Another post by our expert registered dietitian, Susan McFarlane. This time, Susan tackles PRAL centered/alkaline diets and the science behind them.
Potential Renal Acid Load (PRAL) Alkalinity
Although it’s popularity has waned in favour of more recent dietary trends, the alkaline diet continues to be touted by many as the key to preventing disease and improving health. According to proponents of the diet, eating acidifying foods, while failing to combine acid-alkaline foods in the right combination, will rob your body of nutrients and lead to a host of diseases ranging from osteoporosis to heart disease and cancer (1).
With any diet, it’s necessary to look beyond the smoke and mirrors to determine if there is sufficient scientific evidence to support its health claims. So, what does the scientific community have to say about the alkaline diet and its supposed benefits?
Chemistry 101 and the PRAL
Before going any further, we need to pause for a brief chemistry lesson on the pH scale in relation to acidity and alkalinity. Food, and other substances, can be classified as “acidic”, “alkaline”, or “neutral” based on their positioning on the following scale:
BioNinja. “The pH Scale”. BioNinja, n.d., http://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/24-proteins/the-ph-scale.html
The Potential Renal Acid Load or “PRAL” of foods was calculated more than 100 years ago as a way to evaluate a food’s acidifying or alkalinizing potential and is represented by the following equation (2):
PRAL = 0.49 x protein (g/day) + 0.037 x phosphorus (mg/d) – 0.021 x potassium (mg/day) – 0.026 magnesium (mg/day) – 0.013 x calcium (mg/day)
A food with a positive PRAL is considered acid-producing, while a negative PRAL is suggestive of a food that is more basic. In general, fruits, vegetables, potatoes, wine, and soda water are considered alkaline, while grains, meats, dairy, fish, and beer are considered acidic (2).
More recently, the PRAL has been criticized for not considering our bodies ability to neutralize acids through normal body systems (3). In addition, the formula classifies phosphate as acidic and sodium as basic, despite evidence showing the contrary (4,5).
The Alkaline Diet and Human Disease
What’s most important to realize is that we cannot change the pH of our blood simply by eating foods that are acidic or alkaline. In fact, if your blood pH changes even slightly in either direction, metabolic or respiratory acidosis/alkalosis ensues, which damages organs and, if left untreated, can be fatal. However, these conditions only occur if your body’s acid-base buffering organs, the kidneys and lungs become damaged. That said, there is another buffering system with a seemingly unlimited ability to neutralize acid – our bones. As such, a key area of interest is knowing if a more acidic diet will lead to an increase in the loss of bone.
The results of a well-designed 2009 meta-analysis (6), along with two additional systematic reviews (7,8) concluded that diets with a high acid load do not reduce calcium balance or the retention of whole body calcium, despite an increase in the urinary excretion of calcium. This result does not come as a surprise given that osteoporosis is a multi-factorial disease manifested in the interplay between diet, physical activity, environment, and genetics.
Nonetheless, consuming a more alkaline diet, or a diet that is rich in fruits and vegetables and low in animal protein, can improve the overall nutrient density of your diet, your day-to-day health, and reduce your risk for common lifestyle diseases, including diabetes, heart disease, hypertension, and obesity.
- The Acid Alkaline Association. n.d. Accessed 2018 Feb 23 from: http://www.acidalkalinediet.net/acid-alkaline-diet.php
- Remer T, Manz F. Estimation of the renal net acid excretion by adults consuming diets containing variable amounts of protein. Am J Clin Nutr. 1994 Jun;59(6):1356-61. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/8198060.
- Fenton T, Firus S, Naqvi N, Williams R. Knowledge Pathway – Diet Composition: Alkaline Diets. In: Practice-based Evidence in Nutrition [PEN]. 2016. Available from: http://www.pennutrition.com/KnowledgePathway.aspx?kpid=17944. Access only by subscription.
- Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis. Nutr J 2009;8:41. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/19754972
- Dietitians of Canada. What is the optimal ratio of sodium intake to potassium intake for bone health? In: Practice-based Evidence in Nutrition [PEN]. 2005. Available from: http://www.pennutrition.com/home.asp. Access only by subscription.
- Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acidash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009;24(11):1835-40. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/19419322.
- Fenton TR, Tough SC, Lyon AW, Eliasziw M, Hanley DA. Causal assessment of dietary acid load and bone disease: a systematic review & meta-analysis applying Hill’s epidemiologic criteria for causality. Nutr J. 2011 Apr 30;10:41. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/21529374.
- Calvez J, Poupin N, Chesneau C, Lassale C, Tomé D. Protein intake, calcium balance and health consequences. Eur J Clin Nutr. 2012 Mar;66(3):281-95. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/22127335.