Exogenous carbohydrate oxidation from maltose and glucose ingested during prolonged exercise

Summary Intestinal perfusion studies have shown that glucose absorption from maltose occurs faster than from isocaloric glucose. To determine whether ingested maltose might be a superior source of carbohydrate (CHO) for endurance athletes, we compared the rates of gastric emptying, absorption and oxidation of 15 g · 100 ml^–1 solutions of maltose and glucose. Six endurance-trained cyclists drank 1200 ml of either U-¹⁴C maltose or U-¹⁴C glucose as a 400-ml loading bolus immediately before exercise, and as 8 × 100-ml drinks at 10-min intervals during a 90-min ride at 700% of maximal oxygen consumption. The rates of gastric emptying [maltose 690 (SD 119) ml · 90 min^–1; glucose 655 (SD 93) ml · 90 min^–1], the appearance of U-¹⁴C label in the plasma, and the peak rates of exogenous CHO oxidation [maltose 1.0 (SD 0.09) g · min^–1; glucose 0.9 (SD 0.09) g · min^–1] were not significantly different. Further, the 51 (SD 8) g of maltose and the 49 (SD 9) g of glucose oxidised during exercise were similar. Each accounted for approximately 2001o of the total CHO oxidised during the 90 min of exercise. Since only half of the CHO delivered to the intestine was oxidised in the 90-min ride (maltose 49%; glucose 50%), we conclude that neither the rate of gastric emptying, nor digestion limited the rate of ingested CHO utilisation during the early stages of exercise. Rather, we hypothesise that, initially, it could be the rate at which the CHO diffuses across the unstirred water layer of the brush-border of the intestinal villi that limits the utilisation of soluble CHO ingested during prolonged exercise.     

Changes in lipid profile variables in response to submaximal and maximal exercise in trained cyclists

This study examined the effect of prolonged submaximal exercise followed by a self-paced maximal performance test on cholesterol (T-Chol), triglycerides (TG), and high-density lipoprotein cholesterol (HDLC). Nine trained male athletes cycled at 70% of maximal oxygen consumption for 60 min, followed by a selfpaced maximal ride for 10 min. Venous blood samples were obtained at rest, at 30 and 60 min during submaximal exercise, and immediately after the performance test. Lactic acid, haematocrit (Hct), haemoglobin (Hb), T-Chol and TG were measured in the blood, while plasma was assayed for HDL-C. Plasma volume changes in response to exercise were calculated from Hct and Hb values and all lipid measurements were corrected accordingly. In order to ascertain the repeatability of lipid responses to exercise, all subjects were re-tested under identical testing conditions and experimental protocols. When data obtained during the two exercise trials were analysed by two-way ANOVA no significant differences (P > 0.05) between tests were observed. Consequently the data obtained during the two testing trials were pooled and analysed by one-way ANOVA. Blood lactic acid increased non-significantly (P > 0.05) during the prolonged submaximal test, but rose markedly (P < 0.05) following the performance ride. Lipid variables ascertained at rest were within the normal range for healthy subjects. ANOVA showed that blood T-Chol and TG were unchanged (P > 0.05), whereas HDL-C rose significantly (P < 0.05) in response to exercise. Post hoc analyses indicated that the latter change was due to a significant rise in HDL-C after the performance ride. It is concluded that apparent favourable changes in lipid profile variables occur in response to prolonged submaximal exercise followed by maximal effort, and these changes showed a good level of agreement over the two testing occasions.     

Priority of blood flow to splanchnic organs in humans during pre- and post-meal exercise

Cardiac output and superior mesenteric arterial flow in five healthy young men were followed using Doppler ultrasound techniques at rest and during 4 min bouts of bicycle exercise in both a pre- and a post-meal situation. The meal given was mixed and heavy, with an energy content (related to body size) of about 1400–1600 kcal (5.9-6.9 MJ). Two levels of exercise, 50–65 W and 150–200 W (about 75% of Vo_tmax), were tested, with the subjects cycling in a reclining position. Superior mesenteric arterial flow increased threefold, to about 1.11 min^-1, after the meal. During exercise in the fasting situation there were only modest changes in splanchnic vascular conductance, and moderate increases in superior mesenteric arterial flow were actually recorded. Exercise in the post-prandial state caused appreciable reductions in splanchnic vascular conductance, and a 38% reduction was observed during the most heavy exercise. However, not even such a decrease in conductance resulted in any definite reduction in superior mesenteric arterial blood flow, which was maintained at the pre-exercise level. Cardiac output increased by about 1.3 1 min^-1 after the meal. The exercise-induced increases in cardiac output were of the same order in the fasting and in the post-prandial state. Variance analyses showed the high cardiac output levels reached during postprandial exercise to be no different from levels that would be reached by pure summation of the changes caused by eating alone and by exercise alone. It is concluded that blood flow to the splanchnic organs in reclining man retains its high pre- and post-prandial priority during short exercise bouts of up to 75% of Vo_Smax.     

Brief intense exercise followed by passive recovery modifies the pattern of fuel use in humans during subsequent sustained intermittent exercise

The role of work period duration as the principal factor influencing carbohydrate metabolism during intermittent exercise has been investigated. Fuel oxidation rates and muscle glycogen and free carnitine content were compared between two protocols of sustained intermittent intense exercise with identical treadmill speed and total work duration. In the first experiment subjects (n=6) completed 40 min of intermittent treadmill running involving a work : recovery cycle of 6 : 9 s or 24 : 36 s on separate days. With 24 : 36 s exercise a higher rate of carbohydrate oxidation approached significance (P=0.057), whilst fat oxidation rate was lower (P ≤ 0.01) and plasma lactate concentration higher (P ≤ 0.01). Muscle glycogen was lower post‐exercise with 24 : 36 s (P ≤ 0.05). Muscle free carnitine decreased (P ≤ 0.05), but there was no difference between protocols. In the second experiment a separate group of subjects (n=5) repeated the intermittent exercise protocols with the addition of a 10‐min bout of intense exercise, followed by 43 ± 5 min passive recovery, prior to sustained (40 min) intermittent exercise. For this experiment the difference in fuel use observed previously between 6 : 9 s and 24 : 36 s was abolished. Carbohydrate and fat oxidation, plasma lactate and muscle glycogen levels were similar in 6 : 9 s and 24 : 36 s. When compared with the first experiment, this result was because of reduced carbohydrate oxidation in 24 : 36 s (P ≤ 0.05). There was no difference, and no change, in muscle free carnitine between protocols. A 10‐min bout of intense exercise, followed by 43 ± 5 min of passive recovery, substantially modifies fuel use during subsequent intermittent intense exercise.     

Learned control of heart rate during exercise in patients with borderline hypertension

Summary Twelve patients with borderline hypertension [⩽21.33/12.6, ⩾18.6/12.0 kPa (⩽160/ 95; ⩾ 140/90 mm Hg)] participated in an experiment aimed at testing whether they could learn to attenuate heart rate while exercising on a cycle ergometer. Six experimental (E) subjects received beat-to-beat heart-rate feedback and were asked to slow heart rate while exercising; six control (C) subjects received no feedback. Averaged over 5 days (25 training trials) the exercise heart-rate of the E group was 97.8 bt min⁻¹, whereas the C group averaged 107 bt min⁻¹ (P=0.03). Systolic blood pressure was unaffected by feedback training. Generally, changes in rate-pressure product reflected changes in heart-rate. Oxygen consumption was lower in the E than in the C group late in training. We conclude that neurally mediated changes associated with exercise in patients with borderline hypertension can be brought under behavioral control through feedback training.     

Caffeine increases maximal anaerobic power and blood lactate concentration

Summary The aim of this study was to specify the effects of caffeine on maximal anaerobic power (W _max). A group of 14 subjects ingested caffeine (250 mg) or placebo in random double-blind order. TheW _max was determined using a force-velocity exercise test. In addition, we measured blood lactate concentration for each load at the end of pedalling and after 5 min of recovery. We observed that caffeine increasedW _max [964 (SEM 65.77) W with caffeine vs 903.7 (SEM 52.62) W with placebo;P<0.02] and blood lactate concentration both at the end of pedalling [8.36 (SEM 0.95) mmol · l^–1 with caffeine vs 7.17 (SEM 0.53) mmol · l^–1 with placebo;P<0.011 and after 5 min of recovery [10.23 (SEM 0.97) mmol · l^–1 with caffeine vs 8.35 (SEM 0.66) mmol · l^–1 with placebo;P<0.04]. The quotient lactate concentration/power (mmol · l^–1 · W^–1) also increased with caffeine at the end of pedalling [7.6 · 10^–3 (SEM 3.82 · 10^–5) vs 6.85 · 10^–3 (SEM 3.01 · 10^–5);P<0.01] and after 5 min of recovery [9.82·10^–3 (SEM 4.28 · 10^–5) vs 8.84 · 10^–3 (SEM 3.58 · 10^–5);P<0.02]. We concluded that caffeine increased bothW _max and blood lactate concentration.     

The effects of cycle racing on pulmonary diffusion capacity and left ventricular systolic function

The purpose of this study was to examine the effects of a 20 km cycle race (TT) on left ventricular (LV) systolic and pulmonary function in 12 endurance cyclists. Spirometry, single-breath diffusion capacity (DLCO) with partitioning of membrane (DM) and capillary blood volume (Vc) components and 2-D echocardiograms were performed before and after the TT. During the TT mean oxygen consumption was 3.79 +/- 0.5 L x min(-1) (83 +/- 5.5% of VO2max) and mean blood lactate was 8.4 +/- 2.4 mM. Following the TT, spirometry values were unchanged, however, DLCO and DM were significantly (P<0.05) reduced. LV systolic function was increased (P<0.05) immediately after exercise, while end-diastolic area was decreased (P<0.05) at all points during recovery. The reduction in DM was correlated with LV systolic function following the TT. This relationship suggests a cardiovascular contribution to pulmonary diffusion impairment following exercise.     

Effect of alkalosis on performance and lactate formation in supramaximal exercise

Summary An increased base binding power of the blood induced by alkali administration to subjects performing a supramaximal exercise has no appreciable effect neither on the maximal performance time nor on the total amount of lactic acid or its rate of appearance in blood.This work has been supported by a grant from the Italian National Research Council. Thanks are due also to Laboratory Glaxo, S.p.A. for facilities and financial support in the course of the experiments.     

The metabolic response to prolonged walking in fed and fasted men

Summary Six healthy men walked 37 km (23 miles) per day over a 3-lap course for each of 4 consecutive days. Subjects were allowed breakfast and an unrestricted diet was consumed after completion of the walk, but no food was consumed during or between laps. At a later date the same subjects walked over the same course after an overnight fast and without breakfast. Completion time for each lap was 139±1 min (mean ±SE) and exercise intensity was equivalent to 17±1% . Mean 24h energy intake was 14.5±0.8 MJ during the fed walk. Estimated daily energy expenditure was 12.0 MJ. Blood glucose concentration fell significantly on the first, third and fourth days of the fed walk, but no subject became hypoglycaemic. Glucose concentration did not fall during the fasted walk and was significantly higher pre-exercise and at the end of laps one and three when compared to the first day of the fed walk. Blood alanine concentration fell significantly after the end of the first lap of each day of the fed walk but not during the fasted walk. Blood lactate levels did not change during the course of either walk. Plasma free fatty acid, glycerol and blood 3-hydroxybutyrate concentrations were unchanged during the passage of the first lap on each day of the fed walk, but all three had increased significantly by the end of the first lap of the fasted walk. The elevations from rest to the end of each day's walking for these three metabolites were of similar magnitude when comparing each day of the fed walk and the fed and fasted walks. The present experiment indicates that feeding a mixed diet can affect the pattern of substrate mobilization in a similar manner to that seen during more strenuous exercise. It also appears that the pattern of fuel substrate mobilization is very similar from one day to the next providing dietary intake is similar.     

Physical work capacity in dynamic exercise with differing muscle masses in healthy young and older men

Ten young (aged 23–30 years) and nine older (aged 54–59 years) healthy men with similar estimated limb muscle volumes performed, in random order, three different types of ergometer exercise tests (one-arm cranking, two-arm cranking, and two-leg cycling) up to the maximal level. Values for work load (WL), peak oxygen consumption , peak heart rate (HR), peak ventilation , respiratory gas exchange ratio (R), recovery blood lactate concentration [La^–], and rating of perceived exertion (RPE) were compared between the age-groups in the given exercise modes. No significant age-related differences in WL, peak , peak HR, R, [La^–], or RPE were found in one-arm or two-arm cranking. During one-arm cranking the mean peak was 1.65 (SD 0.26)1 · min^–1 among the young men and 1.63 (SD 0.10)1 · min^–1 among the older men. Corresponding mean peak during two-arm cranking was 2.19 (SD 0.32)1 · min-1 and 2.09 (SD 0.18)1 · min^–1, respectively. During one-arm cranking peak was higher (P < 0.05) among the older men compared to the young men. During two-leg cycling the young men showed higher values in WL (P < 0.001), peak (P < 0.001), and peak HR (P < 0.001). The mean peak was 3.54 (SD 0.24)1 · min^–1 among the young men and 3.02 (SD 0.20)1 · min^–1 among the older men. Corresponding mean peak HR was 182 (SD 5) beats · min^–1 and 170 (SD 8) beats · min^–1, respectively. During two-leg cycling, peak , R, [La^–], and RPE did not differ between the two age-groups. In summary, the older men with similar sizes of estimated arm and leg muscle volumes as the young men had a reduced physical work capacity in two-leg cycling. In one-arm or two-arm cranking, no significant difference in work capacity was found between the age-groups. These results indicate, that in healthy men, age, at least up to the 6th decade of life, is not necessarily associated with a decline in physical work capacity in exercises using relatively small muscle groups, in which the limiting factors are more peripheral than central.