On the significance of the respiratory exchange ratio after different diets during exercise in man

Abstract

Total respiratory exchange ratio (R) was compared to the respiratory exchange ratio over the legs (RQL) during exercise after different diets, to evaluate if R (which represents a mean for the whole body) can be used to estimate the relative proportions of fat and carbohydrate oxidation in exercising muscle. One important prerequisite for this is a steady state acid base balance, 7 subjects were studied at rest and during the later part of a 25 min exercise (65% of Vo2 max) on two occasions, the first preceded by a fat rich diet and the second by a carbohydrate rich diet. Oxygen uptake, R and arterial-femoral venous differences for [O2], [CO2], PCO2 and pH and arterial concentrations for lactate and beta-hydroxybutyrate were measured. Respiratory exchange ratio over the exercising legs (RQL) and ventilation/oxygen uptake were calculated. Arterial pH, PCO2, lactate and beta-hydroxybutyrate as well as specific ventilation attained steady levels during the later part of exercise after both diets. Although arterial lactate and beta-hydroxybutyrate differed between the diets, the arterial pH and specific ventilation were the same. Both R and RQL were higher after the carbohydrate than after the fat diet and there was no systematic difference between R and RQL. Therefore, it seems likely that R estimates the proportion of fat and carbohydrate oxidation in skeletal muscle during submaximal exercise after extreme diets.

Improved Gross Efficiency during Long Duration Submaximal Cycling Following a Short-term High Carbohydrate Diet

“…The results presented indicate that a high carbohydrate intake in the three days prior to an exercise assessment increased the efficiency values of trained cyclists by ~4% compared to moderate carbohydrate conditions. These results support the data presented by Jansson [20], Neufer et al, [27] and Dumke et al, [8] who reported alterations in exercise efficiency scores with altered carbohydrate status. As these previous studies utilizsed time to exhaustion tests, their data isare limited for the purposes of calculating GE due to fatigue related reductions in the exercise intensity as the trials progressed.…”

“…Unfortunately, these early papers did not maintain the exercise intensity during their experimental protocols, and as time elapsed and fatigue ensued, the work rates were altered between trials. Altering work rates has been shown to influence the efficiency values obtained [6,11,30], therefore limiting the application of both Jansson and Neufer et al, [20,27]. Additionally, the later work of Dumke et al, [8] alluded to altered efficiency values with nutritional intervention but likewise did not maintain exercise intensity across different conditions.…”

“…This is somewhat problematic because early papers in this field have reported differences in cycling efficiency with altered carbohydrate intake during the period leading up to assessment [20,27]. Unfortunately, these early papers did not maintain the exercise intensity during their experimental protocols, and as time elapsed and fatigue ensued, the work rates were altered between trials.…”

Effects of ingested fluids on exercise capacity and on cardiovascular and metabolic responses to prolonged exercise in man

“…The rate of carbohydrate oxidation was calculated from V0, and the respiratory exchange ratio (R). with the assumptions that the R accurately reflects the respiratory quotient and that no protein oxidation was occurring (Jansson, 1982).…”

Nutritional aspects of exercise-induced fatigue

“…Furthermore these studies have also shown that during exercise of increasing intensity there is an ever increasing contribution from carbohydrate to energy metabolism. Recent studies, using more sophisticated techniques for studying metabolism than the respiratory exchange ratios, have confirmed and extended these basic guidelines on metabolism during exercise (Jansson, 1982).…”