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been reported to be about four times the basal metabolic rate, and similar values have been reported for other very demanding occupations, suggesting that this may be close to the upper limit of physical exercise that can be sustained on a long-term basis. In the short term, sporting activities can involve much higher levels of energy output: the world record for distance run in 24 h is 286 km, which requires an energy expenditure of about 80 MJ (20 000kcal). Such an effort, however, results in considerable depletion of the body's energy reserves, and must be followed by a period of recovery.

For athletes, very high levels of daily energy expenditure are more often a feature of training than of competition, with very high levels of energy intake reported in many sports. Measurements on runners in steady state with regard to training load and body mass show good relationships between energy intake and distance run. There are some competitive events that require high levels of activity to be sustained for many consecutive days, the most obvious examples being the multi-stage cycle tours, of which the most famous is the Tour de France. Measurements on some of the competitors have shown that they manage to maintain body weight in spite of a mean daily energy expenditure of 32 MJ (8000 kcal) sustained over a 3-week period. It was suggested that those cyclists who were unable to meet the daily energy requirement were unable to complete the race.

Measurements of oxygen uptake, heart rate, and other variables made after exercise show that the metabolic rate may remain elevated for at least 12 h and possibly up to 24 h if the exercise is prolonged and close to the maximum intensity that can be sustained. After more moderate exercise, the metabolic rate quickly returns to baseline level. Therefore, it seems likely that the athlete training at near to the maximum sustainable level, who already has a very high energy demand, will find this increased further by the elevation of postexercise metabolic rate: this will increase the difficulties that many of these athletes have in meeting their energy demand. The recreational exerciser, for whom the primary stimulus to exercise is often to control body mass or to reduce body fat content, will not benefit to any appreciable extent from this effect.

The control of food intake in relation to energy expenditure is not well understood, but it is clear that both short-term and long-term regulatory mechanisms exist. These allow the adult body weight to be maintained within fairly narrow limits in spite of wide variations in energy expenditure. It is also clear from the growing prevalence of obesity, that these control mechanisms are not perfect. The acute

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