The therapeutic effect of dietary nitrate supplementation in healthy adults, individuals with type 2 diabetes mellitus and chronic obstructive pulmonary disease

Abstract

Introduction and aim Increases in the bioavailability of nitric oxide have been shown to reduce the oxygen (O2) cost of exercise, improve exercise performance, alter gastric blood flow and mediate glucose uptake in healthy individuals. Aim; does dietary nitrate reduce the O2 cost of exercise, improve walking performance in individuals with type 2 diabetes mellitus (T2DM) and chronic obstructive pulmonary disease (COPD) or alter hepatic diffusion and positively affect glucose homeostasis in healthy adults? Methods Experimental chapters utilised a double-blind, placebo-controlled, randomised, experimental design. Breath by breath pulmonary gas analysis was utilised to assess the O2 cost of exercise in 48 individuals with T2DM and 13 with COPD. Walking performance was assessed via the six minute walk test (6MWT) in cohorts 1 and 2. Magnetic resonance imaging was used to assess portal vein flux, velocity and the apparent diffusion coefficient, in order to assess hepatic microvascular diffusion (apparent diffusion coefficient (ADC)). Blood pressure (BP) was measured in all trials. Results Relative to placebo, beetroot juice resulted in a significant increase in plasma nitrate and nitrite. There were no differences between placebo vs. beetroot juice for the O2 cost of walking (T2DM: placebo; 946 ± 221 vs. beetroot juice; 939 ± 223 ml.min-1; P=0.59) or cycling (COPD: placebo; 933 ± 323 vs. beetroot juice; 939 ± 302 ml: min-1; P=0.88), distance covered in the 6MWT (T2DM: placebo; 550 ± 83 vs. beetroot juice; 554 ± 90m; P=0.17 or COPD: placebo; 456 ± 86 vs. beetroot juice; 449 ± 79 m; P=0.37) or BP (T2DM: systolic: placebo; 134 ± 10 vs. beetroot juice; 132 ± 12 mmHg, P=0.17; diastolic: placebo; 77 ± 7: vs. beetroot juice; 76 ± 11 mmHg, P=0.27. COPD: systolic: placebo; 123 ± 14 vs. beetroot juice; 123 ± 14 mmHg; P=0.91; diastolic: placebo; 77 ± 9 vs. beetroot juice; 79 ± 9 mmHg; P=0.27). No differences were seen between placebo and beetroot juice for ADC (young adults: F(3, 45) = 0.25, P = 0.74; older adults; F(3, 42) = 1.3, P = 0.28), portal vein flux (young adults: F(3, 45) = 0.339, P = 0.79; older adults; F(3, 42) = 1.65, P = 0.19) however, there was an interaction effect in the young adults: (F(3, 45) = 2.9, P = 0.04) but not in the older adults; F(3, 42) = 1.8, P = 0.16) between visits for portal vein velocity. Nitrate supplementation did not reduce plasma glucose concentrations (young adults: F(3, 45) = 0.96, P = 0.42; older adults; F(3, 42) = 0.04, P = 0.99). Nitrate supplementation did not reduce systolic blood pressure (young adults: F(3, 45) = 0.20, P = 0.89; older adults; F(3, 42) = 1.7, P = 0.18) or diastolic blood pressure (young adults: F(3, 45) = 0.25, P = 0.86; older adults; F(3, 42) = 0.45, P = 0.72). Conclusion Dietary nitrate supplementation does not alter the O2 cost of exercise, improve walking performance or reduce BP in individuals with T2DM or COPD. Nitrate supplementation does not alter hepatic diffusion, glucose homeostasis or BP.