Common variations in the composition of Western diets can alter systemic acid/base balance, as reflected by changes in blood pH of ~0.03 pH units, (Giannini et al., 1999; Yancy et al., 2007) and changes in urine pH of ~1.0 pH unit (Buclin et al., 2001). The effect of diet on systemic acid load can be quantified by calculating the potential renal acid load (PRAL) of the diet, which is based on dietary intakes of protein and mineral ions (Remer and Manz, 1995). In general, fruits and vegetables promote systemic alkalinity, while meat, grains, and cheese promote systemic acidity. We previously proposed (Niekamp et al., 2012) that a low-PRAL (alkaline promoting) diet increases circulating bicarbonate availability and as a result, increases the capacity to produce "non-metabolic" C[O.sub .2] from the bicarbonate buffering system during maximal exercise. Because non-metabolic C[O.sub .2] production is responsible for increasing the respiratory exchange ratio (RER) above the 1.00 during maximal intensity exercise (Robergs et al., 2004), this would be expected to increase maximal exercise RER. Indeed, in a cross-sectional study (Niekamp et al., 2012), we found that men and women who habitually consumed a low-PRAL diet had higher RER values at the end of a progressive incremental treadmill exercise test to exhaustion than those who were consuming a high-PRAL (acid promoting) diet (1.21 [+ or -] 0.01 vs. 1.15 [+ or -] 0.01 RER units, p
If dietary acid load does affect acid-base regulation and serum bicarbonate levels during exercise, it is also possible that dietary PRAL could affect the development of acidosis during high-intensity exercise and consequently alter high-intensity exercise performance (for comprehensive review on the effects of acidosis on muscle fatigue, see reference (Cairns, 2006)). More specifically, a low-PRAL diet would be expected to improve anaerobic exercise performance by inducing alkalosis (higher blood pH) and increasing bicarbonate availability. Such effects have been well documented in response to sodium bicarbonate loading (Carr et al., 2011a; McNaughton et al., 2008; Peart et al., 2012). However, to our knowledge, no studies have evaluated the effect of broad dietary patterns, such as high- and low-PRAL diets, for ergogenic effects during high-intensity anaerobic exercise.
The purpose of the present study was to perform an intervention study to confirm and expand on our previous cross-sectional findings (Niekamp et al., 2012). The primary aim was to evaluate the hypothesis that a short term (4-9 days) low-PRAL diet results in a higher respiratory exchange ratio during maximal exercise as compared to that after a high-PRAL diet. A second aim of the present study was to determine if a low-PRAL diet improves anaerobic exercise performance. Finally, as an exploratory aim, we sought to determine if dietary PRAL affects respiratory exchange ratio during submaximal exercise.
Participants and screening
Men and women (n = 10), aged 18-60 years were recruited from the Saint Louis metropolitan area. Volunteers completed a medical history and medications questionnaire, which was used along with criteria from the American College of Sports Medicine, to classify each individual as low, moderate, or high risk for medical complications during exercise (American College of Sports Medicine, 2014). Volunteers were excluded if they were at moderate or high risk for medical complications. Both trained and untrained individuals were eligible to participate, provided that they were willing to undergo maximal exercise testing and undergo substantial dietary alterations for several days with the goal of changing dietary PRAL. The study was reviewed and approved by Saint Louis University Institutional Review Board and all participants provided informed written consent to participate in the study.
The study was a cross-over trial in which participants underwent exercise testing on two occasions, once after following a low-PRAL diet and once after following a high-PRAL diet. The intervention sequence was randomized and counterbalanced such that half of the participants underwent the low-PRAL intervention first and the others underwent the high-PRAL intervention first.
For each of the dietary interventions, the study dietitian provided the subjects with specific instructions on how to modify their habitual diets to achieve a low- or high-PRAL diet. The study dietitian was in contact with the participants (via telephone or email) every day during the dietary interventions to encourage compliance and to provide specific food suggestions as needed. The general strategy used for the low-PRAL diet was to increase the consumption of alkaline-promoting foods such as fruits and vegetables and to reduce the consumption of acid-promoting foods such as meats, cheeses, and grains (Welch et al., 2008). More specifically, participants were instructed to consume 6-8 cups of vegetables and >4 servings of fruit each day. Because there is a tendency for lower energy intake with diets that are rich in fruits and vegetables, such as the low-PRAL diet, participants were instructed to eat frequently and consume energy dense foods during the low-PRAL trial, such as starchy vegetables (e.g. sweet potatoes), dried fruits (e.g. dates and raisins), and plant sources of fat (e.g. avocado, coconut, nuts, seeds). Foods with moderate PRAL values (e.g. legumes, yogurt, egg whites, quinoa) were allowed and were used to ensure that energy and macronutrient intakes were adequate. The participants were also advised to minimize the consumption of all meats, cheeses and common grains (most of which are high-PRAL) during the low-PRAL diet. During the high-PRAL diet, participants were instructed to consume at least 3-4 servings of common grains (e.g. wheat, corn, and oats), 3 servings of meat, and 3 servings of cheese (especially hard cheeses such as parmesan) each day while minimizing the intakes of fruits and vegetables. Moderate PRAL foods were allowed as desired as long as it did not displace highPRAL foods from the diet. In general, the high-PRAL diet required less intensive counseling from the dietitian because it closely resembled the baseline diet of the participants.
The goal during the low PRAL diet was to achieve a dietary PRAL of [less than or equal to] -1 mEq/d and during the high-PRAL diet the goal was PRAL [greater than or equal to] 15 mEq/d; these cut points were based on PRAL values of the high and low PRAL tertiles that were observed in our previous cross-sectional study on 57 middle-aged men and women (Niekamp et al., 2012). Food portions were adjusted as needed during both dietary interventions to maintain energy balance as evidenced by body weight changes. The diets were a minimum of 4 days in duration but were maintained as long as needed to achieve a fasted morning urine pH of [greater than or equal to] 7.0 in the low-PRAL trial and [less than or equal to] 6.0 in the high-PRAL trial, as described below. Participants were instructed to avoid antacid supplements and medications, which have their own effects on systemic acid/base status.
Dietary assessment and calculation of PRAL
Participants recorded 3-day food diaries at baseline and during the dietary interventions. Nutrient analysis of the diaries was performed with Food Processor SQL software (version 10.6.0 ESHA Research, Salem, OR). Dietary PRAL was calculated based on the following equation:
PRAL = P x 0.0366 + Pro x 0.4888 - [K x 0.0205 + Ca x 0.0125 + Mg x 0.0263]
Where the units for PRAL are mEq/d and P is phosphorous in mg/d, Pro is protein in g/d, K is potassium in mg/d, Ca is calcium in mg/d, and Mg is magnesium in mg/d (Remer & Manz, 1995).
Urine pH was measured to the nearest 0.5 pH unit with pH strips (pH 5.0-10.0, catalog #9588, ColorpHast, EMD Chemicals, Inc., Gibbstown, NJ) and used to inform decisions about the duration of the diets and to confirm that the diets adequately altered dietary acid load. The goal during the low-PRAL intervention was to achieve a urine pH of [greater than or equal to] 7.0 and during the high-PRAL intervention the goal was [less than or equal to] 6.0 (Welch et al., 2008). Fasted morning urine pH was self-monitored by the participants every morning during the intervention; if urine pH was not within the desired range, subjects were asked to follow the dietary intervention for additional days as needed and to achieve the urine...