Endurance exercise increases the use of endogenous fuels to provide energy for working muscles. Elderly subjects oxidize more glucose and less fat during moderate intensity exercise. This shift in substrate use is presumably caused by age-related changes in skeletal muscle, including decreased skeletal muscle respiratory capacity, because adipose tissue lipolysis and plasma fatty acid availability are not rate limiting. Endurance training in elderly subjects increases muscle respiratory capacity, decreases glucose production and oxidation, and increases fat oxidation thereby correcting or compensating for the alterations in substrate oxidation associated with aging.
Bettina Mittendorfer and Samuel Klein
Daniel J. Peart, Andy Hensby and Matthew P. Shaw
The purpose of this study was to compare markers of hydration during submaximal exercise and subsequent time trial performance when consuming water (PW) or coconut water (CW). There was also a secondary aim to assess the palatability of CW during exercise and voluntary intake during intense exercise. 10 males (age 27.9 ± 4.9 years, body mass 78.1 ± 10.1kg, average max minute power 300.2 ± 28.2W) completed 60-min of submaximal cycling followed by a 10-km time trial on two occasions. During these trials participants consumed either PW or CW in a randomized manner, drinking a 250 ml of the assigned drink between 10–15 min, 25–30 min and 40–45 min, and then drinking ad libitum from 55-min until the end of the time trial. Body mass and urine osmolality were recorded preexercise and then after 30-min, 60-min, and post time trial. Blood glucose, lactate, heart rate, rate of perceived exertion (RPE; 6–20) and ratings of thirst, sweetness, nausea, fullness and stomach upset (1 =very low/none, 5= very high) were recorded during each drink period. CW did not significantly improve time trial performance compared with PW (971.4 ± 50.5 and 966.6 ± 44.8 s respectively; p = .698) and there was also no significant differences between trials for any of the physiological variables measured. However there were subjective differences between the beverages for taste, resulting in a significantly reduced volume of voluntary intake in the CW trial (115 ± 95.41 ml and 208.7 ± 86.22 ml; p < .001).
Robert A. Robergs, Susie B. McMinn, Cristine Mermier, Guy Leadbetter III, Brent Ruby and Chris Quinn
This study was conducted to compare blood glucose and glucoregulatory hormone responses to the ingestion of solid and liquid carbohydrate (CHO) during prolonged cycling, followed by 30 min of isokinetic cycling. Eight male cyclists randomly completed three cycling trials (LC = liquid CHO, SCE = solid CHO with water equal to LC, SCA = solid CHO + ad libitum water). Each subject cycled for 120 min at 65% of VO2max with CHO ingestion (0.6 g CHO/kg/hr) at 0, 30, 60, 90, and 120 min. Subjects then completed a 30-min maximal isokinetic ride at 90 rpm. There was no significant (p < .05) difference between the trials for plasma glucose, insulin, glucagon, glycerol, lactate, RER, HR, VO2 RPE, and total work performed during the isokinetic ride. However, serum glucose was significantly lower in the SCE and SCA trials compared to LC at 80 min. The ingestion of a solid food containing CHO. protein, and fat with added water produced similar blood glucose, metabolic, glucoregulatory hormone, and exercise performance responses to those seen with the ingestion of liquid CHO.
Stephen R. Stannard, Martin W. Thompson and Janette C. Brand Miller
Consumption of low glycemic index (GI) foods before submaximal endurance exercise may be beneficial to performance. To test whether this may also be true for high intensity exercise. 10 trained cyclists began an incremental exercise test to exhaustion 65 min after consuming equal carbohydrate portions of glucose (HGI), pasta (LGI), and a noncarbohydrate control (PL). Time to fatigue did not differ significantly (p = 0.05) between treatments. Plasma glucose concentration was significantly lower after LGI vs. HGI from 15 to 45 min of rest postprandial. During exercise, plasma glucose concentration was significantly lower after HGI vs. LGI from 200 W until exhaustion. Plasma lactate concentration following HGI was significantly higher than PL from 30 min of rest postprandial through to the end of the 200-W workload. Plasma lactate concentration following LGI was significantly lower than after HGI from 45 min of rest postprandial through to the end of the 100-W workload. At higher exercise intensities, there was no significant difference in plasma lactate levels between treatments. These findings suggest that a high GI carbohydrate meal (1 g/kg body wt) 65 min prior to exercise decreases plasma glucose and increases plasma lactate levels compared to a low GI meal, but not enough to be detrimental to incremental exercise performance.
Matthew David Cook, Stephen David Myers, John Stephen Michael Kelly and Mark Elisabeth Theodorus Willems
Impaired glucose tolerance was shown to be present 48 hr following muscle-damaging eccentric exercise. We examined the acute effect of concentric and muscle-damaging eccentric exercise, matched for intensity, on the responses to a 2-hr 75-g oral glucose tolerance test (OGTT). Ten men (27 ± 9 years, 178 ± 7 cm, 75 ± 11 kg, VO2max: 52.3 ± 7.3 ml·kg-1·min-1) underwent three OGTTs after an overnight 12 hr fast: rest (control), 40-min (5 × 8-min with 2-min interbout rest) of concentric (level running, 0%, CON) or eccentric exercise (downhill running, –12%, ECC). Running intensity was matched at 60% of maximal metabolic equivalent. Maximal isometric force of m. quadriceps femoris of both legs was measured before and after the running protocols. Downhill running speed was higher (level: 9.7 ± 2.1, downhill: 13.8 ± 3.2 km·hr-1, p < .01). Running protocols had similar VO2max (p = .59), heart rates (p = .20) and respiratory exchange ratio values (p = .74) indicating matched intensity and metabolic demands. Downhill running resulted in higher isometric force deficits (level: 3.0 ± 6.7, downhill: 17.1 ± 7.3%, p < .01). During OGTTs, area-under-the-curve for plasma glucose (control: 724 ± 97, CON: 710 ± 77, ECC: 726 ± 72 mmol·L-1·120 min, p = .86) and insulin (control: 24995 ± 11229, CON: 23319 ± 10417, ECC: 21842 ± 10171 pmol·L-1·120 min, p = .48), peak glucose (control: 8.1 ± 1.3, CON: 7.7 ± 1.2, ECC: 7.7 ± 1.1 mmol·L-1, p = .63) and peak insulin levels (control: 361 ± 188, CON: 322 ± 179, ECC: 299 ± 152 pmol·L-1, p = .30) were similar. It was concluded that glucose tolerance and the insulin response to an OGTT were not changed immediately by muscle-damaging eccentric exercise.
Yan Burelle, François Péronnet, Denis Massicotte, Guy R. Brisson and Claude Hillaire-Marcel
The oxidation of 13C-labeled glucose and fructose ingested as a preexercise meal between 180 and 90 min before exercise was measured on 6 subjects when either a placebo or sucrose was ingested during the exercise period. Labeled hexose oxidation, which occurred mainly during the first hour of exercise, was not significantly modified when sucrose was ingested, but exogenous glucose oxidation was significantly higher than exogenous fructose oxidation in both situations. The results suggest that the absorption rate of exogenous hexoses was high when exercise was initiated but diminished thereafter, and that glucose and fructose released from sucrose ingested during exercise did not compete with glucose or fructose ingested before exercise for intestinal absorption, for conversion into glucose in the liver (for fructose), or for uptake and oxidation of glucose in peripheral tissues. However, as already shown, in terms of availability for oxidation of carbohydrates provided by the preexercise meal, glucose should be favored over fructose.
Thomas D. Fahey, James D. Larsen, George A. Brooks, William Colvin, Steven Henderson and Darrel Lary
Five trained, fasted male cyclists rode a cycle ergometer three times at 50% of
Athletes often train or compete in athletic events that significantly reduce muscle and liver glycogen reserves. Carbohydrate ingestion before or during endurance exercise enhances performance by maintaining blood glucose levels and carbohydrate utilization. Also, an adequate intake of carbohydrate following endurance exercise helps to restore muscle and liver glycogen. This paper reviews the physiologic and performance benefits of solid versus liquid carbohydrate feedings before, during, and following endurance exercise. Solid and liquid carbohydrates are equally effective in raising blood glucose and enhancing performance when consumed during endurance exercise. Also, both forms of carbohydrate are similarly beneficial in promoting muscle glycogen synthesis after exercise. It is unclear whether solid and liquid carbohydrate feedings have the same effect on serum glucose and performance when consumed before exercise. Although limited research suggests that a low glycemic solid carbohydrate may represent the best preexercise meal choice, further research is needed to support this hypothesis.
William M. Sherman, Julie M. Lash, John C. Simonsen and Susan A. Bloomfield
Because muscle damage from eccentric exercise has been associated with alterations in muscle glycogen metabolism, this study determined the effects of exercise on the insulin and glucose responses to an oral glucose tolerance test (OGTT). In a repeated-measures design, 11 subjects undertook either no exercise, 2 min of isokinetic leg exercise, or 50 min of level or downhill running. No exercise was performed and diet was controlled during the 48 hrs after the treatments and before the OGTT. Ratings of muscle soreness and CK activity were significantly elevated 48 hrs after downhill running. Level running also increased CK activity but did not induce muscle soreness. Isokinetic exercise did not affect either one. Blood glucose responses to the OGTT were similar among the treatments. In contrast, the insulin responses to the OGTT following downhill running were significantly increased. These results suggest that eccentric exercise associated with downhill running that results in delayed muscle soreness is associated with the development of a mild insulin-resistant condition.
Randall L. Wilber and Robert J. Moffatt
Ten trained male runners performed a treadmill exercise test at 80%