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Keywords:

  • antioxidants;
  • endothelial dysfunction;
  • oxidative stress;
  • postprandial lipoproteins;
  • triglyceride-rich lipoproteins

Abstract

Background  Endothelial dysfunction (ED) has been described in Type 2 diabetes (T2DM). We have described previously a diminution of flow-mediated arterial dilatation and, by implication, further ED in T2DM in response to postprandial lipaemia (PPL) at 4 h. This is possibly mediated by oxidative stress/alteration of the nitric oxide (NO) pathway. T2DM subjects tend to exhibit both exaggerated and prolonged PPL. We therefore studied the relationship of PPL to the duration of ED in T2DM subjects and oxidative stress with or without the antioxidant, vitamin C.

Methods  Twenty subjects with T2DM with moderate glycaemic control (mean HbA1c 8.4%) were studied. After an overnight fast, all subjects consumed a standard fat meal. Endothelial function (EF), lipid profiles, and venous free radicals were measured in the fasting, peak lipaemic phase (4 h) and postprandially to 8 h. The study was repeated in a double-blinded manner with placebo, vitamin C (1 g) therapy for 2 days prior to re-testing and with the fat meal. Oxidative stress was assessed by lipid-derived free radicals in plasma, ex vivo by electron paramagnetic resonance spectroscopy (EPR) and by markers of lipid peroxidation (TBARS). Endothelial function was assessed by flow-mediated vasodilatation (FMD) of the brachial artery.

Results  There was a significant decrease in endothelial function in response to PPL from baseline (B) 1.3 ± 1.3% to 4 h 0.22 ± 1.1% (P < 0.05) and 8 h 0.7 ± 0.9% (P < 0.05) (mean ± sem). The endothelial dysfunction seen was attenuated at each time point with vitamin C. Baseline EF with vitamin C changed from (fasting) 3.8 ± 0.9–2.8 ± 0.8 (at 4 h) and 2.9 ± 1.3 (at 8 h) in response to PPL. Vitamin C attenuated postprandial (PP) oxidative stress significantly only at the 4-h time point [301.1 ± 118 (B) to 224.7 ± 72 P < 0.05] and not at 8 h 301.1 ± 118 (B) to 260 ± 183 (P = NS). There were no changes with placebo treatment in any variable. PPL was associated with a PP rise in TG levels (in mmol/l) from (B) 1.8 ± 1 to 2.7 ± 1 at 4 h and 1.95 ± 1.2 at 8 h (P = 0.0002 and 0.33, respectively).

Conclusion  PPL is associated with prolonged endothelial dysfunction for at least 8 h after a fatty meal. Vitamin C treatment improves endothelial dysfunction at all time points and attenuates PPL-induced oxidative stress. This highlights the importance of low-fat meals in T2DM and suggests a role for vitamin C therapy to improve endothelial function during meal ingestion.