Influence of Caffeine on Exercising Muscle Blood Flow and Exercise Tolerance in Type II Diabetes
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BACKGROUND: Exercise is a critical treatment modality in persons with Type II Diabetes Mellitus (T2DM), however people with this disease experience chronic fatigue and a decreased exercise capacity, which affects their ability or willingness to participate in physical activity. Studies suggest that this exercise intolerance may be partly due to a reduced exercising muscle blood flow (MBF), and in particular to a reduced ability of red blood cells (RBCs) to evoke ATP-mediated vasodilation and an increase in MBF as they traverse areas of high O2 demand. Additional evidence suggests that caffeine may attenuate this impairment by enhancing the release of ATP from RBCs. HYPOTHESIS: Persons with T2DM would have reduced Forearm Blood Flow (FBF), oxygen consumption (VO2), and exercise tolerance responses to exercise compared to control (CON) subjects, and caffeine would attenuate these impairments. METHODS: T2DM (n = 4) and CON (n = 4) participants performed rhythmic forearm handgrip exercise at an intensity equivalent to 17.5 kg until “task failure” or 20 minutes of exercise was reached, after having consumed either a caffeine (5mg/kg; Caff) or placebo (Pl) capsule. FBF (Doppler and Echo ultrasound of the brachial artery), VO2 and lactate efflux (deep venous blood sampling), forearm vascular conductance (FVK), mean arterial pressure (MAP) and heart rate (HR) were quantified for each minute of exercise. RESULTS: Steady state FBF was similar across groups and treatment conditions (mean ± SE ml/min; CONCaff 553.80 ± 82.35, CONPl 583.42 ± 112.62, T2DMCaff 523.33 ± 105.39, T2DMPl 569.08 ± 134.20, NS), and this was due to similar MAP and FVK (across groups and treatment conditions, NS). VO2 and Time to Task Failure (TTF) were not different between groups and treatment conditions (NS), although TTF tended to be improved with caffeine versus placebo (10.00 ± 2.02 vs 8.24 ± 1.79 min, P=0.295). There was a strong positive relationship between FBF and TTF (r2=0.763; P=0.005). CONCLUSIONS: In the exercise model utilized, persons with T2DM do not have impaired cardiovascular responsiveness or reduced exercise tolerance, and caffeine does not provide any benefit. Differences in exercising MBF may be an underlying mechanism regarding differences in exercise tolerance.