Muscle glycogen utilization during prolonged exercise on successive days

Improvement of insulin sensitivity by short-term exercise training in hypertensive African American women

African American women have a high prevalence of insulin resistance, non-insulin-dependent diabetes mellitus, obesity, and hypertension that may be linked to low levels of physical activity. We sought to determine whether 7 days of aerobic exercise improved glucose and insulin metabolism in 12 obese (body fat >35%), hypertensive (systolic blood pressure > or =140 and/or diastolic blood pressure > or =90 mmHg) African American women (mean age 51+/-8 years). Insulin-assisted frequently-sampled intravenous glucose tolerance tests were performed at baseline and 14 to 18 hours after the 7th exercise session. There was no significant change in maximal oxygen consumption, body composition, or body weight after the 7 days of aerobic exercise. The insulin sensitivity index increased (2.68+/-0.45 x 10[-5] to 4.23+/-0.10 x 10[-5] [min(-1)/pmol/L], P=.02). Fasting (73+/-9 to 50+/-9 pmol/L, P=.02) and glucose-stimulated (332+/-58 to 261+/-45 pmol/L, P=.05) plasma insulin levels decreased. Additional measures related to the insulin resistance syndrome also changed with the 7 days of exercise: basal plasma norepinephrine concentrations were reduced (2.46+/-0.27 to 1.81+/-0.27 nmol/L, P=.02) and sodium excretion rate increased from 100+/-13 to 137+/-7 mmol/d (P=.03); however, there was no change in potassium excretion or 24-hour ambulatory blood pressure. We conclude that a short-term aerobic exercise program improves insulin sensitivity in African American hypertensive women independent of changes in fitness levels, body composition, or body weight. The present study indicates that short-term exercise can improve insulin resistance in hypertensive, obese, sedentary African American women and confirms previous reports that a portion of the exercise-induced improvements in glucose and insulin metabolism may be the result of recent exercise.

Effects of meal frequency on body composition during weight control in boxers

The effects of meal frequency on changes in body composition by food restriction were investigated. Twelve boxers were divided between a two meals day-1 group (the 2M group) and a six meals day-1 group (the 6M group). Both groups ingested 5.02 MJ (1200 kcal) day-1 for 2 weeks. Although there was no difference in change of body weight by food restriction between the two groups, the decrease in lean body mass (LBM) was significantly greater in the 2M group than in the 6M group. The decrease in urinary 3-methylhistidine/creatinine was significantly greater in the 6M group than in the 2M group. These results suggest that the lower frequency of meal intake leads to a greater myoprotein catabolism even if the same diet is consumed.

Blood hormones as markers of training stress and overtraining

An imbalance between the overall strain experienced during exercise training and the athlete's tolerance of such effort may induce overreaching or overtraining syndrome. Overtraining syndrome is characterised by diminished sport-specific physical performance, accelerated fatiguability and subjective symptoms of stress. Overtraining is feared by athletes yet there is a lack of objective parameters suitable for its diagnosis and prevention. In addition to the determination of substrates (e.g. lactate, ammonia and urea) and enzymes (e.g. creatine kinase), the possibilities for monitoring of training by measuring hormonal levels in blood are currently being investigated. Endogenous hormones are essential for physiological reactions and adaptations during physical work and influence the recovery phase after exercise by modulating anabolic and catabolic processes. Testosterone and cortisol are playing a significant role in metabolism of protein as well as carbohydrate metabolism. Both are competitive agonists at the receptor level of muscular cells. The testosterone/cortisol ratio is used as an indication of the anabolic/catabolic balance. This ratio decreases in relation to the intensity and duration of physical exercise, as well as during periods of intense training or repetitive competition, and can be reversed by regenerative measures. Correlations have been noted with the training-induced changes of strength. However, it seems more likely that the testosterone/cortisol ratio indicates the actual physiological strain in training, rather than overtraining syndrome. The sympatho-adrenergic system might be involved in the pathogenesis of overtraining. Overtraining appears as a disturbed autonomic regulation, which in its parasympathicotonic form shows a diminished maximal secretion of catecholamines, combined with an impaired full mobilisation of anaerobic lactic reserves. This is supposed to lead to decreased maximal blood lactate levels and maximal performance. Free plasma adrenaline (epinephrine) and noradrenaline (norepinephrine) may provide additional information for the monitoring of endurance training. While prolonged aerobic exercise conducted at intensities below the individual anaerobic threshold lead to a moderate rise of sympathetic activity, workloads exceeding this threshold are characterised by a disproportionate increase in the levels of catecholamines. In addition, psychological stress during competitive events is characterised by a higher catecholamines to lactate ratio in comparison with training exercise sessions. Thus, the frequency of training sessions with higher anaerobic lactic demands or of competition, should be carefully limited in order to prevent overtraining syndrome. In the state of overtraining syndrome and overreaching, respectively, an intraindividually decreased maximum rise of pituitary hormones (corticotrophin, growth hormone), cortisol and insulin has been found after a standardised exhaustive exercise test performed with an intensity of 10% above the individual anaerobic threshold.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of carbohydrate supplementation during intense training on dietary patterns, psychological status, and performance

The purpose of this study was to determine the effects of carbohydrate supplementation during intense training on dietary patterns, psychological status, and markers of anaerobic and aerobic performance. Seven members of the U.S. National Field Hockey Team were matched to 7 team counterparts (N = 14). One group was blindly administered a carbohydrate drink containing 1 g.kg-1 of carbohydrate four times daily, while the remaining group blindly ingested a flavored placebo during 7 days of intense training. Subjects underwent pre- and posttraining aerobic and anaerobic assessments, recorded daily diet intake, and were administered the Profile of Mood States (POMS) psychological inventory prior to and following each practice. Results revealed that the carbohydrate-supplemented group had a greater (p < .05) total energy intake, carbohydrate intake, and change (pre vs. post) in time to maximal exhaustion following training while reporting less postpractice psychological fatigue. However, no significant differences were observed in remaining psychological, physiological, or performance-related variables.

Nutritional practices of athletes: are they sub-optimal?

Athletes' nutritional needs are principally determined by their training load (the intensity x frequency x duration of daily workouts) and body mass. Analyses of the diets of track and field competitors and marathon runners reveal a macronutrient composition similar to that of weight-matched, inactive individuals. Male athletes generally ingest adequate dietary energy to meet their daily energy expenditure and all vitamin and mineral needs. However, the energy intake of most female athletes is less than might be anticipated based on their training load. As a result, intakes of iron, calcium, vitamin B12 and zinc are often below the recommended daily allowances. Compared with the recommendations of sports nutritionists and exercise physiologists, the majority of athletes consume a diet which might be considered significantly deficient in carbohydrate (CHO). Although there is currently little scientific support for increasing the proportion of daily energy intake from CHO above the 45-55% (approximately 5 g kg BM-1 day-1) chronically consumed by most athletes, such a regimen would probably improve an athlete's training capacity, especially when rapid recovery from intense activity is required.

Endurance exercise training improves body composition and plasma insulin responses in 70- to 79-year-old men and women

Forty-two men and women aged 70 to 79 years were studied to assess the effects of 6 months of endurance or resistance training and subsequent cessation of training on glucose tolerance, plasma insulin responses, serum triglyceride and cholesterol levels, and plasma dehydroepiandrosterone (DHEA) levels. The endurance training group (n = 16) exercised at 75% to 85% heart rate reserve for 35 to 45 minutes three times per week; the resistance training group (n = 17) completed one set of eight to 12 repetitions on 10 Nautilus machines three times per week. No significant changes in any variables occurred in a control group (n = 9). Maximal oxygen consumption (VO2max) increased by 20% with endurance training, but did not change with resistance training. Upper- and lower-body strength increased in the resistance training group, but did not change with endurance training. Neither group changed their body weight with training, but the endurance training group elicited a significant reduction in their sum of seven skinfolds and percent body fat. Neither group altered their glucose tolerance with training; however, the endurance training group had lower plasma insulin responses after training compared with the other two groups. Serum lipid and plasma DHEA levels did not change in either the endurance or resistance training groups. Ten days of no exercise following training did not significantly alter body weight or composition, glucose tolerance, plasma insulin responses, or plasma DHEA levels in either the endurance training (n = 10) or resistance training (n = 14) group.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemic effects of ingesting varying amounts of a commercial carbohydrate beverage postexercise

OBJECTIVE

Although the role of postexercise carbohydrate intake in the replenishment of muscle glycogen is well established, large amounts of carbohydrate may affect other systems which are recovering from exercise as well.

METHODS

We varied the timing and amount of a commercial glucose polymer/fructose (CHO) beverage ingested postexercise in 2 groups of 8 normotensive men following 1 hour of cycling exercise. In Study A the subjects ingested 1 L of a 200 g CHO solution or 1 L of water (W) immediately postexercise. The participants in Study B consumed 1 L of a 1.5 g/kg CHO solution, or W, immediately and 2 hours postexercise.

RESULTS

Recovery systolic blood pressure was elevated after 200 g CHO as compared to water, but not after 1.5 g/kg CHO. Diastolic blood pressure was decreased, while heart rate, insulin and glucose increased following both doses of CHO. Despite the potassium (K) content of the beverages, serum K decreased in Study A and B, while a trend was noted following CHO for decreased urinary K excretion at 2 hours and for increased sodium excretion at 4 hours in Study B. Post CHO aldosterone declined more rapidly than after W, and urine volumes were decreased compared to W in both studies 2 hours after CHO.

CONCLUSIONS

We speculate that hyperinsulinemia contributed to the rapid decline in K and aldosterone by creating a flux of K to the intracellular space. It appears that CHO ingestion postexercise results in systemic effects that are related to the amount and timing of CHO consumed.

Seven consecutive days of exercise lowers plasma insulin responses to an oral glucose challenge in sedentary elderly

OBJECTIVE

To assess the effects of 1 and 7 consecutive days of exercise on glucose and insulin responses to an oral glucose challenge.

DESIGN

Intervention group assessed at baseline and after 1 and 7 days of exercise.

SETTING

Academic medical institution.

PARTICIPANTS

Nine healthy 60 to 80-year-old men and women.

INTERVENTION

Seven days of 50 minutes of exercise at 70% VO2 max.

MEASUREMENTS

Body weight, body composition, and glucose and insulin levels and responses to an oral glucose challenge at baseline and after 1 and 7 days of exercise.

MAIN RESULTS

Fasting plasma insulin levels and plasma insulin responses to an oral glucose challenge were reduced by 15% and 20%, respectively, with 7 consecutive days of exercise that resulted in no change in body weight or body composition. No changes in glucose or insulin levels or responses to the oral glucose challenge were evident after a single day of exercise.

CONCLUSION

The hyperinsulinemia associated with aging can be blunted significantly by repeated bouts of exercise in the elderly, independent of any changes in body composition.

Future directions for performance-related sports science research: an interdisciplinary approach

The present paper is based on a review which was commissioned by the Sports Council (London) on behalf of the Open Section of the British Association of Sports Sciences (BASS). This was one of four such reviews which were collectively designed to provide information pertinent to the formulation of a strategy that would guide fundamental sports science research in the UK until the year 2000. All of the reviews were expected to focus on research that was relevant to the performance of the elite athlete and the specific brief of the Open Section Review was to concentrate on interdisciplinary research. The current paper established the unique value of interdisciplinary sports science research. Four themes were considered in some detail in order to review the extant interdisciplinary research and propose directions for future research involving an interdisciplinary approach. The four topics were talent identification, adherence, injuries and peaking. A critical review of each area revealed a lack of interdisciplinary research and recommendations for future research priorities were made. The paper is concluded with a brief outline of a strategy that would facilitate the development and expansion of interdisciplinary sports science research.

Effects of protein supplementation during prolonged exercise at moderate altitude on performance and plasma amino acid pattern

The effects of two levels of protein intake on muscle performance and energy metabolism were studied in humans submitted to repeated daily sessions of prolonged exercise at moderate altitude. For this purpose, 29 healthy males, were exposed to seven successive stages of ski-mountaineering at altitudes between 2500 and 3800 m, and to an isocaloric diet (4000 kcal.day-1, 16,760 kJ.day-1) with either 1.5 g.kg-1.day-1 (C group, n = 14), or 2.5 g.kg-1.day-1 (PR group, n = 15) protein intake. Measurements made after the ski-mountaineering programme did not show any change in body mass. The peak torque during maximal isometric voluntary contraction (MVC) of the quadriceps muscle was unaffected by the repeated exercises, whereas the endurance time at 50% MVC was decreased in PR subjects (-26.8%, P < 0.001). Increased levels of both free fatty acids (+ 147%, P < 0.001) and glycerol (+ 170%, P < 0.001) observed in C subjects would suggest that lipolysis was enhanced after the repeated exercise. The plasma amino acid pattern was altered after completion of the ski-mountaineering programme; the plasma concentration of the three branched-chain amino acids (BCAA) was significantly decreased in C subjects, whereas the higher level of protein intake (PR group) greatly minimized the exercise-induced decrease in serum BCAA.

Glucose ingestion before and during exercise does not enhance performance of daily repeated endurance exercise

The effect of glucose (Glc) ingestion before and during daily, repeated, prolonged exercise on metabolism and performance was tested. Seven young, healthy males performed cycling exercise in two series, with 1 month interval. Each exercise series consisted of 1 h/day on 3 successive days. On the 3rd day, exercise was continued until exhaustion. The intensity was 73.4 (7.7)% [mean (SD)] of maximal oxygen uptake (VO2max). Glucose (Glc) or placebo (P) drink was ingested 15 min before the start, and at 15 and 45 min of each daily exercise. The total amount of Glc ingested was 43.1 (4.2) g. During exercise, blood Glc concentrations were significantly higher (P < 0.05) when Glc was ingested than when P was ingested [Glc 5.14 (0.32) and P 4.12 (4.17) mmol.l-1 at exhaustion]. However, Glc ingestion did not improve performance time to exhaustion [Glc 92.05 (29.55) and P 98.07 (27.33) min]. Free fatty acid concentrations were significantly lower when Glc was ingested than when P was ingested [Glc 0.63 (0.21) and P 1.39 (0.46) mmol.l-1 at exhaustion]. There were no significant differences in exercise heart rate, VO2, respiratory exchange ratio, blood lactate concentrations or rating of perceived exertion between the conditions nor were there any significant differences in these parameters on different days of exercise. It seems that ingestion of small amounts of Glc does not increase the metabolism of carbohydrate or improve the performance of intensive endurance exercise of poorly trained subjects, even when the exercise is repeated daily.

Adenosine in exercise adaptation

By influencing the regulation of the mechanisms of angiogenesis, erythropoietin production, blood flow, myocardial glucose uptake, glycogenolysis, systolic blood pressure, respiration, plasma norepinephrine and epinephrine levels, adenosine may exert a significant effect on the body's adaptation response to exercise. However, adenosine's possible influence over the vasodilatory response to exercise in skeletal muscle is controversial and more research is required to resolve this issue. Various popular exercise training methods, such as cyclic training, interval training, and the 'warm down' from training may increase adenosine levels and thereby might enhance the response of adenosine-influenced adaptive mechanisms. Among the several classes of drugs which may enhance extracellular adenosine levels and thereby might augment adenosine-influenced adaptive mechanisms, are the anabolic steroidal and some readily available non-steroidal anti-inflammatory drugs (NSAIDs).

Nutrition and exercise determinants of postexercise glycogen synthesis

During the initial hours of recovery from prolonged exhaustive lower body exercise, muscle glycogen synthesis occurs at rates approximating 1-2 mmol.kg-1 wet wt.hr-1 if no carbohydrate is consumed. When carbohydrate is consumed during the recovery, the maximal rate of glycogen synthesis approximates 7-10 mmol.kg-1 wet wt.hr-1. The rate of post-exercise glycogen synthesis is lower if the magnitude of glycogen degradation is small, if less than 0.7 gm glucose.kg-1 body wt.hr-1 is ingested, when the recovery is active, and when the carbohydrate feeding is delayed. The rate of postexercise glycogen synthesis is not reduced during the initial hours (< 4) after eccentric exercise. For studies evaluating muscle glycogen synthesis in excess of 12 hours of recovery, average rates of glycogen synthesis are below 4 mmol.kg-1 wet wt.hr-1. Glycogen synthesis is known to be impaired for time periods in excess of 24 hours following exercise causing eccentric muscle damage. Following intense exercise resulting in high concentrations of muscle lactate, muscle glycogen synthesis occurs at between 15-25 mmol.kg-1 wet wt.hr-1. These synthesis rates occur without ingested carbohydrate during the recovery period and are maintained when a low intensity active recovery is performed.

Nutritional concerns of female athletes: a case study

Some of the nutritional concerns of female athletes are highlighted in this case study of a 20-year-old woman who wants to lose 16% of her body weight to qualify for the position of coxswain on a national crew team. These concerns include adequacy of vitamin, mineral, protein, and carbohydrate intake as well as amenorrhea and pathogenic eating behaviors.

Nutritional considerations for ultraendurance performance

The nutritional considerations of the ultraendurance athlete center around proper caloric and nutrient intake during training as well as adequate energy and fluid replacement during competition to maintain optimal performance. Energy needs of ultraendurance athletes during training vary widely, depending upon duration, intensity, and type of exercise training. These athletes may train several hours daily, thus risking inadequate caloric intake that can lead to chronic fatigue, weight loss, and impaired physical performance. It is not known whether protein needs are increased in ultraendurance athletes as a result of extended exercise training. Additionally, micronutrient needs may be altered for these athletes while dietary intake is generally over the RDA because of high caloric intake. Prior to competition, ultraendurance athletes should consider glycogen supercompensation and a prerace meal eaten 4 hrs before as a means of improving performance. Carbohydrate feedings during prolonged exercise can significantly affect performance. During events lasting over several hours, sodium sweat losses and/or the consumption of sodium-free fluids may precipitate hyponatremia.

Regulation of glycogen resynthesis following exercise. Dietary considerations

With the cessation of exercise, glycogen repletion begins to take place rapidly in skeletal muscle and can result in glycogen levels higher than those present before exercise. Understanding the rate-limiting steps that regulate glycogen synthesis will provide us with strategies to increase the resynthesis of glycogen during recovery from exercise, and thus improve performance. Given the importance of muscle glycogen to endurance performance, various factors which may optimise glycogen resynthesis rate and insure complete restoration have been of interest to both the scientist and athlete. The time required for complete muscle glycogen resynthesis after prolonged moderate intensity exercise is generally considered to be 24 hours provided approximately 500 to 700g of carbohydrate is ingested. Muscle glycogen synthesis rate is highest during the first 2 hours after exercise. Ingestion of 0.70g glucose/kg bodyweight every 2 hours appears to maximise glycogen resynthesis rate at approximately 5 to 6 mumol/g wet weight/h during the first 4 to 6 hours after exhaustive exercise. Further enhancement of glycogen resynthesis rate with ingestion of greater than 0.70g glucose/kg bodyweight appears to be limited by the constraints imposed by gastric emptying. Ingestion of glucose or sucrose results in similar muscle glycogen resynthesis rates while glycogen synthesis in liver is better served with the ingestion of fructose. Also, increases in muscle glycogen content during the first 4 to 6 hours after exercise are greater with ingestion of simple as compared with complex carbohydrate. Glycogen synthase activity is a key component in the regulation of glycogen resynthesis. Glycogen synthase enzyme exists in 2 states: the less active, more phosphorylated (D) form which is under allosteric control of glucose-6-phosphate, and the more active, less phosphorylated (I) form which is independent of glucose-6-phosphate. There is generally an inverse relationship between glycogen content in muscle and the percentage synthase in the activated (I) form. Exercise and insulin by themselves activate glycogen synthase by conversion to glycogen synthase I. Although small changes in the activity ratio (% I form) can lead to large changes in the rate of glycogen synthesis, glycogen synthase I appears to increase very little (approximately 25%) in response to glycogen depletion and returns to pre-exercise levels as glycogen levels return to normal. Thus glycogen resynthesis, which may increase 3- to 5-fold, may also be influenced by glucose-6-phosphate, which can activate glycogen synthase in the D form.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscle glycogen synthesis before and after exercise

The importance of carbohydrates as a fuel source during endurance exercise has been known for 60 years. With the advent of the muscle biopsy needle in the 1960s, it was determined that the major source of carbohydrate during exercise was the muscle glycogen stores. It was demonstrated that the capacity to exercise at intensities between 65 to 75% VO2max was related to the pre-exercise level of muscle glycogen, i.e. the greater the muscle glycogen stores, the longer the exercise time to exhaustion. Because of the paramount importance of muscle glycogen during prolonged, intense exercise, a considerable amount of research has been conducted in an attempt to design the best regimen to elevate the muscle's glycogen stores prior to competition and to determine the most effective means of rapidly replenishing the muscle glycogen stores after exercise. The rate-limiting step in glycogen synthesis is the transfer of glucose from uridine diphosphate-glucose to an amylose chain. This reaction is catalysed by the enzyme glycogen synthase which can exist in a glucose-6-phosphate-dependent, inactive form (D-form) and a glucose-6-phosphate-independent, active form (I-form). The conversion of glycogen synthase from one form to the other is controlled by phosphorylation-dephosphorylation reactions. The muscle glycogen concentration can vary greatly depending on training status, exercise routines and diet. The pattern of muscle glycogen resynthesis following exercise-induced depletion is biphasic. Following the cessation of exercise and with adequate carbohydrate consumption, muscle glycogen is rapidly resynthesised to near pre-exercise levels within 24 hours. Muscle glycogen then increases very gradually to above-normal levels over the next few days. Contributing to the rapid phase of glycogen resynthesis is an increase in the percentage of glycogen synthase I, an increase in the muscle cell membrane permeability to glucose, and an increase in the muscle's sensitivity to insulin. The slow phase of glycogen synthesis appears to be under the control of an intermediate form of glycogen synthase that is highly sensitive to glucose-6-phosphate activation. Conversion of the enzyme to this intermediate form may be due to the muscle tissue being constantly exposed to an elevated plasma insulin concentration subsequent to several days of high carbohydrate consumption. For optimal training performance, muscle glycogen stores must be replenished on a daily basis. For the average endurance athlete, a daily carbohydrate consumption of 500 to 600g is required. This results in a maximum glycogen storage of 80 to 100 mumol/g wet weight.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypoglycemia and endurance exercise: dietary considerations

Until recently, common dietary prescription for chronic hypoglycemia has been a high-protein, low-carbohydrate regimen (Airola, 1977; Danowski, 1978). Increasing evidence suggests, however, that a diet rich in complex carbohydrates may be more suitable for those involved in endurance exercise (Costill & Miller, 1980; Sherman & Costill, 1984). Although little original research has been undertaken which deals with the effects of performance-enhancing nutritional techniques on the hypoglycemic exerciser, such practices need to be examined in order to understand the mechanisms involved. Specifically, carbohydrate loading would seem to be as important, if not more so, to the hypoglycemic individual as a means of supercompensating glycogen stores prior to endurance performance. The roles of pre-exercise supplements and carbohydrate feedings during exercise in this context are less clear. Although results are mixed, increasing evidence (Snyder et al., 1983; Okano et al., 1988) suggests that carbohydrates may be consumed before exercise with beneficial effects on performance. Because of rapid gastric emptying characteristic of reactive hypoglycemia, it would appear that pre-exercise supplementation may be of particular value to the hypoglycemic exerciser. Further, recent studies (Bergstrom & Hultman, 1967; Coyle et al., 1983; Foster et al., 1986; Leatt & Jacobs, 1986; Horton, 1988) indicate that carbohydrate solutions taken during exercise are effective in maintaining serum glucose levels and improving endurance performance. Careful monitoring of nutritional factors would appear to be critical in creating a suitable dietary environment for the hypoglycemic endurance exerciser.

Effect of glucose polymer diet supplement on responses to prolonged successive swimming, cycling and running

Fifteen male endurance athletes were studied to determine the effect of a glucose polymer (GP) diet supplement on physiological and perceptual responses to successive swimming, cycling and running exercise. Thirty min of swimming, cycling and running at 70% VO2max, followed by a run to exhaustion at 90% VO2max was performed after one week of training under two dietary conditions: 1) GP (230 g of GP consumed daily) and 2) placebo (P, saccharin-sweetened supplement consumed daily). During GP, daily carbohydrate (CHO) intake was higher (p less than 0.05) by 173 g or 14% of energy intake than during P, but total energy intake was not significantly different. During 90 min of exercise, CHO utilization and blood glucose were significantly higher under GP than P by an average of 20.2% and 14.5%, respectively, but heart rate, ventilation, oxygen uptake, ratings of perceived exertion, and plasma lactate were not different. Run time to exhaustion at 90% VO2max was significantly longer by 1.2 min (23%) under GP. The results suggest that a GP diet supplement may be of value during endurance exercise by increasing the availability of CHO.

The effects of previous severe exercise upon the respiratory Vco2/Vo2 exchange ratio as a predictor of maximum oxygen uptake

The present study examined the effect of previous severe exercise upon (i) respiratory exchange during maximal exercise, and (ii) the respiratory Vco2/Vo2 exchange ratio (R) as a predictor of maximum oxygen uptake (Vo2max). Thirteen healthy males performed a progressive treadmill test to Vo2max: at rest (T1); after a 1 h run on the level treadmill at a speed corresponding 82.4 +/- 7.3% of their Vo2max (T2); after 1 h recovery (T3); and after 24 h recovery (T4). Respiratory gases were continuously monitored. No changes in average work Vo2, Vo2max or maximum heart rate were found between trials. Average work Vco2 was lower in T2 (2.055 +/- 0.093 1.min-1, p less than 0.001), T3 (2.080 +/- 0.087 1.min-1, p less than 0.001) and T4 (2.337 +/- 0.154 1.min-1, NS) compared with T1 (2.360 +/- 0.147 1.min-1). This resulted in lower average R values in T2 (0.81 +/- 0.02, p less than 0.001), T3 (0.83 +/- 0.02, p less than 0.001) and T4 (0.94 +/- 0.02, NS) in relation to T1 (0.95 +/- 0.02). Analysis of the %Vo2max/R relationship over the final 5 min of each test showed a shift to the left during T2 (p less than 0.001), T3 (p less than 0.001) and T4 (NS) compared with T1. As a result predictions of Vo2max based on R (Vo2max/R) were similar to recorded Vo2max in T1 (+ 0.6%) and T4 (+ 2.2%). But higher in T2 (+ 8.7%, p less than 0.001) and T3 (+ 6.9%, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

The respiratory Vco2/Vo2 exchange ratio during maximum exercise and its use as a predictor of maximum oxygen uptake

The purpose of this study was to define carefully the dynamic relationship between oxygen uptake (as % Vo2max) and the respiratory Vco2/Vo2 exchange ratio (R) during maximum progressive treadmill exercise in trained and untrained men, and to determine if this relationship could be used to predict Vo2max. Respiratory gases were continuously monitored and the %Vo2max/R time profile calculated at 15 sec intervals over the final 5 min of each test. Young sedentary men (controls, n = 122) and over-60y sedentary men (n = 30) shared the same %Vo2max/R relationship but the latter group had lower R values at Vo2max (1.06 +/- 0.03 vs 1.08 +/- 0.03, p less than 0.01) than controls. Endurance trained men (n = 45) had a lower %Vo2max/R relationship and higher R at Vo2max (1.11 +/- 0.02, p less than 0.001), team athletes (n = 98) had a lower %Vo2max/R relationship but lower R at Vo2max (1.06 +/- 0.03, p less than 0.001) and the weight trained (n = 19) had a higher %Vo2max/R relationship and lower R at Vo2max (1.01 +/- 0.02, p less than 0.001) all compared to controls. From the %Vo2max/R time profile, the following formulae were devised for the estimation of Vo2max (Vo2maxR): Young Sedentary, Vo2maxR = Vo2R (3.000-1.874 R); Over-60y Sedentary, Vo2maxR = Vo2R (3.457-2.345 R); Endurance Trained, Vo2max = Vo2R (1.980-0.912 R); Team Athletes, Vo2maxR = Vo2R (2.805-1.726 R); Weight Trained, Vo2maxR = Vo2R (4.236-3.191 R).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of marathon training on the plasma lactate response to submaximal exercise in middle-aged men

Twenty-one previously sedentary male volunteers (aged 35-50 years) undertook a defined marathon training programme lasting 30 weeks. At weeks 0 (T1), 15 (T2) and 30 (T3) they underwent measurement of maximal oxygen uptake (VO2 max), submaximal VO2 and submaximal plasma lactate concentration during cycle ergometry. No exercise was taken for 24-48 hours prior to testing. During training aerobic power increased significantly (p less than 0.001) from an initial VO2 max at T1 of 33.9 +/- 6 (mean +/- sd) ml.kg-1min-1 to 39 +/- 5.6 ml.kg-1min-1 at T2 but the T3 value of 39.2 +/- 5.2 ml.kg-1min-1 was not significantly different from that at T2. Plasma lactate concentration of 4 mmol.l-1 (OBLAw) occurred at a significantly (P less than 0.05) higher workload (155 +/- 28 w) at T2 compared with T1 (132 +/- 30 w) but the T3 figure was 137 +/- 34 w. OBLA VO2 at T1 was 2.04 +/- 0.42 l.min-1, at T2 was 2.24 +/- 0.04 l.min-1 but at T3 was 2.03 +/- 0.30 l.min-1 (T1:T2 P less than 0.05, T1:T3 NS). OBLA % VO2 max at T1 was 75 +/- 12%, at T2 was 73 +/- 11% but at T3 was 62 +/- 10% (T1:T2 NS, T1:T3 P less than 0.01).

Detection of human muscle glycogen by natural abundance 13C NMR

Natural abundance 13C nuclear magnetic resonance spectroscopy was used to detect signals from glycogen in the human gastrocnemius muscle. The reproducibility of the measurement was demonstrated, and the ability to detect dynamic changes was confirmed by measuring a decrease in muscle glycogen levels after exercise and its subsequent repletion. Single frequency gated 1H decoupling was used to obtain decoupled natural abundance 13C NMR spectra of the C-1 position of muscle glycogen.

Carbohydrate feeding and glycogen synthesis during exercise in man

In 7 male cyclists glycogen synthesis during exercise and rest was studied. Each subject did two exercise trials (A and B), in random order. In both trials, after determining the maximal workload (Wmax), intermittent exercise was given to exhaustion. After the exhaustive exercise and taking a muscle biopsy the subjects either exercised at 40% Wmax for 3 h (trial A) or rested for 3 h (trial B), during which they consumed approximately 2 l of a 25% malto-dextrine drink in both trials. After 3 h rest (trial A) or 3 h of mild exercise (trial B) a second muscle biopsy was taken for total glycogen and histochemistry (ATPase and PAS). Blood glucose and insulin levels were elevated during the first 2 h of exercise (p less than 0.05). Glycogen depletion was most pronounced in type I and to a less extent in type IIA fibers. In trial A muscle glycogen increased from 136 +/- 66 to 199 +/- 71 mmol/kg DW, and in trial B from 145 +/- 56 to 257 +/- 79 mmol/kg DW. During exercise glycogen repletion was restricted to type IIA and IIB fibers, whereas during rest glycogen synthesis occurred both in type I and type II fibers. The present study demonstrates that oral carbohydrate administered during exercise may not only provide substrate for energy metabolism, but can also be utilized for glycogen synthesis in the non-active muscle fibers.

Metabolic response of endurance athletes to training with added load

Endurance athletes were divided into experimental (n = 12) and control (n = 12) groups to investigate the effects of extra-load training on energy metabolism during exercise. A vest weighing 9%-10% body weight was worn every day from morning to evening for 4 weeks including every (n = 6) or every other (n = 6) training session. After 4 weeks the control group had a lower blood lactate concentration during submaximal running, whereas the experimental group had significantly higher blood lactate and oxygen uptake (p less than 0.01--p less than 0.05), and a lower 2 mmol lactate threshold (p less than 0.05) and an increased blood lactate concentration after a short running test to exhaustion (p less than 0.05). Those experimental subjects (n = 6) who used the added load during every training session had a lower 2 mmol lactate threshold, improved running time to exhaustion, improved vertical velocity when running up stairs and an increased VO2 during submaximal running after the added load increased anaerobic metabolism in the leg muscle during submaximal and maximal exercise. An increased recruitment and adaptation of the fast twitch muscle fibres is suggested as the principal explanation for the observed changes.

Carbohydrate, Muscle Glycogen, and Improved Performance

In brief: Muscle glycogen is the body's chief source of energy for prolonged exercse at moderately high intensity (65% to 85% of Vo2 max); glyocgen reserves therefore limit endurance at this pace. Endurance athletes who train daily must maintain normal glycogen stores to support their training and thereby improve performance. Glycogen synthesis after prolonged exercise is directly related to the amount of dietary carbohydrate; accordingly, athletes who engage in intense daily training should consume 70% of their calories as carbohydrate. Further, athletes can increase their endurance by overloading their glycogen stores. This is done by tapering training the week before a competition and increasing carbohydrate intake the last three days before the event.

The Physiology of Marathon Running

In brief: The marathon is one of the greatest tests of human endurance. This review article describes the physiological demands and responses of the respiratory, cardiovascular, and muscular systems to marathon running, focusing on the chain of oxygen transport needed to fulfill the aerobic requirements of marathon running. During the race, runners use about 75% of VO2 max. Increased levels of ventilation (> 80 liters· min(-1)) have been observed during the marathon. This can lead to decrements in forced vital capacity and respiratory muscle fatigue. Prolonged muscle activity during the run can cause muscle injury such as inflammation and necrosis of muscle fibers. Techniques for augmenting performance include endurance and interval training, diet manipulation, and racing strategy.

Physiologic changes during a marathon, with special reference to magnesium

In a single case study of a moderately trained, healthy man, physiologic changes during a marathon are reported. Blood was drawn prior to the race, at 1 hour and 2 hours into the race, at the end of the race, and after 1 hour of recovery. By 1 hour into the race, norepinephrine, epinephrine, and dopamine had increased nearly nine-fold, two-fold and five-fold, respectively. After 1 hour of recovery, epinephrine had returned to the pre-race value but norepinephrine and dopamine were still elevated. Cortisol increased gradually and was more than doubled by the end of the race. It was still elevated after 1 hour of recovery. White blood cells gradually increased, reaching their maximum value at the end of the race; a four-to-five-fold increase. Thromboxane B2, which had an inverse relationship to serum magnesium, was below the pre-race value for the first 2 hours but increased nine-fold by the end of the race. Serum magnesium increased from 1.44 meq/l to 1.68 meq/l at 2 hours into the marathon, dropped to 1.07 meq/l by the end of the race, and returned to its pre-race value by 1 hour of recovery. The decrease in serum magnesium at the end of the race may be associated with increased plasma free fatty acid levels.

Nutrition and exercise

The nutritional aspects of exercise are topics of popular interest, misconception, and active research. In this article, the authors review basic concepts of muscle metabolism; information concerning the role of exercise in weight loss; dietary supplements for athletes, including nutrition for competition; and eating disorders among those performing vigorous exercise.

The marathon: dietary manipulation to optimize performance

Despite pronounced involvement of the cardiovascular and respiratory systems during the marathon, the limiting factor for this event revolves around the supply and utilization of intramuscular and extramuscular fuel reserves. The single most consistently observed factor contributing to fatigue at work intensities selected by marathon runners is the depletion of muscle's endogenous carbohydrate, glycogen. Dietary manipulations which reduce the rate of muscle glycogen degradation will, therefore, spare this important fuel and delay fatigue. The purpose of this paper will be to review dietary factors which are pertinent to fuel utilization in order to optimize marathon performance.

Diminished dietary thermogenesis in exercise-trained human subjects

The influence of exercise-training on dietary-induced thermogenesis (DIT) was investigated in humans. The resting metabolic rate was identical in trained and non-trained subjects, but the response to a meal containing 1,636 kcal (6.9 MJ) was markedly lower in trained subjects. Mean dorsal skin temperature, as measured by thermography, was not influenced by training. A significant correlation was observed between postprandial RQ and DIT, which indicates that the reduced energy expenditure noted in trained subjects is related to a greater lipid oxidation. This sparing effect of exercise-training on energy utilization in the form of carbohydrate, is interpreted as adaptive in the sense that energy is preserved for the purpose of producing work.

Physical work capacity and lung function in competitive swimmers

Competitive swimmers require a high aerobic capacity to support the sustained performance of severe exercise. Maximal oxygen uptake values and blood lactate concentrations were measured in ten male and fifteen female swimmers from the Scottish National and Youth Squads 1981-82. Lung function tests were also performed to determine pulmonary efficiency. The results of these studies were compared with other investigations of international competitive swimmers and indicate a high level of fitness among Scottish national swimmers.

Changes in onset of blood lactate accumulation (OBLA) and muscle enzymes after training at OBLA

Eight well-trained middle and long distance male runners added to their regular training program a weekly 20-min treadmill run at a velocity calculated to elicit a blood lactate concentration of 4 mmol X 1-1. VO2 max, the running velocity eliciting 4 mmol X 1-1 blood lactate (VOBLA), and the activities of citrate synthase (CS), phosphofructokinase (PFK), lactate dehydrogenase (LDH) and LDH isozymes in the M. vastus lateralis were determined before and after 14 weeks of this training. Significant increases were observed in VOBLA and the relative fraction of heart-specific LDH, while the activity of PFK and the ratio of PFK/CS decreased after training. The change in VOBLA was negatively correlated to the mean rate of blood lactate accumulation during the last 15 min of the treadmill training runs, and positively correlated to the percentage of slow twitch fibers in the M. vastus lateralis. The data support the hypothesis that a steady state training intensity which approximates VOBLA will increase VOBLA, and will result in measureable local metabolic adaptations in the active skeletal muscles of well-trained runners without a significant change in maximal aerobic power. Muscle fiber type composition may be an indicator of the "trainability" of the musculature.

Post-competition blood lactate concentrations in collegiate swimmers

The purpose of this investigation was to quantitate post-competition lactate (LA) concentrations of swimmers during a competitive collegiate meet. Blood LA was measured by an enzymatic method on 23 subjects 5 min after each race event. The largest mean LA concentration of 25.7 mM/L was observed in swimmers after competing in the 200-yd individual medley. Swimmers in the 200-yd butterfly, back, breast and freestyle races had similar mean blood LA concentrations (ranging from 16.4 to 20.6 mM/L). Swimmers in the two longest events, the 500-yd and 1,000-yd free style races, had mean LA concentrations of 15.6 and 10.0 mM/L, respectively. To account for the effects of motivation, LA concentrations were measured following maximal effort noncompetitive 100 and 200-yd swims. LA concentrations were slightly greater in conjunction with faster performances for the competitive as compared to the noncompetitive 100 and 200-yd swims.

Inhibition of glucose uptake and glycogenolysis by availability of oleate in well-oxygenated perfused skeletal muscle

The effects of exogenous oleate on glucose uptake, lactate production and glycogen concentration in resting and contracting skeletal muscle were studied in the perfused rat hindquarter. In preliminary studies with aged erythrocytes at a haemoglobin concentration of 8g/100ml in the perfusion medium, 1.8mm-oleate had no effect on glucose uptake or lactate production. During these studies it became evident that O(2) delivery was inadequate with aged erythrocytes. Perfusion with rejuvenated human erythrocytes at a haemoglobin concentration of 12g/100ml resulted in a 2-fold higher O(2) uptake at rest and a 4-fold higher O(2) uptake during muscle contraction than was obtained with aged erythrocytes. Rejuvenated erythrocytes were therefore used in subsequent experiments. Glucose uptake and lactate production by the well-oxygenated hindquarter were inhibited by one-third, both at rest and during muscle contraction, when 1.8mm-oleate was added to the perfusion medium. Addition of oleate also significantly protected against glycogen depletion in the fast-twitch red and slow-twitch red types of muscle, but not in white muscle, during sciatic-nerve stimulation. In the absence of added oleate, glucose was confined to the extracellular space in resting muscle. Addition of oleate resulted in intracellular glucose accumulation in red muscle. Contractile activity resulted in accumulation of intracellular glucose in all three muscle types, and this effect was significantly augmented in the red types of muscle by perfusion with oleate. The concentrations of citrate and glucose 6-phosphate were also increased in red muscle perfused with oleate. We conclude that, as in the heart, availability of fatty acids has an inhibitory effect on glucose uptake and glycogen utilization in well-oxygenated red skeletal muscle.

The adipokinetic effect of hyperthermic stress in man

The purpose of this investigation was to assess the alteration in serum free fatty acid concentrations during heat stress and dehydration. Each subject was exposed to heat stress in an environment chamber on 2 separate occasions. During the first exposure the subjects remained seated until the core temperature was elevated 1.4degreesC resulting in a mean weight loss of 1.66 kg due to dehydration. The second condition involved water replacement equal to the weight loss of the initial dehydration condition. Blood samples were obtained prior to heat exposure, when the core temperature was elevated 0.7degreesC and 1.4degreesC. They were subsequently analyzed for free fatty acids (FFA), glucose and lactin acid. Heart rates and core temperatures were monitored at 4 min intervals. During the dehydration condition the mean change in serum FFA was 0.9 muEq/ml in contrast to 0.2 muEq/ml for the rehydration condition. Serum levels of glucose increased moderately throughout the exposure (8 mg-%).

A sparing effect of increased plasma fatty acids on muscle and liver glycogen content in the exercising rat

Increasing plasma free fatty acids decreased the degree of glycogen depletion, and increased the citrate concentration, in slow-red (soleus) and fast-red (deep portion of vastus lateralis) muscle during exercise (approx. 50% depletion of glycogen, as against 75% in control animals). There was no effect in fast-white muscle (superficial portion of vastus lateralis). Glycogen concentration in the liver decreased by 83% in controls, but only by 23% in animals with increased free fatty acids during exercise. The decreased glycogen depletion may be partly explained by the findings that (a) plasma-insulin concentration was two- to three-fold higher in animals with increased plasma free fatty acids and (b) the exercise-induced increase in plasma glucagon was lessened by increased free fatty acids. Blood glucose was higher in the animals with increased free fatty acids after the exercise. The rats with increased plasma free fatty acids utilized approx. 50% as much carbohydrate as did the controls during the exercise.

Continuous in vivo measurement of creatine kinase variations in man during an exercise

In order to obtain information concerning the CPK turnover during exercise, we have elaborated a technique to measure its activity continuously in vivo. A colorimetric method has been adapted to total blood. CPK was measured continuously in 5 physically fit athletes exercised on an ergometric bicycle for 30 min. There was practically no increase in enzymatic activity. The starting level was very high, corresponding perhaps to a "permanent state of enzyme release"; the exercise was not hard enough because of the fitness of the athletes or the maximal increase did not occur immediately after the exercise.