Shaun A. Mason

Antioxidant supplements are commonly consumed by endurance athletes to minimize exercise-induced oxidative stress, with the intention of enhancing recovery and improving performance. There are numerous commercially available nutritional supplements that are targeted to athletes and health enthusiasts that allegedly possess antioxidant properties. However, most of these compounds are poorly investigated with respect to their in vivo redox activity and efficacy in humans. Therefore, this review will firstly provide a background to endurance exercise-related redox signalling and the subsequent adaptations in skeletal muscle and vascular function. The review will then discuss commonly available compounds with purported antioxidant effects for use by athletes. N-acetyl cysteine may be of benefit over the days prior to an endurance event; while chronic intake of combined 1000 mg vitamin C + vitamin E is not recommended during periods of heavy training associated with adaptations in skeletal muscle. Melatonin, vitamin E and α-lipoic acid appear effective at decreasing markers of exercise-induced oxidative stress. However, evidence on their effects on endurance performance are either lacking or not supportive. Catechins, anthocyanins, coenzyme Q10 and vitamin C may improve vascular function, however, evidence is either limited to specific sub-populations and/or does not translate to improved performance. Finally, additional research should clarify the potential benefits of curcumin in improving muscle recovery post intensive exercise; and the potential hampering effects of astaxanthin, selenium and vitamin A on skeletal muscle adaptations to endurance training. Overall, we highlight the lack of supportive evidence for most antioxidant compounds to recommend to athletes.

BACKGROUND Evidence suggests that vitamin C supplementation could be a potential therapy in type 2 diabetes. However, its effectiveness and evidence quality require further evaluation. PURPOSE To investigate the efficacy of oral vitamin C supplementation in improving glycemic control, cardiovascular risk factors, and oxidative stress in people with type 2 diabetes. DATA SOURCES Databases (PubMed, Embase, Scopus, Cochrane Library) and clinical trial registries were searched for randomized controlled trials up to 8 September 2020. STUDY SELECTION Trials in adults with type 2 diabetes were included. Trials were excluded if supplements were not exclusive to vitamin C and if <2 weeks in duration. DATA EXTRACTION Primary outcomes were HbA1c, glucose, cholesterol, triglycerides, and blood pressure (BP). Data were extracted for changes in outcomes between vitamin C and control groups. Evidence certainty was assessed using Grading of Recommendations, Assessment, Development, and Evaluation methods. DATA SYNTHESIS Twenty-eight studies (N = 1,574 participants) were included in the review. Outcomes that changed to a statistically and clinically significant extent with vitamin C were systolic BP (mean difference −6.27 [95% CI −9.60, −2.96] mmHg; P = 0.0002), with moderate evidence certainty, and HbA1c (−0.54% [−0.90, −0.17]; P = 0.004) and diastolic BP (−3.77 [−6.13, −1.42] mmHg; P = 0.002) with very low evidence certainty. LIMITATIONS Studies were predominantly short term (<6 months) with a small number of participants (n < 100). CONCLUSIONS While evidence from short-term studies suggests that vitamin C supplementation may improve glycemic control and BP in people with type 2 diabetes, vitamin C supplementation cannot currently be recommended as a therapy until larger, long-term, and high-quality trials confirm these findings.

Aim The primary aim of this study was to investigate whether ascorbic acid (AA) supplementation improves postprandial glucose responses under free‐living conditions in individuals with type 2 diabetes. A secondary aim was to investigate the effect of AA supplementation on blood pressure. Materials and methods A total of 31 individuals with type 2 diabetes (26 males and 5 females; aged 61.8 ± 6.8 years; duration of diabetes, 5.6 ± 4.6 years; HbA1c, 7.6% ± 0.7% [mean ± SD]) were enrolled in a randomized cross‐over study involving 4 months of supplementation with oral AA (2 × 500 mg/d) or placebo. Participants wore continuous glucose monitors for 48 hours and consumed standardized meals pre‐ and post‐supplementation. Measurements included postprandial glucose incremental areas under the curve (iAUC), duration of day in hyper‐ and hypo‐glycaemia status, average 24‐hour and daily postprandial glucose concentrations, HbA1c, insulin, blood pressure (BP) and oxidative stress (F 2 ‐isoprostanes). Results Following AA supplementation, significant decreases were observed in daily postprandial glucose iAUC (−36%), in duration of day with hyperglycaemia (−2.8 h/d) and postprandial hyperglycaemia (−1.7 h/d), in average 24‐hour glucose (−0.8 mmol/L) and daily postprandial glucose (−1.1 mmol/L) concentrations, in systolic (−7 mm Hg) and diastolic (−5 mm Hg) blood pressures and in a specific fraction of free plasma F 2 ‐isoprostanes (−47 pg/mL) as compared to placebo. Conclusions Individuals with type 2 diabetes experienced improved postprandial and 24‐hour glycaemia and decreased BP after 4 months of AA supplementation as compared to placebo. These findings offer evidence for the proposed use of AA as an adjunct therapy to improve glycaemic and BP control in individuals with type 2 diabetes.