The Acid-Ash Hypothesis states that a diet which includes the consumption of protein and grain foods creates an “acidic environment” (net acid load) in the body. It also suggests that to buffer this acid, our body releases calcium from bones which could potentially lead to osteoporosis and other diseases.
In the common language, this hypothesis is promoted as the “alkaline diet”. Basically, the people who promote this hypothesis believe that diets rich in animal protein, dairy and grains are detrimental to human health.
But does this hypothesis stand the test of the scientific method?
1) In 1999, Munger et al. examined the relationship between protein intake and the risk of incurring a hip fracture in postmenopausal women. 44 cases of hip fracture incidences were studied. The study concluded that intake of dietary protein, including animal sources, was associated with a reduced incidence of hip fractures. http://ajcn.nutrition.org/content/69/1/147.abstract
2) Similarly, in 2000, Hannan et al. showed that subjects with a lower protein intake had increased bone loss. They also concluded that protein intake is important in maintaining bone or minimizing bone loss in elderly persons. Further, higher intake of animal protein did not appear to affect the skeleton adversely. This result is in directly contradiction to the alkaline diet.
3) In the same vein, Promislow et al. (2002) showed that animal protein plays a protective role in the skeletal health of elderly women.
4) One of the earliest observations which led to the formation of the hypothesis was the higher amount of calcium found in the urine post the consumption of an “acidic” meal. A study conducted in 2005 (Kerstetter et al.) examined the effects of a high protein intake on the amount of calcium excreted in the urine. The results, “The high-protein diet caused a significant reduction in the fraction of urinary calcium of bone origin and a non-significant trend toward a reduction in the rate of bone turnover. There were no protein-induced effects on net bone balance. These data directly demonstrate that, at least in the short term, high-protein diets are not detrimental to bone.”
5) A meta-analysis which examined the relationship between protein and bone health in humans was conducted in 2009 (Darling, Millward, Torgerson, Hewitt, & Lanham-New). The researchers concluded that protein has a slight protective effect on bone health but may not reduce the risk of fracture in the long term. In simpler words, higher protein intake does not turn you into Superman. http://ajcn.nutrition.org/content/90/6/1674.abstract
6) Another meta-analysis conducted in 2009 (Fenton et al.) found that changes in acid-base intakes were directly proportional to the amount of calcium found in the urine but unrelated to net calcium balance in the body. Therefore, changes in calcium excretion do not accurately represent changes in calcium balance. http://doi.wiley.com/10.1359/jbmr.090515
7) Similarly, another meta-analysis from Fenton and colleagues examined the effects of phosphate on bone loss in healthy individuals on the same parameters as the previous study. The authors concluded, “Dietary advice that dairy products, meats, and grains are detrimental to bone health due to “acidic” phosphate content needs reassessment. There is no evidence that higher phosphate intakes are detrimental to bone health.” https://www.ncbi.nlm.nih.gov/pubmed/19754972
8) Fenton and his tribe were at it again in 2010. This time, to determine if a low urine pH and acid excretion in the urine can predict the risk of osteoporosis. The changes in bone density of the participants of this study were recorded for a period of 5 years. The result – “Urine pH and urine acid excretion do not predict osteoporosis risk.”
9) Later that year, they also showed that that milk does not make the body acidic. Further, net acid excretion is not an important influence on calcium metabolism.
10) Caroll et al. grabbed the baton in 2011 and published a review which studied the relationship between the intake of dairy products and bone health. They concluded that milk and dairy are good and bioavailable sources of calcium. https://www.ncbi.nlm.nih.gov/pubmed/22032348.
11) Schwalfenberg (2011) studied the available literature on the relationship between an alkaline diet and various diseases and systems of the body such as muscular and skeletal. They also examined the relationship between an alkaline diet and urine pH. It was found that alkaline diets result in a more alkaline urine pH and may result in reduced calcium in the urine. However, as seen in some recent reports, this may not reflect total calcium balance because of other buffers such as phosphate. No substantial evidence was found that this improves bone health or protects from osteoporosis.
12) A study by Hanley & Whiting in 2013 also examined the relationship between an alkaline diet and urine pH as well as calciuria and its effect on the bone. The findings were then used to give recommendations which were consistent with the evidence. It was found that an acidic diet featuring a high protein intake may be associated with an increase in calciuria but the evidence that it plays a role in the development of osteoporosis is inconsistent.
13) In 2014, Nicoll & McLaren Howard examined the effect of dietary acid intake with varying levels of calcium intake. The findings led them to conclude that an acid producing diet maybe detrimental to the bone if combined with a low calcium intake. But if calcium intake is high, the acid producing diet may be protective.
14) In the same year, Rizzoli observed the role of dairy products on reduction of fracture risks. It was found that dairy products could improve bone health and reduce the risk of fractures in later life.
15) Recently the alkaline diet has also been promoted for its “cancer fighting properties”. The logic behind this is that cancerous cells thrive in an acidic environment and therefore it must be the case that increasing the body’s pH (making it alkaline) will lead to a cure. Fenton & Huang (2016) studied the alkaline diet in the context of cancer prevention and concluded, “This systematic review of the literature revealed a lack of evidence for or against diet acid load and/or alkaline water for the initiation or treatment of cancer. Promotion of alkaline diet and alkaline water to the public for cancer prevention or treatment is not justified”
16) Besides cancer, the alkaline diet has also been promoted as a cure to a multitude of other diseases such as obesity, type 2 diabetes and cardiovascular disease. A recent review by Thorning et al. (2016) puts the final nail in the coffin. The objective was to review several studies which have studied the relationship between dairy intake and the diseases mentioned above and all-cause mortality. The results speak for themselves:
“A diet high in milk and dairy products reduces the risk of childhood obesity and improves body composition in adults. This likely contributes to lower the risk of developing type 2 diabetes. Finally, there is increasing evidence suggesting that especially the fermented dairy products, cheese and yoghurt, are associated with a reduced risk of type 2 diabetes”
“The overall evidence indicates that a high intake of milk and dairy products, that is, 200–300 ml/day, does not increase the risk of cardiovascular disease. Specifically, there is an inverse association with risk of hypertension and stroke.”
In conclusion, the acid-ash hypothesis is promoted by an over-zealous and passion driven community. And like most information which comes from these communities, it has little foundation in scientific evidence and there’s a mountain of data which completely contradicts its many promises.
Moral of the Story:
Always be skeptical of the claims you come across, especially when the claims rely less on scientific knowledge and more on scare tactics and fear mongering.