Oxidation of carbohydrate ingested during prolonged endurance exercise.

Classic studies conducted in the 1920s and 1930s established that the consumption of a high carbohydrate (CHO) diet before exercise and the ingestion of glucose during exercise delayed the onset of fatigue, in part by preventing the development of hypoglycaemia. For the next 30 to 40 years, however, interest in CHO ingestion during exercise waned. Indeed, it was not until the reintroduction of the muscle biopsy technique into exercise physiology in the 1960s that a series of studies on CHO utilisation during exercise appeared. Investigations by Scandinavian physiologists showed that muscle glycogen depletion during prolonged exercise coincided with the development of fatigue. Despite this finding, attempts to delay fatigue during prolonged exercise focused principally on techniques that would increase muscle glycogen storage before exercise. The possibility that CHO ingestion during exercise might also delay the development of muscle glycogen depletion and hence, at least potentially, fatigue, was not extensively investigated. This, in part, can be explained by the popular belief that water replacement to prevent dehydration and hyperthermia was of greater importance than CHO replacement during prolonged exercise. This position was strengthened by studies in the early 1970s which showed that the ingestion of CHO solutions delayed gastric emptying compared with water, and might therefore exacerbate dehydration. As a result, athletes were actively discouraged from ingesting even mildly concentrated (greater than 5 g/100ml) CHO solutions during exercise. Only in the early 1980s, when commercial interest in the sale of CHO products to athletes was aroused, did exercise physiologists again begin to study the effects of CHO ingestion during exercise. These studies soon established that CHO ingestion during prolonged exercise could delay fatigue; this finding added urgency to the search for the optimum CHO type for ingestion during exercise. Whereas in the earlier studies, estimates of CHO oxidation were made using respiratory gas exchange measurements, investigations since the early 1970s have employed stable 13C and radioactive 14C isotope techniques to determine the amount of ingested CHO that is oxidised during exercise. Most of the early interest was in glucose ingestion during exercise. These studies showed that significant quantities of ingested glucose can be oxidised during exercise. Peak rates of glucose oxidation occur approximately 75 to 90 minutes after ingestion and are unaffected by the time of glucose ingestion during exercise. Rates of oxidation also appear not to be influenced to a major extent by the use of different feeding schedules.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fat and carbohydrate metabolism during submaximal exercise in children

During exercise, the contribution of fat and carbohydrate to energy expenditure is largely modulated by the intensity of exercise. Age, a short- or long-term diet enriched in carbohydrate or fat substrate stores, training and gender are other factors that have also been found to affect this balance. These factors have been extensively studied in adults from the perspective of improving performance in athletes, or from a health perspective in people with diseases. During the last decade, lifestyle changes associated with high-energy diets rich in lipid and reduced physical activity have contributed to the increase in childhood obesity. This lifestyle change has emerged as a serious health problem favouring the early development of cardiovascular diseases, insulin resistance or type 2 diabetes mellitus. Increasing physical activity levels in young people is important to increase energy expenditure and promote muscle oxidative capacity. Therefore, it is surprising that the regulation of balance between carbohydrate and lipid use during exercise has received much less attention in children than in adults. In this review, we have focused on the factors that affect carbohydrate and lipid metabolism during exercise and have identified areas that may be relevant in explaining the higher contribution of lipid to energy expenditure in children when compared with adults. Low muscle glycogen content is possibly associated with a low activity of glycolytic enzymes and high oxidative capacity, while lower levels of sympathoadrenal hormones are likely to favour lipid metabolism in children. Changes in energetic metabolism occurring during adolescence are also dependent on pubertal events with an increase in testosterone in boys and estrogen and progesterone in girls. The profound effects of ovarian hormones on carbohydrate and fat metabolism along with their effects on oxidative enzymes could explain that differences in substrate metabolism have not always been observed between girls and women. Finally, although the regulatory mechanisms of fat and carbohydrate balance during exercise are quite well identified, there are a lack of data specific to children and most of the evidences reported in this review were drawn from studies in adults. Isotope tracer techniques and nuclear magnetic resonance will allow non-invasive investigation of the metabolism of the different substrate sources in skeletal muscle.     

Oxidation and metabolic effects of fructose or glucose ingested before exercise

The aim of this study was to compare the effects of fructose (F) and glucose (G) intake before exercise on oxidation of the ingested substrate, glycogen utilization, work output, and metabolic changes. Ten trained subjects ingested F or G (1 g/kg), both of which were naturally enriched in 13C. After 1 h of rest, they exercised on an ergometer at 61% of their maximal oxygen uptake (VO2 max) for 45 min, which was immediately followed by 15 min at their maximal voluntary output. During the resting hour, blood insulin and glucose were lower (p less than 0.05) and respiratory quotient and blood lactate higher (p less than 0.01) after F. During exercise, the differences disappeared, apart from a transient but moderate (4.3 mmol/l) hypoglycemia after G compared to F. No difference between F and G was observed for uric acid, glycerol, FFA, and glucagon. Glycogen decrements in the vastus lateralis muscle were 67 +/- 9 (F) and 97 +/- 15 (G) mmol/kg, values not significantly different from each other (P greater than 0.05). The maximal voluntary work produced during the last 15 min did not differ between treatments. During the 2 h after sugar ingestion, 30 +/- 3 g of F and 26 +/- 3 g of G were oxidized to 13CO2. These findings indicate that fructose ingested before exercise was utilized at least as well as glucose, allowed a more stable glycemia, and did not modify performance.     

The effect of exercise on lipid metabolism in men and women

Lipoprotein abnormalities constitute a major risk for development of cardiovascular disease. These substances, which are comprised of various lipids and proteins (apoproteins), are influenced by specific enzymes which effect their concentrations. It has been demonstrated that elevated total cholesterol and LDL cholesterol are directly associated with the development of coronary artery disease, whereas HDL cholesterol has an inverse relationship with coronary heart disease (CHD). Although more controversial, triglycerides may also be directly associated with coronary atherosclerosis. Favourable changes in lipid levels have been shown to reduce coronary mortality. Exercise may constitute a non-pharmacological approach to lipoprotein therapy. Many exogenous factors also influence lipoprotein concentrations. Changes in diet, body composition, age, as well as medication and alcohol usage may directly alter lipid levels. In addition, they can be artificially affected by the analytical method. The immediate effects of one to several bouts of physical activity appear to influence lipoprotein level. A reduction in triglycerides has been shown after physical exertion, especially among trained individuals and those with hypertriglyceridaemia. These acute changes may reflect the utilisation of both muscle and plasma triglycerides as fuels during exertion. After more prolonged training, changes in lipoproteins may also occur. However, since exercise is accompanied by many co-variables which also favourably alter these levels (e.g. lower percentage of body fat, dietary alterations), it is difficult to determine the direct effect of regular physical activity. Initial studies of exercise training's effects on total cholesterol did not differentiate changes in HDL and LDL cholesterol. Subsequent research has observed these specific cholesterol fractions. Consistent reduction in LDL cholesterol levels have not been convincingly demonstrated. Although HDL cholesterol has been shown to increase in certain studies, the response has been variable in other investigations. These latter responses may have been due to the fact that HDL cholesterol changes may be dependent on levels prior to conditioning. Assessment of HDL cholesterol subfractions (HDL2 and HDL3), which could additionally impact on cardiovascular risk reduction, have shown favourable increases in HDL2, but as yet these HDL moieties have not been adequately investigated. Reductions in triglyceride levels after training among those with elevated values and beneficial apoprotein changes post-training have been reported, although few studies exist.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of carbohydrate and vitamin B6 on fuel substrates during exercise in women

Pyridoxal 5'-phosphate is an essential co-factor for glycogen phosphorylase and certain enzymes in the alanine-glucose cycle. Glycogen phosphorylase is also proposed as a storage reservoir for vitamin B6. To examine the effect of vitamin B6 and carbohydrate on fuel substrates during exercise, 5 young/trained, 5 young/un-trained, and 5 post-menopausal/un-trained women were alternately fed four diets (varying carbohydrate and B6 level) over a 7-wk period. Subjects were exercised at the end of each dietary period at 80% VO2max for 20 min on a cycle ergometer. Blood was drawn pre-, post-, post-30, and post-60 min of exercise and analyzed for plasma pyridoxal 5'-phosphate, glucose, free fatty acid, and lactate. ANOVA showed no difference among groups or diets for lactate, although lactate was significantly different over time (P less than 0.0001) on all diets. ANOVA showed significant time x group x diet inter-actions for free fatty acid (P less than 0.05) and significant diet x time (P less than 0.04) and time x group (P less than 0.03) inter-actions for glucose. Supplementation and/or increased carbohydrate resulted in lower free fatty acid during exercise in all groups. ANOVA showed no difference in pyridoxal 5'-phosphate for groups with respect to diet or time, but did show a significant increase from pre- to post-exercise and a significant decrease from post- to post-60 min of exercise. These results indicate that a high-carbohydrate diet and/or supplemental vitamin B6 can alter plasma fuel substrates during exercise in women; however, the effect depends on age and level of training.     

Metabolic responses to exercise after carbohydrate loads in healthy men and women

PURPOSE: This study aimed to investigate gender differences in i) pancreatic insulin secretory (beta-cell sensitivity) and whole body insulin sensitivity responses to an intravenous carbohydrate (CHO) load, and (ii) metabolic responses to exercise after both intravenous and oral CHO loads. METHODS: Seven untrained healthy men and seven age-, body mass-, and VO2max-matched women performed two trials. In one trial they cycled for 60 min at 50% VO2max, starting 60 min after ingestion of a carbohydrate-rich meal (ME trial). In the other trial, subjects were infused with 20% dextrose solution to maintain blood glucose concentration at approximately 8 mmol x L(-1) for 60 min (INF trial), then the infusion rate was maintained constant during the following 60 min while exercising at 50% VO2max. RESULTS: There was no gender effect on beta-cell sensitivity (serum insulin: 161 +/- 37 and 159 +/- 28 pmol x L(-1) for men and women, respectively) and whole body insulin sensitivity (155 +/- 24 and 135 +/- 29 mg x KgFFM(-1) x min(-1) per pmol x L(-1) x 100 for men and women, respectively). This may explain the similarity in glycemic, substrate oxidation and other metabolic responses to exercise after both intravenous and oral CHO loads in men and women. CONCLUSION: These results suggest that moderate exercise performed in the postprandial state presents a similar challenge to the ability of healthy, untrained men and women to perform exercise without a substantial decline in plasma glucose concentration below fasting values.     

Effect of carbohydrate ingestion on exercise endurance and metabolism after a 1-day fast

Fasting before an exercise event has been demonstrated to decrease endurance. The purpose of this study was to investigate whether this decrement in performance after fasting could be reversed by ingestion of a carbohydrate solution before and during exercise. Nine fit male subjects ran to exhaustion at approximately 70% VO2max in two counterbalanced trials. The subjects were fasted for 21 h before both trials, and the trials were arranged so that the subjects ingested either a carbohydrate (CHO) or placebo (PL) solution. Although ratings of perceived exertion were significantly lower in the CHO trial, there were no differences in endurance time to exhaustion in the two trials (102 +/- 8 min in the PL trial and 106 +/- 8 min in the CHO trial). There were no differences between trials for the VO2, heart rate, and blood lactate concentrations. As expected, the blood glucose and insulin concentrations were higher in the CHO trial. The respiratory exchange ratio was significantly higher in the CHO trial at 40 min of exercise and tended to be higher at all other times, suggesting a greater reliance on carbohydrate and less on fat as an energy source. This seemed to be confirmed by the significantly lower plasma glycerol concentration, which suggested less fat mobilization in the CHO trial. Ingestion of a glucose polymer solution increased carbohydrate utilization in fasted subjects, but exercise performance was not improved.     

The effects of supplemental carbohydrate ingestion on intermittent isokinetic leg exercise.

BACKGROUND: The purpose of this investigation was to examine the effects of carbohydrate (CHO) supplementation on isokinetic leg extension/flexion exercise performance, blood glucose responses, blood free fatty acid (FFA) responses, and blood lactate (La) responses. METHODS: Eight resistance trained males (mean+/-SEM, age: 23.7+/-1.3 yrs, height: 180.0+/-3.5 cm, bodymass: 94.9+/-4.9 kg) participated in a randomized, double blind protocol with testing sessions separated by 7-d. Subjects were given CHO or placebo (P) while performing 16 sets of 10 repetitions at 120 degrees x s(-1) on a Cybex isokinetic dynamometer. Performance variables measured were; total work (TW), average work (AW), peak torque (PT) and average torque (AT). Plasma glucose (PG), FFA, and La were measured prior to testing (PRE), after set 8 (MID), and 16 (POST). RESULTS: Results indicated that the CHO treatment elicited significantly (p<0.05) more TW (CHO: 41.1+/-3.9 kJ; P: 38.1+/-3.9 kJ) and AW (CHO: 2.6+/-0.2 kJ; P: 2.4+/-0.2 kJ). There were no differences (p<0.05) between treatments for PT of the hamstrings (CHO: 91.6+/-6.5 Nm; P: 87.4+/-8.5 Nm) and quadriceps (CHO: 129.7+/-9.5 Nm; P: 123.0+/-10.6 Nm). The AT of the hamstrings (CHO: 77.8+/-5.2 Nm; P: 75.7+/-8.7 Nm) and quadriceps (CHO: 116.9+/-8.9 Nm; P: 110.0+/-8.5 Nm) were not statistically different (p>0.05) between the treatments. PG was significantly higher at the POST blood draw in the CHO treatment. No significant differences (p>0.05) were observed between the treatments for FFA and La concentrations. CONCLUSIONS: The data from this investigation indicate that the use of CHO supplementation during isokinetic leg exercise allows for the performance of more work.     

Puberty effects on NK cell responses to exercise and carbohydrate intake in boys

Previous research has demonstrated that younger versus older animals and humans experience smaller perturbations in natural killer (NK) cells in response to physiological stress. PURPOSE: To determine whether the smaller perturbations in NK cells induced by strenuous exercise and carbohydrate (CHO) intake, previously reported in children, are influenced by puberty. METHODS: Twenty 12-yr-old boys, distinguished as prepubertal (Tanner (T) 1, N = 7), early pubertal (T2, N = 7), or pubertal (T3-5, N = 6), cycled for 60 min at 70% VO(2max) while drinking 6% CHO (CT) or flavored water (WT). Blood was collected at rest and during (30 and 60 min) and following (30 and 60 min) exercise to identify NK cells as CD3(-)CD56(dim) or CD3(-)CD56(dim). CD69 expression on CD3(-)CD56(+) cells was also determined. RESULTS: A puberty x CHO x exercise interaction was found for the proportion, but not number, of CD56(dim) cells (P = 0.06). CD56(dim) cell counts were lower in CT versus WT (P < 0.001). Responses of CD56(bright) proportions (P = 0.007) and counts (P = 0.03) depended on pubertal status, but not CHO. The CD56(bright):CD56(dim) ratio remained stable during exercise, but during recovery was higher in T1 and T3-5 versus T2 (P = 0.08) and in CT versus WT (P = 0.04). During recovery, CD3(-)CD56(+) cells expressed higher levels of CD69 (P = 0.01), with no change in the proportion of CD69(+) cells. CONCLUSION: These results confirm the influence of puberty on the distribution of NK cell subsets in response to exercise and CHO intake. Increased CD69 expression suggests that NK cells increase activation status during recovery from physiological stress.     

Acute effects of resistance exercise on parameters of lipoprotein metabolism

Ten healthy, trained males (25.4 +/- 3.1 yr) were studied before and after 90 min of resistance exercise to determine the acute effects of high volume (HV) and low volume (LV) sessions on alterations in lipid and lipoprotein concentrations as well as the activity of lecithin: cholesterol acyltransferase (LCAT). The HV session involved the use of 8-12 repetition maximum (RM) loads performed to exhaustion with 60 s rest intervals between sets, while the LV session involved the use of 1-5RM loads with 3 min rest intervals between sets. Fasting blood samples were drawn from an antecubital vein immediately before and after exercise as well as 24, 48, and 72 h post-exercise. Following adjustment for changes in plasma volume, significant changes were only found following the HV session. These included increases in HDL-C (11%) and HDL3-C (12%) 24 h post-exercise. Modifications in HDL-C were significantly different from both the LV and control sessions. In contrast, triglycerides and LCAT were significantly depressed 24 h post-exercise following the HV session when compared with immediate post-exercise values. There were no significant changes in either total cholesterol or HDL2-C at any time. The results of this study suggest that the potential of resistance exercise to affect acute modifications in the lipoprotein profile resides in the volume of exercise performed.     

Exogenous carbohydrate spares muscle glycogen in men and women during 10 h of exercise

PURPOSE: The purpose of the study was to evaluate the effects of carbohydrate (CHO) supplementation on whole-body and net muscle substrate use during 10 h of discontinuous exercise, simulating occupational settings in men and women. METHODOLOGY: Recreationally trained subjects (N = 7 males, N = 6 females) performed a graded exercise test on a treadmill (TM) and cycle ergometer (CE) to determine ventilatory threshold (VT) and V O2peak. In a double-blind, randomized crossover design, subjects received either CHO [20% maltodextrin (0.6 g.kg FFM.h)] or flavored placebo (PLA) drink each hour across 10 h of exercise. Exercise intensity was 71.3 +/- 3% and 72.4 +/- 4% VT for TM and CE, respectively. Hourly exercise included 9 min of upper-body ergometery, 19 min of cycling, and 20 min of treadmill walking, with a 1-min transition between modes, followed by a 10-min rest and feeding period. The protocol was selected to simulate arduous occupational settings. Vastus lateralis biopsies were obtained before and after exercise. Expired gases were collected every other hour to establish average rates of whole-body CHO and fat oxidation. Blood glucose (BG) was measured continuously. RESULTS: Whole-body CHO oxidation was maintained during CHO trial compared with the PLA trial. Net muscle glycogen use was 52% higher for the PLA trial (176.0 +/- 16.7, 117.0 +/- 20.9 and 164.5 +/- 11.0, 133.8 +/- 10.9 mmol.kg w.w. for PLA and CHO, respectively, P < 0.05). There were no significant sex-specific differences in glycogen use, whole-body substrate oxidation, or BG values. CONCLUSION: The ingestion of CHO during long-duration exercise decreases net muscle glycogen use while better maintaining whole-body carbohydrate oxidation, and potentially increasing performance in field settings. There are limited differences in sex-specific substrate oxidation.     

The acute effects of exercise duration on serum lipoprotein metabolism

The purpose of this study was to determine the effect of exercise duration on lipoprotein responses. Twenty two normolipidemic male volunteers, ages 19-31 yrs (X +/- SEM = 23.1 +/- 2.94) participated in the study. Each was novice a runner (training less than 5 mi/wk). Subjects were assessed for baseline lipid measures of high density lipoprotein (HDL-C), low density lipoprotein (LDL-C), total cholesterol (TC) and triglycerides (TG). They were then evaluated for VO2max and ventilatory threshold (VT). Later they were matched for VO2max and randomly assigned to one of three groups which exercised for 15, 30 or 45 min respectively at a VO2 20% below VT. Subjects were evaluated again for HDL-C, LDL-C, TC and TG from blood samples taken pre-exercise and immediately post-exercise, as well as 1, 24 and 48 hrs post-exercise. A 3 X 4 split plot ANOVA found no difference for any lipid measure during the baseline period. A 3 X 5 split-plot ANOVA (covarying for pre-exercise measures) and post-hoc comparisons of pre- and post-exercise lipid levels indicated no significant differences occurred for either TC, TG or LDL-C measures (p less than 0.05). With respect to HDL-C, the 30 min group had significantly lower HDL-C at the 24 hr measure than did 45 min group (46.41 +/- 1.70 vs 53.34 +/- 1.73 mg.dl-1 respectively). No other differences were found. These findings indicate exercise duration will have an effect on acute responses of lipoprotein following exercise of low intensity.     

Effect of carbohydrate feeding during recovery from prolonged running on muscle glycogen metabolism during subsequent exercise

This study examined the effect of carbohydrate (CHO) intake during a 4 h recovery from prolonged running on muscle glycogen metabolism during subsequent exercise. On 2 occasions, 7 male subjects ran for 90 min at 70 % maximum oxygen uptake VO(2 max) on a motorized treadmill (R1) followed by a 4 h rest period (REC) and a 15 min run (R2) consisting of 5 min at 60 % and 10 min at 70 % VO(2 max) During REC, each subject ingested a total of 2.7 l of an isotonic solution containing either 50 g of CHO (LOW) or 175 g of CHO (HIGH). Biopsy samples were obtained from the vastus lateralis immediately after R1, REC and R2. During REC, a higher muscle glycogen resynthesis was observed in HIGH when compared with LOW trial (75 +/- 20 vs. 31 +/- 11 mmol x kg dry matter (dm) -1, respectively; p < 0.01). Muscle glycogen utilization during R2 was similar between the HIGH and LOW trials (39 +/- 10 vs. 46 +/- 11 mmol x kg dm -1, respectively). These results suggest that ingestion of a large amount of CHO at frequent intervals during recovery from exercise does not affect the rate of muscle glycogen utilization during subsequent exercise.     

The effects of a walking exercise program on fall-related fitness, bone metabolism, and fall-related psychological factors in elderly women

The purpose of this study was to determine the effects of a 3-month walking exercise program with ankle weights on fall-related fitness, bone metabolism, and fall-related psychological factors. Fall-related fitness was determined from strength, balance, agility, aerobic endurance, muscle mass, and fat mass measures. Bone metabolism was measured using bone density, hormones, and biochemical markers. Fall-related psychological factors included fear of falling and falls efficacy. A 2 × 2 factorial with repeated measures design was used. All subjects were community-dwelling elderly women who volunteered to participate, and randomly were assigned to either an exercise group (n = 11) or a control group (n = 10). Results revealed significant changes in upper body strength, leg strength, aerobic endurance, and body composition. Additionally, hormones and biochemical markers changed significantly over time. Trunk fat and fear of falling changed differently among the two groups. In conclusion, this study suggests that a 3-month walking exercise program with ankle weights may have positive effects on fall-related fitness, bone metabolism, and fall-related psychological factors.     

A high carbohydrate diet negates the metabolic effects of caffeine during exercise

We tested the hypothesis that unrecognized nutritional factors might influence the serum free fatty acid response to caffeine. The time course and extent of the serum free fatty acid response to a fatty meal alone, to caffeine ingestion after a high carbohydrate meal, or to caffeine in combination with either a fatty meal or a high carbohydrate meal (the latter following 3 d of a high carbohydrate diet) were studied in six trained runners. The metabolic response to 120 min of exercise at 75% of maximum oxygen consumption after caffeine ingestion was also studied in runners after a high carbohydrate diet and ingestion of a high carbohydrate meal. Serum free fatty acid levels were highest 3 h after caffeine ingestion alone and were lower following a fatty meal with or without caffeine ingestion (P less than 0.05). The high carbohydrate diet combined with the ingestion of a high carbohydrate meal prevented the expected rise in serum free fatty acid levels following caffeine ingestion. The metabolic response to prolonged submaximal exercise was also not influenced by the ingestion of caffeine by subjects who had eaten a high carbohydrate diet. Nutritional factors, in particular the state of the body carbohydrate stores and the simultaneous ingestion of carbohydrate, influence the response of serum free fatty acid levels to caffeine ingestion.     

The effects of water-based strength exercise on quality of life in young women

Background

Exercise improves quality of life (QoL). However, little is known concerning the effects of different volumes of strength exercise on QoL. The aims of this study are to: (1) evaluate the effects of water-based strength exercise on QoL of healthy young women and (2) compare the effects of different volumes of water-based strength exercises on QoL of healthy young women.

Methods

Sixty-six participants were randomly allocated into four groups with different volumes of exercise. The participants performed water-based strength exercises for 20 weeks, two times a week, supervised by trained physiologists.

Results

A significant improvement was found in overall QoL (F = 5.96; p = 0.018) and in physical (F = 22.01; p < 0.001), psychological (F = 8.408; p = 0.006) and environment domains (F = 8.34; p = 0.006). In addition, a significant decrease of depressive symptoms was found (F = 22.32; p < 0.001). No difference was found between groups in any domain of QoL or depressive symptoms.

Conclusion

Water-based strength exercise improves specific domains of QoL and decreases depressive symptoms of young healthy women. Different volumes of exercise promote similar effects on QoL and depressive symptoms.

     

Use of transgenic models to understand effects of exercise on glucose metabolism

This review will focus on the achievements obtained using transgenic animals in our understanding of exercise-induced insulin sensitivity as well as the regulation of glycogen storage in skeletal muscle, including the effects of acute exercise and insulin on the major determinants: glycogen synthesis and glucose uptake.