Acid ash hypothesis

The acid-ash hypothesis is a medical hypothesis which suggests that excessively acidic diets may result in a number of identifiable health effects, including an increased risk of osteoporosis.[1] It has received some attention in the lay community, and has been used to support the fad diet known as the Alkaline diet.[1] According to the hypothesis, acid ash is produced by meat, poultry, cheese, fish, eggs, and grains. Alkaline ash is produced by fruits and vegetables, except cranberries, prunes and plums. Since the acid or alkaline ash designation is based on the residue left on combustion rather than the acidity of the food, foods such as citrus fruits that are generally considered acidic are actually considered alkaline producing in this diet.[2]

Research

Recent systematic reviews have been published which have methodically analyzed the weight of available scientific evidence, and have found no significant evidence to support the acid-ash hypothesis in regard to prevention of osteoporosis. A meta-analysis of studies on the effect of dietary phosphate intake contradicted the expected results under the acid-ash hypothesis with respect to calcium in the urine and bone metabolism. This result suggests use of this diet to prevent calcium loss from bone is not justified.[3] Other meta-analyses which have investigated the effect of total dietary acid intake have also found no evidence that acid intake increases the risk for osteoporosis as would be expected under the acid-ash hypothesis.[4][5] A review looked at the effects of dairy product intake, which have been hypothesized to increase the acid load of the body through phosphate and protein components. This review found no significant evidence suggesting dairy product intake causes acidosis or increases risk for osteoporosis.[6] A meta-analysis on the effects of alkaline potassium salts on calcium metabolism and bone health found that supplementation with alkaline potassium salts reduces loss of calcium in urine and reduces acid secretion.[7]

Relation to alkaline diets
Main article: Alkaline diet

The acid ash hypothesis suggests that diets high in "acid ash" (acid producing) elements would cause the body to try to buffer (or counteract) any additional acid load in the body by breaking down bone, leading to weaker bones and increased risk for osteoporosis. Consequently, "alkaline ash" (alkaline producing) elements would hypothetically decrease the risk of osteoporosis. This hypothesis has been advanced in a position statement of the Academy of Nutrition and Dietetics,[2] in a publication of the U.S. National Academy of Sciences,[8] as well as other scientific publications,[4] which have stated foods high in potassium and magnesium such as fruits and vegetables may decrease the risk of osteoporosis through increased alkaline ash production. However, this acceptance of the acid-ash hypothesis as a major modifiable risk factor of osteoporosis by these publications was largely made without significant critical review by high quality systematic analysis.[3]

It has also been speculated that an alkaline diet may have an effect on muscle wasting, growth hormone metabolism or back pain, though there is no conclusive evidence to confirm these hypotheses.[9][10][11] Given an aging population, the effects of an alkaline diet on public health may offer some benefits due to its focus on an increase in fresh fruits and vegetables, but there are limited scientific studies on the topic.[11]

References
  1. Fenton, Tanis. "The alkaline diet, the acid-ash hypothesis and bone health". Dairy Nutrition.
  2. Cunningham E (October 2009). "What impact does pH have on food and nutrition?". J Am Diet Assoc. 109 (10): 1816. doi:10.1016/j.jada.2009.08.028. PMID 19782182.
  3. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA (2009). "Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash diet hypothesis". Nutr J. 8: 41. doi:10.1186/1475-2891-8-41. PMC 2761938. PMID 19754972.
  4. Fenton TR, Tough SC, Lyon AW, Eliasziw M, Hanley DA (2011). "Causal assessment of dietary acid load and bone disease: a systematic review & meta-analysis applying Hill's epidemiologic criteria for causality". Nutr J. 10: 41. doi:10.1186/1475-2891-10-41. PMC 3114717. PMID 21529374.
  5. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA (November 2009). "Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance". J. Bone Miner. Res. 24 (11): 1835–40. doi:10.1359/jbmr.090515. PMID 19419322.
  6. Fenton TR, Lyon AW (October 2011). "Milk and acid-base balance: proposed hypothesis versus scientific evidence". J Am Coll Nutr. 30 (5 Suppl 1): 471S–5S. doi:10.1080/07315724.2011.10719992. PMID 22081694.
  7. Lambert H, Frassetto L, Moore JB, Torgerson D, Gannon R, Burckhardt P, Lanham-New S (2015). "The effect of supplementation with alkaline potassium salts on bone metabolism: a meta-analysis" (PDF). Osteoporos Int. 26 (4): 1311–8. doi:10.1007/s00198-014-3006-9. PMID 25572045.
  8. Food and Nutrition Board. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (2005), page 189. National Academies Press.
  9. Pizzorno J, Frassetto LA, Katzinger J (April 2010). "Diet-induced acidosis: is it real and clinically relevant?". Br. J. Nutr. 103 (8): 1185–94. doi:10.1017/S0007114509993047. PMID 20003625.
  10. "Alkaline Diets". WebMD. Retrieved 5 February 2014.
  11. Schwalfenberg GK (2012). "The alkaline diet: is there evidence that an alkaline pH diet benefits health?". J Environ Public Health (Review). 2012: 1–7. doi:10.1155/2012/727630. PMC 3195546. PMID 22013455.
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