Fatigue mechanisms in sedentary and endurance trained adults: effects of nutritional countermeasures

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O'Dea, Namrita Kumar
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Physical inactivity is a major risk factor for chronic disease; yet the majority of the population does not meet physical activity recommendations, with fatigue being a primary underlying reason. Common nutritional supplements such as caffeine (CAF) and carbohydrate (CHO) are frequently taken prior to and during exercise and can delay fatigue, particularly in trained athletes; but, whether these same reductions in fatigue translate to habitually sedentary individuals is less clear. The main goal of this research was to investigate the influence of nutritional aids: CAF, low-calorie CHO, and their combination CAF+CHO to delay physical and mental fatigue in healthy but sedentary men and women compared to endurance-trained counterparts. The minimum amount of CHO that could elicit a benefit without eliciting a metabolic response when ingested remains unclear. Thus, our first aim was to evaluate glycemic response after ingestion of CHO drinks ranging from 0-6% CHO; and, to determine effects of two methods of CHO administration (ingest and oral rinse without ingestion) on sustained attention during a mentally fatiguing cognitive task. In agreement with evidence that “low calorie beverages” do not appear to alter blood glucose, a 0.4% CHO solution (<2 g CHO) elicited the most similar response to artificially sweetened placebo and was evaluated further. Compared to oral rinse of a small amount (<2 g) of CHO or control (0 g CHO), ingestion of an equivalent “dose” (<2 g CHO) in the 0.4% CHO solution maintained sustained attention, providing evidence that ingesting a low-CHO drink may be more effective than simply rinsing, in a fasted state. Oral rinse of CHO did not, however, provide any benefit compared to rinsing a control solution. Our second aim was to evaluate the efficacy of ingesting a moderate dose of CAF (3 mg/kg), low-CHO, and the combination of CAF+CHO on exercise capacity in trained (ET) versus sedentary (SED) groups. As expected, CAF reduced perceived effort during exercise and increased endurance capacity (longer total time to volitional fatigue and longer duration before rating vigorous intensity exercise as “very hard”), for ET and SED. However, addition of <2 g CHO did not provide further improvement with CAF; or any benefit when ingested alone, compared to placebo. CAF and CHO did not influence blood glucose but CAF resulted in higher blood lactate compared to no-CAF. As expected, ET had higher fat oxidation than SED; and, CAF increased CHO oxidation but not fat oxidation. CAF tended to maintain maximal voluntary contractile (MVC) strength in the quadriceps after exercise; but, CAF did not influence voluntary muscle activation or appear to have a direct effect on skeletal muscle since electrically evoked strength was not altered with CAF. Since acute exercise also benefits cognition independent of CAF or CHO, our third aim was to determine: (1) the effect of moderate intensity exercise (MOD-EX) on sustained attention in comparison to seated rest; and 2) whether CAF provides additional benefit to sustained attention and perceptual measures when combined with exercise. As expected, compared to an equivalent duration of rest, MOD-EX improved sustained attention following mental fatigue in both groups; and, when coupled with CAF, provided greater benefit to sustained attention and perceived mental energy. Although CAF’s beneficial effect on sustained attention persisted after exercise to volitional fatigue, it did not improve perceptual measures of reduced mental energy and increased mental fatigue at the point of physical fatigue. Our research investigated nutritional aids that are not only beneficial for athletes, but also widely consumed by the general population despite their lack of regular physical activity. In response to public health recommendations for reducing sugar intake, low-sugar/low-calorie drinks have been heavily marketed to the population despite much research to support their efficacy. Our findings suggest that low-CHO can be ergogenic during a sedentary mental task in a fasted state, perhaps through central mechanisms. Although CHO mouth rinse is evidenced to be efficacious for exercise performance without ingestion, we did not find that ingestion of low-CHO provides any significant benefit during moderate to vigorous exercise following a small meal. However, a moderate dose of CAF was ergogenic for both ET and SED. Future work should not only investigate additional exercise interventions; but also different CHO and CAF doses and administration protocols, and their relative metabolic implications.
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