Abstract
Trans fatty acids (TFA) could affect cell membrane functions, and may therefore influence peripheral insulin sensitivity and the risk of developing type 2 diabetes. It is important to understand whether low amounts of TFA consumed during long periods may promote insulin resistance and have clinically relevant effects on diabetes risk. Data from controlled intervention studies examining the effects of TFA on insulin sensitivity and type 2 diabetes are reviewed. The results show no consistent effect of TFA on insulin sensitivity in lean healthy subjects, but there is some evidence that TFA could impair insulin sensitivity more than unsaturated fat in subjects with insulin resistance or type 2 diabetes. In particular, conjugated TFA, i.e. certain isomers of conjugated linoleic acid (CLA), impair insulin sensitivity and could promote metabolic disorders. The effect of CLA (trans10cis12) on insulin sensitivity and lipid peroxidation is the most dramatic adverse effect described for a dietary fatty acid. CLA isomers are found in relatively low amounts, but long-term exposure may, in theory, have unwanted health effects. The mechanisms of CLA effects are still not completely understood, but may involve increased oxidative stress and inflammation, as well as endothelial dysfunction and direct down-regulating effects on transcription factors required for optimal insulin sensitivity. The inconsistent effect of TFA as a group may be due to methodological limitations (e.g. few studies, with short duration and small sample size) and differences between studies in design, and the type and amount of TFA used. Large controlled trials have been required to demonstrate adverse effects of saturated fat on insulin sensitivity, and similar efforts will be needed to clarify the effect of TFA on insulin sensitivity and diabetes risk. CLA isomers are a group of TFA with potentially adverse effects on glucose metabolism. There are no data to suggest that TFA in general impair insulin sensitivity in practice, compared with such an effect of the much more abundant saturated fatty acids. Keywords: CLA; controlled trial; inflammation; insulin resistance; insulin sensitivity; oxidative stress; trans fatty acids
Highlights
Epidemiological data from the Nurses Health Study indicate that trans fatty acids (TFA) increase the risk of developing type 2 diabetes [1]
There are several points and questions that should be considered when interpreting the data published so far on the health effects of TFA. (i) Only shortterm data with varying duration are available so far. (ii) Do the studies have sufficient power? (iii) What is the role of the background diet? (iv) Are there differences between types of TFA? (v) Do lean and obese subjects respond differently? (vi) What is the importance of the various amounts of TFA used? and (vii) Few studies have used the goldstandard method for determining insulin sensitivity
The literature suggests that TFA have no significant effect on insulin sensitivity in lean, healthy subjects
Summary
Epidemiological data from the Nurses Health Study indicate that trans fatty acids (TFA) increase the risk of developing type 2 diabetes [1]. Since insulin resistance is a major contributor to type 2 diabetes, it is of great interest to study the effect of TFA on insulin sensitivity. There is fairly convincing evidence from controlled studies to suggest that saturated fatty acids (SFA) impair insulin sensitivity when substituted for unsaturated fats [2]. The mechanisms behind such effects are unclear, but could involve several possible components such as altered physiochemical membrane properties, fuel partitioning (via altered fat oxidation and carbohydrate oxidation) or altered gene expression. Recent animal experiments indicate that TFA impair fat cell membrane fluidity and insulin sensitivity, but TFA may down-regulate peroxisome proliferator-activated receptor-g (PPAR-g) in adipose tissue [3]. The aim of this paper was to review intervention studies that have examined the effects of TFA on insulin sensitivity
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