individual SFA are involved in protein fatty acid acetylation, which may have various regulation functions in the cell, as recently reviewed by Legrand and Rioux. Dietary myristic acid (14:0) may be the limiting factor for this process, known as myristoylation. Palmitoylation is the corresponding process for 16:0 and is involved in protein–membrane interactions and signal transduction. Dietary 14:0 has also been demonstrated to increase the conversion of α-linolenic acid (18:3n-3) to very-LC n-3 fatty acids in experimental studies in rats and also in human feeding studies. In humans, this seems to follow a U-shaped association with maximum effect at 1.2% of daily energy intake as 14:0.
Clin Lipidology. 2011;6(2):209-223. © 2011 Future Medicine Ltd.