How endurance training helps women keep their weight down

Endurance training leads to metabolic changes which favour leanness in women. That is the main conclusion of a Canadian study which compared the thermogenic (energy-burning) response to food and other metabolic changes after eating in 12 endurance-trained and 13 untrained women.

The trained subjects were distance runners or triathletes, who had been training for at least two years and had taken part in provincial and national competitions. Crucially they and their untrained counterparts, recruited through advertisements, were matched for body size to facilitate analysis of the effect of body composition on metabolism. Energy expenditure and substrate (glucose and fatty acid) oxidation were measured before and for six hours after an oral test meal and then again after the same meal administered directly to the stomach via a nasogastric tube. The point of delivering the food in these two different ways was to calculate the difference between the ‘obligatory’ component of the thermogenic response to food (TRF) – the increase in energy expenditure arising directly from the process of digestion, absorption and storage of nutrients – and the ‘facultative’ component of TRF which is related to stimulation of the mouth and throat. The fact that previous research in this area has been inconclusive may be due to the fact that these two components of energy consumption after eating are not routinely separated.

The researchers point out: ‘Understanding the relationships between physical training and TRF is important for the evaluation of long-term benefits of exercise for weight regulation. Although TRF represents a small proportion of 24-hour energy expenditure, a permanent increase in this component of the energy balance equation is likely to be significant. The objective of this study was, therefore, to measure both components of the TRF in trained and untrained women and to calculate the relationship of these measurements to VO2 max and body fat.’

The two groups were matched for age, weight, height and BMI. Body fat, fat mass and resting heart rate were predictably lower in trained subjects, while fat-free mass, resting energy expenditure and VO2 max were predictably higher. A three-day dietary recall showed higher total energy intake but a lower proportion of energy derived from fats in the trained subjects.

Testing after the oral and tube-fed meals revealed the following findings:

* Both groups exhibited significantly lower energy expenditure after tube feeding – an expected finding since the facultative component had been removed;
* Trained subjects had higher total TRF (+22%) and higher OTRF (+32%) than untrained subjects, but the facultative component of TRF was similar in both groups; * Fatty acid oxidation was significantly higher in trained subjects after the oral meal but not after the tube-fed meal;
* OTRF was significantly related to VO2 max but not to percentage body fat;
* Resting metabolic rates were higher in the trained subjects even after adjustment for fat-free mass.

‘In conclusion’, state the researchers, ‘this study shows that trained women have higher resting and postprandial energy expenditure than untrained women of similar body size. The greater TRF in trained subjects is related to a higher cost of nutrient metabolism, and this energy expenditure is correlated with VO2 max. Fat oxidation is also higher in trained women after an overnight fast and postprandially. These findings have important implications for understanding the relationships between physical fitness, body weight regulation and the efficiency of nutrient digestion, absorption and storage.’

Am J Physiol Endocrinol Metab 279: E601-E607, 2000

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