Acidity and alkalinity are measured on the pH scale, which ranges from 1 to 14, with 1 being the most acidic, 14 the most alkaline. Most foods have a pH below the neutral measurement of 7 but are further categorized as being low- or high-acid foods depending on whether their pH is above or below 4.6.
The pH level of food and its impact on acid-base balance plays an important role in several areas of nutrition, including food safety. Clostridium botulinum is a bacterium that naturally occurs in soil and is found on the surfaces of most fresh produce. Washing produce, removing peels and cooking items helps to reduce the number of botulinum spores, keeping the numbers low enough to reduce the likelihood of illness. However, some botulinum spores may remain and can reproduce rapidly in certain conditions, such as moist, low-acid foods – especially when sealed in a low-oxygen environment. Canned foods provide an ideal environment for these spores, so certain steps should be taken when canning at home. When undertaking home canning, it's important to consider the pH of foods as it will affect recommended methods of canning, time and temperature controls, as well as additives that may be required to make the canned foods more acidic.
In addition to food safety, pH can play a role in medical nutrition therapy. While controversy surrounds some of the claims made regarding acid-base balance and its potential impact on health, the kidneys work hard to prevent acidosis or alkalosis and to maintain the body's homeostasis. The renal acid load of daily food choices may influence the pH content of urine; however, changes to the body's serum pH are life-threatening events, so the body goes to extreme measures to maintain the pH within a narrow range (between 7.35 and 7.45). Therefore, pH levels of the blood do not readily change based on daily dietary intake when the kidneys are functioning normally.
The importance of this balance and the role diet plays can be seen when determining the potential renal acid load (PRAL) for patients with chronic kidney disease (CKD). PRAL is calculated based on dietary intake, and if the value is positive, then it is considered to be acid-forming, while the opposite is true for a negative value, which would be considered base-forming. Given that most foods have a pH below 7, it may come as a surprise that many foods that are commonly described as being acidic, such as citrus fruits, do not create a positive PRAL for the kidneys. When it comes to dietary contributions, meat, fish, poultry and cheese are the primary acid-producing foods. Foods such as vegetables and fruits tend to have a more alkalinizing effect. This can be broken down further to say phosphorus and protein (or even specific amino acids) have a higher PRAL and release acid, whereas calcium, magnesium and potassium are alkalinizing and have a negative PRAL.
The 2020 update to the KDOQI Clinical Practice Guidelines for Nutrition in Chronic Kidney Disease (CKD), which resulted from a collaboration between the Academy and the National Kidney Foundation addresses how to manage acid load through increased fruit and vegetable intake for patients with CKD (Stages 1-4). The guidelines also describe the effect vegetable-based protein diets can have on reducing acid load; although more evidence is needed in this population.
In addition to the kidneys, bone also has a protective role in maintaining systemic pH due to its capacity to provide minerals that act as a buffering system during metabolic acidosis. However, this is thought to occur at the bone's expense resulting in the inability to form new bone and the potential for increased risk of fracture in patients with CKD.
Another condition influenced by pH is nephrolithiasis, better known as kidney stones. The pH of urine can contribute to the formation, as well as treatment, of several types of kidney stones. For example, the development of uric acid stones may be attributed to "an abnormally low urinary pH" and a consequence of metabolic syndrome, especially in males. Increasing the urine's alkalinity by eating more plant-based foods and fewer sources of animal protein can aid in its management.
While studies in this area are ongoing as researchers continue to investigate the scientific basis of the relationship between diet and acid-base homeostasis, the benefits of consuming more fruits and vegetables, a message consistent with the 2020 - 2025 Dietary Guidelines for Americans, are well supported by a plethora of evidence.
- McGlynn, W. The Importance of Food pH in Commercial Canning Operations. Oklahoma University State Extension. Published July 2016. Accessed June 2022.
- USDA Complete Guide to Home Canning, 2015 revision. Accessed June 1, 2022.
- Noce, A., Marrone, G., Wilson Jones, G., Di Lauro, M., Pietroboni Zaitseva, A., Ramadori, L., Celotto, R., Mitterhofer, A. P., & Di Daniele, N. (2021). Nutritional Approaches for the Management of Metabolic Acidosis in Chronic Kidney Disease. Nutrients, 13(8), 2534. https://doi.org/10.3390/nu13082534
- Escott-Stump S. Section 16: Kidney Disorders. In: Nutrition & Diagnosis-Related Care, Ninth Edition. Chicago, IL: Academy of Nutrition and Dietetics; 2022:1140-1188.
- Ikizler TA, Burrowes JD, Byham-Gray LD, et al; KDOQI Nutrition in CKD Guideline Work Group. KDOQI clinical practice guideline for nutrition in CKD: 2020 update. Am J Kidney Dis. 2020;76(3)(suppl 1):S1-S107.
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