The fatty acid composition of chylomicron remnants influences their propensity to oxidate

The chronic effects of fish oil with exercise on postprandial lipaemia and chylomicron homeostasis in insulin resistant viscerally obese men

Background

Visceral obesity and insulin resistance are associated with a postprandial accumulation of atherogenic chylomicron remnants that is difficult to modulate with lipid-lowering therapies. Dietary fish oil and exercise are cardioprotective interventions that can significantly modify the metabolism of TAG-rich lipoproteins. In this study, we investigated whether chronic exercise and fish oil act in combination to affect chylomicron metabolism in obese men with moderate insulin resistance.

Methods

The single blind study tested the effect of fish oil, exercise and the combined treatments on fasting and postprandial chylomicron metabolism. Twenty nine men with metabolic syndrome were randomly assigned to take fish oil or placebo for four weeks, before undertaking an additional 12 week walking program. At baseline and at the end of each treatment, subjects were tested for concentrations of fasting apo B48, plasma lipids and insulin. Postprandial apo B48 and TAG kinetics were also determined following ingestion of a fat enriched meal.

Results

Combining fish oil and exercise resulted in a significant reduction in the fasting apo B48 concentration, concomitant with attenuation of fasting TAG concentrations and the postprandial TAG_IAUC response (p < 0.05). Fish oil by itself reduced the postprandial TAG response (p < 0.05) but not postprandial apo B48 kinetics. Individual treatments of fish oil and exercise did not correspond with improvements in fasting plasma TAG and apo B48.

Conclusion

Fish oil was shown to independently improve plasma TAG homeostasis but did not resolve hyper-chylomicronaemia. Instead, combining fish oil with chronic exercise reduced the plasma concentration of pro-atherogenic chylomicron remnants; in addition it reduced the fasting and postprandial TAG response in viscerally obese insulin resistant subjects.

Phospholipid peroxidation: lack of effect of fatty acid pairing

Phospholipids where both fatty acids are polyunsaturated are very rare. Most organisms prefer to couple their polyunsaturated fatty acids (PUFA) with either a saturated (SAT) or a monounsaturated (MUFA) fatty acid. This study examined if these natural couplings are there to protect PUFA from themselves. Specifically, does the coupling of PUFA to SAT or MUFA reduce the potential for increased rates of peroxidation by shrouding these highly peroxidisable fatty acids with less peroxidisable fatty acids? The influence of head group was examined by using the two most common phospholipids found in vertebrate membranes i.e. phosphatidylcholine and phosphatidylethanolamine species. Fatty acid pairings included 16:0/18:2 versus 18:2/18:2 and 16:0/22:6 versus 22:6/22:6. All phospholipids were incorporated into liposomes that were matched for their total PUFA content i.e. 25% PUFA/PUFA or 50% SAT/PUFA with phosphatidylcholine 16:0/16:0 used as the background phospholipid. An iron initiator (Fe²⁺/H₂O₂) was used to induce peroxidation and lipid hydroperoxide production was used to measure peroxidation. The results show that coupling of PUFA together on the same molecule does not increase peroxidation rates and therefore does not support the proposed hypothesis. The lower than expected levels of peroxidation measured for some phospholipid species (e.g. PtdEtn 22:6/22:6) is possibly due to the partitioning of these molecular species into the inner leaflet of the bilayer.

Long-chainn-3 polyunsaturated fatty acid from fish oil modulates aortic nitric oxide and tocopherol status in the rat

In spite of their high oxidisability, long-chainn-3 PUFA protect against CVD. Dietary fatty acids modulate the fatty acid composition of lipoproteins involved in atherosclerosis. We thought that if long-chainn-3 PUFA were able to increase NO production by the aorta, then by its antioxidant activity the NO will prevent lipid peroxidation. However, the beneficial effect of NOin vivoon VLDL+LDL oxidation would only be possible if NO could diffuse to their lipidic core. Rats were fed maize oil- or fish oil as menhaden oil- (MO) rich diets for 8 weeks, to study the effects of MO on aortic NO production, NO diffusion into VLDL+LDL, the extent of oxidation in native VLDL+LDL and their oxidisabilityex vivo. Aortic NO production and its α-tocopherol content were increased andn-3 PUFA were incorporated into the VLDL+LDL. In spite of the higher peroxidisability and the low α-tocopherol in native VLDL+LDL from rats fed MO, native VLDL+LDL from the two groups shared similar electrophoretic patterns, conjugated dienes, thiobarbituric acid-reactive substances, total antioxidant capacity, and NO diffusibility on VLDL+LDL, indicative of anin vivoprotection against oxidation. However, these results do not correlate with theex vivooxidisability of VLDL+LDL, as NO is lacking. Thus, thein vivobeneficial effects can be explained by increased α-tocopherol in aorta and by a compensatory effect of NO on VLDL+LDL against the low α-tocopherol levels, which may contribute to the anti-atherogenic properties of fish oil.