The effects of dietary manipulation upon the respiratory exchange ratio as a predictor of maximum oxygen uptake during fixed term maximal incremental exercise in man

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Summary

This study examined the effects of dietary manipulation upon the respiratory exchange ratio ( \(R = \dot V_{{\text{CO}}_{\text{2}} } /\dot V_{{\text{O}}_{\text{2}} } \) ) as a predictor of maximum oxygen uptake ( \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) ). Seven healthy males performed fixed term maximal incremental treadmill exercise after an overnight fast on three separate occasions. The first test took place after the subjects had consumed their normal mixed diet (45±5% carbohydrate (CHO)) for a period of three days. This test protocol was then repeated after three days of a low CHO diet (3±2% CHO), and again after three days of a high CHO diet (61±5% CHO). Respiratory gases were continuously monitored during each test using an online system. No significant changes in mean exercise oxygen uptake ( \(\dot V_{{\text{O}}_{\text{2}} } \) ), \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) or maximum functional heart rate (FHRmax) were found between tests. Mean exercise carbon dioxide output ( \(\dot V_{{\text{CO}}_{\text{2}} } \) ) and R were significantly lower than normal after the low CHO diet (bothp<0.001) and significantly higher than normal after the high CHO diet (bothp<0.05). Moreover, compared with the normal CHO diet, the R-time relationship during exercise was at all times significantly (p<0.001) shifted to the right after the low CHO diet, and shifted to the left, being significantly so (p<0.05) over the final 5 min of exercise, after the high CHO diet. As a result, predictions of \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) based on the R-time relationship were similar to recorded \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) after the normal CHO dietary condition (-1.5±1.9%), but higher after the low CHO diet (+14.8±3.9%,p<0.001) and lower after the high CHO diet (−7.0±4.5%,p<0.01). These results indicate that dietary manipulation can significantly affect respiratory gas exchanges during fixed term maximal incremental exercise, and by doing so can significantly influence predictions of \(\dot V_{{\text{O}}_{{\text{2}} {\text{max}}} } \) based on R.