The acidity or alkalinity of foods may have a profound effect on retaining muscle, health and endurance performance.

By Kris Walker MD

Intro: Acid-base homeostasis in humans is critical to health.  An important property of blood is its degree of acidity or alkalinity.  Body acidity increases when the level of acidic compounds in the body rises (through increased intake or production, or decreased elimination) or when the level of basic (alkaline) compounds in the body falls (through decreased intake or production, or increased elimination).   Body alkalinity increases with the reverse of these processes.  The body’s balance between acidity and alkalinity is referred to as acid-base balance.  The acidity or alkalinity of any solution, including blood, is indicated on the pH scale.

PH-review: Here is a quick review of some acid-base concepts.  Acids generally taste sour and bases generally taste bitter.  Both strong acids and strong bases are dangerous and can burn your skin.  There are a few different ways to define an acid or a base.  In an aqueous (liquid) solution, an acid generates or donates a hydrogen ion (H+) and a base accepts a hydrogen ion (H+) or donates a hydroxy ion (OH-).  A salt is formed when an acid and a base are mixed and the acid releases H+ ions and the base releases OH- ions.  Example:  HCl (acid)+ NaOH(base) = NaCl(salt) + H2O(water).  This is called a neutralization reaction.  The pH of the salt depends on the strength of the original acid and base being mixed.  pH is the measure of the strength of the acid or base.  The measurement ranges from 0 to 14, with 0 being the most acidic and 14 being the most basic and 7 being neutral.  A pH over 7 is considered basic or alkalotic and under 7 is acidic.  Strong acids have lower pHs and weaker acids are closer to 7, while strong bases have very high pHs and weaker ones are closer to neutral 7.

PH regulation: The blood’s acid-base balance is precisely controlled in the body, because even a minor deviation from the normal range can severely affect many organs.  The body uses 3 different mechanisms to control acid-base balance:  through the respiratory system, chemical buffers, and the excretion of acid by the kidneys.    pH is maintained in the blood between 7.38 and 7.42.  Muscle pH is lower, about 7.0.  The first mechanism that the body has to maintain pH is through buffers..  This is very quick, taking a few seconds.  Bicarbonate or citrate (bases) accepts a proton to form a neutral salt.  The second way is through pulmonary ventilation, which excretes H+ through increased ventilation.  H + HCO3  forms H2CO3 which forms H2O (water) and CO2 (carbon dioxide), which is eliminated through respiration. This takes a few minutes.  The third mechanism is long-term and is through the kidneys ability to excrete acid.  The kidneys cannot excrete urine more acidic than 4.4, so hydrogen acceptors, or buffers, are required.  The main buffer is ammonia, NH3, which accepts a hydrogen molecule and is excreted as NH4, or ammonium.(14)

Protein’s effect on PH: With aging comes a loss of lean tissue mass, both bone and muscle, approximately 2 kilograms per decade after age 50.(4)   Protein is an essential element of muscle and bone and severe protein deficiency causes muscle wasting.  Studies show a positive association between protein intake and lean body mass.  Diets high in  protein, however, cause a net acid load in the body and an increased secretion of nitrogen, which can be an indication of muscle wasting, that is, high protein diets are acidogenic and chronic metabolic acidosis stimulates muscle breakdown.  Increased acidosis with a stable protein intake causes an increased renal acid load.(3)  Protein intake does increase the ability of the kidney to excrete ammonium to regulate acid-base balance, but the increased acidosis increases muscle and bone breakdown and may lead to a loss of lean body mass.(11)  A higher protein diet leads to increased urinary nitrogen partly from muscle breakdown and partly from the increased ability of the kidney to buffer acid as NH4.   As we age, kidney function declines.  Glomerular filtration is reuced by 50% from age 20 to age 80 and most of this decline occurs after age 50.  Day to day stability of acid-base status is dependent on the kidneys’ ability to excrete acid.   Increased age is also associated with an increase in metabolic acidosis, with an increase in serum H+ and a decrease in serum bicarbonate and a decrease in PCO2 because of the respiratory adaptations to this acidosis.  Serum H+ is 6-7% higher and serum HCO3 is 12-16% lower in an 80 year old than in a 20 year old.(7)  Acidosis increases with age and with renal function decline.   Proposed and proven effects of deleterious effects of even mild acidosis include increased bone turnover and loss, muscle wasting and loss of lean body mass, and nephrolithiasis (kidney stones). Manipulation of acid-base balance can be done with supplementation and also with diet; however, reducing dietary acid load is not sufficient to prevent the age-related worsening acidosis and the resulting loss of bone mass, muscle breakdown, increased incidence of kidney stones, and possible decrease in renal function.  However, maybe eating a lower acid-producing diet or supplementation with alkaline agents could slow age-related acidogenesis.  (15)

Decreasing acidosis  may decrease these proposed deleterious effects, as well as increase athletic performance, especially in high-intensity efforts.  Sodium bicarbonate and sodium citrate increase the blood and muscles alkalinity and have been shown to be ergogenic aids, increasing athletic performance in very high intensity efforts.  A dose of 0.3 g/kg NaHCO3 or 0.5 g/kg sodium citrate result in a higher serum pH which may result in increased transfer of hydrogen ions out of working muscles and increased average and peak power output and increased total work.  (20)

PH of Foods: Diet can affect pH balance and the American diet, rich in animal products and grains, produces a net acid load.  With this acid load, renal acid excretion is increased to a new steady state and this decreases plama bicarbonate and increases acidity.(6) The determination of the acid producing effect of food depends on many factors.  First of all the differential absorption of salts by the intestine will determine the effect  on pH.  For instance, with MgCl2, one third of the magnesioum is absorbed in the intestine, but 95% of the chloride is absorbed, so ingestion of magnesium chloride results in an excess of Cl2-.  The primary cation to bind these anions is sodium, which comes from pancreatic release of sodium bicarbonate (NaHCO3).  This results in a net loss of NaHCO3 and a resulting acid load.  The liver also plays a part in acid-base status.  It oxidizes sulfur-containing amino acids and organic acids and produces hydrogen and alkali ions.  It also uses amino acids in the bloodstream to make glutamine.  The kidneys use this glutamine to make ammonia which accepts H+ and is excreted as ammonium. (16,4)

To determine the acid/base effect of foods, several methods have been proposed.  Measuring 24 hour urine pH is one way, but it is not practical for large populations.  A model based on only 4 nutrients has been shown to predict renal acid load in children.  Protein, phosphate, potassium, and magnesium were used to calculate potential renal acid load and correlated well with urine pH.  A simpler method has been proposed to estimate dietary acid load by measuring the ratio of protein to potassium content of food.  Remer and Manz tested foods and their effect on urine pH and listed foods according to their potential renal acid load (PRAL).  Fats and oils are neutral.  Fish, meats, eggs, grains, nuts, and dairy products have positive PRALs.  Fruits, vegetables, and to a lesser extent, beverages, have negative PRALs.(19)  See attached table, adapted from their article.  Substituting just a few lower acid producing foods or base-forming foods for higher acid-forming foods results in a lower PRAL which increases urine pH.  Another study analyzed 20 diets with protein content between 39 to 193 grams per day and potassium content between 40 to133 mEq per day.  The ratio of protein to potassium content varied over a 5-fold range, from 0.45 to 2.21 and these ratios correlated very well with the data presented by Remer and Manz. This suggests that acid load can be predicted by simply determining the protein and potassium content of the diet.(8)

Acidbase PRAL of selected foods

Acidbase average PRAL of food groups

Recommendations: Supplementation also can decrease the negative effects of metabolic acidosis.  Potassium bicarbonate has been shown to decrease urinary nitrogen in subjects age 54-80, implying less muscle breakdown. (3)  Higher urinary potassium, implying a higher intake of potassium is associated with an increased percentage of lean body mass in post-menopausal women.  A potassium intake of 134 mmol per day is associated with a lean body mass 1.64 kg higher.than those who took half that dose.  (4)  At least one study has been done on a plant-based supplement which has been marketed to increase alkalinity.  This supplement, “Green+” was shown to increase urine pH when taken as directed for at least one week.(2)

Summary: In summary, acid-base balance is critical to health and may have important implications in athletic performance.  The standard American diet produces a mild metabolic acidosis and this seems to worsen with age and renal function decline.  Correction of this metabolic acidosis may have positive effects on athletic performance:  increased availability of buffers which could improve performance in high intensity efforts and increased lean body mass through decreased muscle and bone breakdown.  The reduction in acidosis can be accomplished through supplementation–potassium bicarbonate, sodium bicarbonate, sodium citrate, or commercial plant-based supplements have been shown to increase urinary pH and decrease acidosis.  Dietary changes are also effective.  Substituting less acid-producing foods(fruits and vegetables) for higher acid-producing foods (meats and cheese) increases urine pH and reduces acidosis.  Simply increasing intake of potassium-rich foods seems to help maintain lean body mass.  Fruits and vegetables are not only nutritious, they may be ergogenic aids.


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