A maximal-exercise RER (RER(max) ≥ 1.10 is commonly used as a criterion to determine whether a "true" maximal oxygen uptake (V˙O(2max)) has been attained during maximal-effort exercise testing. Because RER(max) is heavily influenced by CO2 production from acid buffering during maximal exercise, we postulated that dietary acid load, which affects acid-base regulation, might contribute to variability in RER(max). PURPOSE: The study's purpose was to determine whether a habitual dietary intake that promotes systemic alkalinity results in higher RER(max) during V˙O(2max) testing. METHODS: Sedentary men and women (47-63 yr, n = 57) with no evidence of cardiovascular disease underwent maximal graded treadmill exercise tests. V˙O(2max) and RER(max) were measured with indirect calorimetry. Habitual diet was assessed for its long-term effect on systemic acid-base status by performing nutrient analysis of food diaries and using this information to calculate the potential renal acid load (PRAL). Participants were grouped into tertiles on the basis of PRAL. RESULTS: The lowest PRAL tertile (alkaline PRAL) had higher RERmax values (1.21 ± 0.01, P ≤ 0.05) than the middle PRAL tertile (1.17 ± 0.01) and highest PRAL tertile (1.15 ± 0.01). There were no significant differences (all P ≥ 0.30) among PRAL tertiles for RER at submaximal exercise intensities of 70%, 80%, or 90% V˙O2max. After controlling for age, sex, V˙O(2max), and HRmax, regression analysis demonstrated that 19% of the variability in RER(max) was attributed to PRAL (r = -0.43, P = 0.001). Unexpectedly, HRmax was lower (P ≤ 0.05) in the low PRAL tertile (164 ± 3 beats·min⁻¹) versus the highest PRAL tertile (173 ± 3 beats·min⁻¹). CONCLUSIONS: These results suggest that individuals on a diet that promotes systemic alkalinity may more easily achieve the RER(max) criterion of ≥ 1.10, which might lead to false-positive conclusions about achieving maximal effort and V˙O(2max) during graded exercise testing.
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