The main finding of the present study is that diets resulting in a low systemic acid load (low PRAL) are associated with the attainment of a higher respiratory exchange ratio at the end of maximal-intensity treadmill exercise tests (~1.20 vs. 1.14). As a result, individuals who habitually consume low PRAL diets might achieve the RER ≥ 1.10 criterion for a “true” VO2
max at submaximal exercise intensities and VO2
max would be underestimated if the test stopped when RER reached 1.10. Alternatively, individuals consuming an acid-promoting diet, which is common in the United States (19
), would be less likely to achieve the true VO2
max criterion of RER ≥ 1.10. In our study, all of the 19 participants with negative (alkaline) PRAL values reached an RER ≥ 1.10, while 34 of 38 participants (89%) with positive (acid) PRAL values achieved an RER ≥ 1.10, although these frequencies were not statistically different (p=0.34). It is conceivable diets that result in a greater acid load than those observed in the present study, or other conditions that increase systemic acid load (e.g. medications) might have more extreme effects on RERmax
. Others have reported on the substantial heterogeneity of RERmax
values from graded exercise tests (17
), thereby bringing into question the use of RER ≥ 1.10 as a criterion for a valid or “true” VO2
max. However, the factors that contribute to this variability in RERmax
have been unknown. Our study demonstrates that habitual dietary patterns that influence systemic acid load account for 19% of the variability in RERmax
. Further studies are needed to determine if the PRAL-related effects on RERmax
are associated with alterations in pH, CO2
pressure, and bicarbonate levels in arterial blood.
PRAL, as a measure of dietary acid load, reflects the tendency for food to alter systemic pH, or the amount of acid that must be cleared or buffered in order to prevent pH changes. It is based on the absorption of specific nutrients and the capacity of these nutrients to produce anions and cations in circulation (18
), but is not necessarily related to the acidity of the food ingested. For example, lemon juice is acidic (pH ~ 2) outside of the body but it has a modestly low (alkaline) PRAL (-2.5 mEq/100 g) and therefore has an acid-load lowering effect on systemic pH (21
). Furthermore, PRAL does not necessarily reflect systemic pH; rather, it reflects the physiologic burden to maintain the optimal systemic pH, with the kidney being the main organ responsible for long-term pH homeostasis (13
). If the kidneys (and other pH control systems such as the bicarbonate buffering system) are able to maintain systemic pH in the face of a high systemic acid load, the urine will become acidic and optimal blood pH will be maintained. However, if the acid load exceeds the systemic capacity to excrete acid (for example, with compromised kidney function), systemic pH will decrease with an acid-promoting diet (11
It is biologically plausible that a low PRAL diet would permit greater non-metabolic CO2
production during maximal exercise, resulting in a greater RERmax
. In presence of a low systemic acid load (i.e. alkaline-promoting diet), circulating bicarbonate levels are elevated (7
), thereby increasing bicarbonate availability for acid buffering during high-intensity, acid-producing exercise (13
). This would allow for more H+
buffering and greater CO2
production during high-intensity exercise, thereby increasing maximal exercise VCO2
and RER (15
). In support of this proposition, Peronnet et al. (16
) demonstrated that during a ramp exercise test to exhaustion, bicarbonate infusion prevented the exercise-induced reductions in bicarbonate and pH and resulted in significantly higher RERmax
values compared to a control condition (RERmax
: 1.21 vs. 1.13). We did not measure circulating bicarbonate or blood pH in our preliminary study; although plasma CO2
content is partly reflective of plasma bicarbonate levels (9
), we did not observe an association between PRAL and plasma CO2
In contrast to maximal exercise RER values, RER values during submaximal exercise were not associated with PRAL. Bicarbonate buffering system activity is a major determinant of maximal exercise RER values; however, it has little or no influence on RER during submaximal exercise. Therefore, the finding that PRAL is associated with maximal but not submaximal exercise RER suggests dietary PRAL influences RERmax through effects on bicarbonate buffering.
An unexpected finding was that low-acid diets were associated with lower maximal heart rates. One possible explanation for this would be that, by chance, the subjects consuming a low PRAL diet did not give as much physical effort during maximal exercise. However, if this were the case, the RERmax
differences among PRAL tertiles would be underestimated. Indeed, after accounting for differences in HRmax
among PRAL tertiles, the differences in RERmax
became slightly larger. Another explanation is that variations in PRAL might have altered plasma electrolyte concentrations, which could have cardiac effects; however, we saw no evidence of associations between PRAL and plasma electrolytes. Lastly, it is possible that diet-related alterations in blood pH had direct or indirect (for example, involving sympathetic nervous system activity) cardiac effects. At rest, acidosis has been shown to decrease contractility (14
) and to either increase (6
) or decrease (1
) heart rate. However, to the best of our knowledge, the effects of acid-base alterations on maximal exercise cardiac function have not been studied.
It cannot be determined, based on our study, whether the effects of PRAL on RERmax
are attributable to acute or chronic effects because we assessed habitual diet and these dietary patterns were presumably practiced by the participants for many years. In one respect, it seems likely that the effect of PRAL on RERmax
would occur rapidly (i.e. in hours or days), as acute changes in diet have been shown to alter blood and urinary pH (5
). However, it is also possible that chronic exposure to mild acidosis/alkalosis has effects that develop over months, years, or decades. For example, acidosis in humans has clear effects on the growth hormone (GH)/insulin-like growth factor (IGF)-1 axis in humans (4
). Because the GH/IGF-1 system has major effects on the heart (e.g. effects on cardiac growth and development and myocardial substrate metabolism and contractility) and has been implicated in clinically relevant cardiac dysfunction (reviewed in (22
)), it is possible that chronic sub-clinical acidosis could affect cardiovascular function by altering long-term GH/IGF-1 function.
Because of the preliminary nature of this study, there are limitations. First, this was a cross-sectional study involving observation of habitual dietary intakes. Therefore, unidentified confounding factors might be responsible for some or all of the reported effects. To advance these preliminary cross-sectional findings, we are initiating an intervention study in which we are increasing PRAL (by using an isoenergetic diet rich in meats, cheeses and grains and low in fruits and vegetables) or decreasing PRAL (by using an isoenergetic diet rich in fruits and vegetable and low in meats, cheeses, and grains) to determine if 7 days of a low or high PRAL diet also alters RERmax. Another limitation is that we depended on estimates of dietary acid load from food diaries and nutrient analysis rather than direct measures of systemic pH or urinary measures of anion/cation content or pH. However, information from this study can be used to justify more advanced studies involving better measures of acid load and physiologic responses to exercise and randomization to controlled feeding interventions.
In conclusion, dietary qualities that result in a low systemic acid load (i.e. alkaline diets) are associated with the attainment of higher peak values for respiratory exchange ratio during maximal-intensity exercise testing. Such diets would typically be very rich in vegetables and fruits and low in meats, grains, and dairy. The implications of this finding are twofold. First, because maximal exercise RER ≥ 1.10 is commonly used as a criterion for determining whether a “true” VO2max has been attained during an exercise test, this finding brings into question the use of maximal RER as a true VO2max criterion. Secondly, although more preliminary, this finding also suggests that dietary acid load affects acid-base regulation during high-intensity exercise. In this context, future studies to investigate the possibility that dietary acid load affects physical performance during acidosis-inducing exercise are perhaps warranted.