Physiological and metabolic effects of a 25 km race in female athletes

Recommendations for treatment of hyponatraemia at endurance events

This review focuses on possible pathophysiology of exercise-associated hyponatraemia and its implication on evaluation and treatment of collapsed athletes during endurance events. Rehydration guidelines and field care have traditionally been based on the belief that endurance events create a state of significant fluid deficit in athletes, which must be corrected by liberal hydration. Beliefs in the necessity of liberal hydration may have contributed to cases of hyponatraemia. Assumptions that fluid loss accounts for the entire weight loss during exercise and that fluid ingestion is the only source of water gain during exercise may lead to an overestimation of the degree of volume depletion and the amount of fluid needed for replacement. Increasing evidence suggests that hyponatraemic athletes are fluid overloaded; ingestion of large amount of hypotonic fluid in combination with inappropriate or inadequate physiological responses leads to excessive retention of free fluid. Risk factors include hot weather, female sex, slower finishing time, and possibly the use of nonsteroidal anti-inflammatory medications. Symptoms of hyponatraemia can be subtle and can mimic those of other exercise-related illnesses, thereby complicating its diagnosis and leading to possible inappropriate treatment. Most athletes who collapse at the finish line experience exercise-associated collapse, a benign and transient form of postural hypotension that can be treated simply by continued ambulation after finishing or elevation of legs while in a supine position for those who cannot walk. Care providers should consider the use of intravenous hydration with normal saline carefully since it is not needed by most collapsed athletes and may worsen the condition of patients with unsuspected hyponatraemia. Historic information and clinical signs of volume depletion should be elicited prior to its use. Most hyponatraemic athletes will recover uneventfully with careful observation while awaiting spontaneous diuresis. Use of hypertonic saline should be reserved for patients with severe symptoms. Moderate consumption of carbohydrate-electrolyte solution during exercise may allow the maintenance of adequate hydration and the prevention of hyponatraemia.

Magnesium deficiency and blood 2,3-diphosphoglycerate concentrations in sedentary and exercised male Osborne-Mendel rats

Magnesium deficiency lowers endurance capacity of untrained rats by an as yet unknown mechanism. We tested the hypothesis that low plasma and blood Mg concentrations would reduce blood 2,3-diphosphoglycerate (2,3-DPG) concentrations, thus diminishing the oxygen delivery system of the Mg-deficient rat. Untrained male Osborne-Mendel rats (95 g) were studied at rest or following exhaustive exercise after three weeks of dietary treatment (400 or 100 micrograms Mg/g diet; deionized or high Mg water [greater than 85 micrograms Mg/ml]). Magnesium deficiency resulted in lowered plasma and red blood cell (RBC) Mg concentrations and reduced endurance capacity compared with controls. Magnesium supplementation normalized these parameters. 2,3-DPG was elevated by 10% in rats acutely exercised compared with sedentary rats; however, 2,3-DPG levels were not affected by dietary Mg intake. Acute exercise tended to increase erythrocyte Mg concentrations in control rats and lower erythrocyte Mg levels in deficient rats. In conclusion, the reduced endurance capacity of Mg-deficient rats is not due to impaired 2,3-DPG production.

Dietary magnesium intake influences exercise capacity and hematologic parameters in rats

The effects of feeding varying concentrations of dietary Mg (50, 100, 200, or 400 ppm) for 22 days on exercise capacity and hematologic parameters were investigated in male rats. Five-week-old male Osborne-Mendel rats fed diets containing 50 or 100 ppm Mg developed transitory priapism and hyperemia, signs of Mg deficiency. Based on a treadmill test, these rats showed a markedly lower exercise endurance capacity (four hours) than rats fed the higher levels of dietary Mg (six hours). They also showed a mild macrocytic anemia. Consumption of a mineral water containing 85 ppm Mg prevented these signs of Mg deficiency. These results show that a reduction in exercise capacity can be an early effect of Mg deficiency. Hematologic changes that occurred with Mg deficiency such as macrocytic anemia may be responsible, at least in part, for the depression of exercise observed in these rats. These data are important because they illustrate the potential significance of even a marginal deprivation of dietary magnesium.

Levels of serum creatine kinase and myoglobin in women after two isometric exercise conditions

The purpose of this study was to measure serum creatine kinase (CK) activity and serum myoglobin (MG) concentrations in women after two unilateral isometric knee extension exercises. Forty maximal voluntary contractions (MVC) were held for 10 s, with either a 5 s (10:5) or 20 s 10:20 exercise (349.4 +/- 66.1 mU . ml-1) and 6 h and MG values were measured pre, 0, 3, 6, and 18 h post exercise. For CK, the highest post exercise values were observed at 6 h following the 10:20 exercise (349.4 +/- 66.1 mU . ml-1) and 6 h following the 10:5 exercise (194.1 +/- 18.6 mU . ml-1). For MG, the highest values were found 3 h after the 10:20 exercise (148.9 +/- 61.7 ng . ml-1) and 6 h after the 10:5 exercise (67.3 +/- 10.9 ng . ml-1). Serum CK and MG levels were significantly greater (p less than 0.01) after the 10:20 exercise bout. The data demonstrate that CK and MG values for women increase significantly after isometric exercise. Since greater tension levels were maintained during the 10:20 exercise it is hypothesized that increased serum CK and MG values after isometric exercise may be related to the tension generated by the contracting muscle.

Some physiological demands of a half-marathon race on recreational runners

The purpose of this study was to assess the physiological demands of a half-marathon race on a group of ten recreational runners (8 men and 2 women). The average running speed was 223.1 +/- 22.7 m.min-1 (mean +/- SD) for the group and this represented 79 +/- 5% VO2 max for these runners. There was a good correlation between VO2 max and performance time for the race (4 = -0.81; p less than 0.01) and an even better correlation between running speed equivalent to a blood lactate concentration of 4 mmol.l-1 and performance times (r = -0.877; p less than 0.01). The blood lactate concentration os 4 of the runners at the end of the race was 5.65 +/- 1.42 mmol.l-1 (mean +/- SD) and the estimated energy expenditure for the group was 6.22 M.J. While there was only a poor correlation between total energy expenditure and performance time for the race, the correlation coefficient was improved when the energy expenditure of each individual was expressed in KJ.kg-1 min-1 (r = 0.938; p less than 0.01).