Bone acts as a reservoir of alkali and calcium can liberated from bone to buffer high acidic levels and restore acid-base balance .
The acid-ash hypothesis described above, is often used to suggest that higher protein diets produce a greater dietary acid load which leads to a loss of bone calcium, increased risk of bone fracture and osteroperosis.
However, the researchers behind the hypothesis are clear to point out the linear association between changes in calcium excretion in response to experimental changes in net acid excretion is not evidence that the source of the excreted calcium is bone or that this calciuria contributes to the development of osteoporosis .
The same researchers have implemented evidence-based statistical criteria for causality and showed clearly that “a causal association between dietary acid load and osteoporotic bone disease is not supported by evidence” .
Thankfully, acid-base balance in our blood and tissues is one of the most vital, intricately controlled physiological states. When foods are digested, the acids produced are buffered by bicarbonate ions in the blood. This reaction produces carbon dioxide, which is exhaled through the lungs. The other by-product, the salt (no not the table kind, the hydrogen of the acid replaced by a metal or other ‘cation’), are easily and efficiently excreted by the kidneys.
That's probably why no investigations have been able to link a cause and effect association regarding a protein intake and calcium loss.[1, 2, 3, 4, 5, 7]
In fact, a substantial amount of research suggests that dietary protein is a nutrient that is supportive of bone health [3-7]. While the research is less clear regarding what source of protein might be best, it's a great idea to consume a variety of plant and animal based protein sources.
One aspect is clear, much of the research on bone health and dietary interventions points towards adequate calcium intake as being a key, as is keeping mobile and incorporating weight baring exercise.
Regardless of where your dietary protein and calcium comes from, the take home message is, it's more important to give your body a reason to keep that nutrition within your bones so they remain strong.
That means being physically active on a daily basis and performing a structured resistance exercise program, two or three times a week.
2. 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
3. Mangano KM, Sahni S, Kerstetter JE. 2014. Dietary protein is beneficial to bone health under conditions of adequate calcium intake: an update on clinical research. Curr. Opin. Clin. Nutr. Metab. Care. 17(1): 69-74
4. Massey, L.K. (2003) Dietary animal and plant protein and human bone health: a whole foods approach. Journal of Nutrition 133, 862S-865S.
5. Promislow, J.H.E., Goodman-Gruen, D., Slymen, D.J. and Barrett-Connor, E. (2002) Protein consumption and bone mineral density in the elderly. American Journal of Epidemiology 155, 636-644.
6. Dawson-Hughes, B. (2003) Calcium and protein in bone health. Proclamations of the Nutrition Society 62, 505-509.
7. Protein "requirements" beyond the RDA: implications for optimizing health. Phillips SM, Chevalier S, Leidy HJ. Appl Physiol Nutr Metab. 2016 May;41(5):565-72.