Dietary protein causes bone loss… we’ve all heard this or read it, or perhaps been told it by someone ‘knowledgeable’ on the topic. But is it true? I mean is there evidence to support this claim? The basic observation that lead to this claim is that higher protein intake, particularly of sources of protein rich in sulphur-containing amino acids (meat and grains), leads to acidification of the blood and that causes resorption of calcium from bones, which leads to higher blood calcium, which leads to calciuria (more calcium being excreted in the urine). This is a very simplistic explanation, but it works for what I’m about to talk about. There is also a consideration of phosphorous in here, but it gets little more complicated when we bring it into the equation. Nonetheless, the basic hypothesis for this mechanism has been termed the acid-ash hypothesis: more acid = more ash (calcium) excretion. From an excellent meta-analysis (in ‘science-speak’) these authors (http://www.ncbi.nlm.nih.gov/pubmed/19419322) say: The acid-ash hypothesis posits that protein [especially meat and dairy] and grain foods, with a low potassium intake [vegetables], produce a diet acid load, net acid excretion (NAE), increased urine calcium, and release of calcium from the skeleton, leading to osteoporosis. So where’s the evidence? Well instead of reviewing individual studies and cherry picking your favourite one we’re going to use and evidence-based approach (I’m at McMaster – the touted birthplace of EBM – and if I don’t say Evidence-Based Medicine I am roundly jeered by my colleagues). So let’s take a look…
First, one of the better analyses is from Dr. Tanis Fenton and was published in 2008 (http://www.ncbi.nlm.nih.gov/pubmed/18842807) in which they reviewed 25 studies that met their inclusion criteria and concluded “Evidence suggests a linear association between changes in calcium excretion in response to experimental changes in net acid excretion. However, this finding is not evidence that the source of the excreted calcium is bone or that this calciuria contributes to the development of osteoporosis.” Interesting? So more calcium is excreted in the urine and yet it may not be coming from bone? Interestingly, from a prior mechanistic standpoint Dr. Jane Kerstetter at Yale showed that in highly detailed studies actually tracing calcium uptake and excretion that with increasing protein intake there was greater gut calcium uptake (http://www.ncbi.nlm.nih.gov/pubmed/15546911) and “The high-protein diet caused a significant reduction in the fraction of urinary calcium of bone origin and a nonsignificant 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.” So onto further EBM to answer the question. In another meta-analysis (http://www.ncbi.nlm.nih.gov/pubmed/19419322) the conclusion was that, “The acid-ash hypothesis posits that protein and grain foods, with a low potassium intake, produce a diet acid load, net acid excretion (NAE), increased urine calcium, and release of calcium from the skeleton, leading to osteoporosis… There is no evidence from superior quality balance studies that increasing the diet acid load promotes skeletal bone mineral loss or osteoporosis. Changes of urine calcium do not accurately represent calcium balance. Promotion of the "alkaline diet" to prevent calcium loss is not justified.” Essentially, there are more meta-analyses on this topic (http://www.ncbi.nlm.nih.gov/pubmed/19754972, http://www.ncbi.nlm.nih.gov/pubmed/20459740), but all have reached the same conclusion, “A causal association between dietary acid load and osteoporotic bone disease is not supported by evidence and there is no evidence that an alkaline diet is protective of bone health.” In fact, some evidence (not conclusive) suggests that “Dietary protein is beneficial to bone health under conditions of adequate calcium intake…” (http://www.ncbi.nlm.nih.gov/pubmed/24316688). There’s nothing like hyperbole and pseudo-science and the occasional ‘cherry-picked’ study to support the protein = bone loss argument, but it’s not evidence-based and it’s CERTAINLY not supported by science. When one looks hard enough and reads (http://www.ncbi.nlm.nih.gov/pubmed/21529374 ) then you have to read deeper that someone’s blog. In this paper “Causal assessment of dietary acid load and bone disease: a systematic review & meta-analysis applying Hill's epidemiologic criteria for causality.” The authors looked to “The objective of this systematic review was to evaluate causal relationships between dietary acid load and osteoporosis using Hill's criteria.” Basically Hill’s criteria is interesting and relates to a paper written in 1965 (yes that’s right) entitled “The environment and disease: Association or causation?” If you’re interested give it a read (http://www.ncbi.nlm.nih.gov/pubmed/14283879). The bottom line here is that these authors reviewed a lot of high quality trial evidence in this area and concluded as I stated above “A causal association between dietary acid load and osteoporotic bone disease is not supported by evidence and there is no evidence that an alkaline diet is protective of bone health.” Most interesting to me in this paper is the use of Hill’s criteria and really of Hill’s 4th criteria “plausibility.” In this area the authors state that “Regarding Hill's plausibility criterion that a theory fit with current biological knowledge, the mechanism for diet acid load induced mineral resorption at the bone is not well described. Some researchers assert that bone is dissolved, releasing skeletal calcium and bicarbonate to neutralize the systemic acidemia… Others hypothesize that the effect occurs at the kidney: calcium is lost in the urine as urinary bicarbonate is reabsorbed from the distal nephron to compensate for the excretion of anions. However, none of the in vitro studies supported these concepts since not one of these studies reported studies of bone demineralization or any adverse effects (such as activation of osteoclasts or enzymes) within the physiological range (7.35 to 7.45).” So basically, there’s not event good biological support for the relationship! The article is a good one and one that I think can serve as a blueprint for how to systemically critique published literature; give it a read it’s a free download! (http://www.nutritionj.com/content/10/1/41#B31)
But, what about milk? I mean it’s the worst right, that’s why we should drink soy ‘milk’ and almond ‘milk’ and rice ‘milk’ (note: the word milk implies that something came from a mammary gland so since neither soy, nor almonds, nor rice have those then they're not milk). I’ll leave the final word to the person who’s done a lot of work in this area Dr. Tanis Fenton who in this paper (http://www.ncbi.nlm.nih.gov/pubmed/22081694) entitled “Milk and acid-base balance: proposed hypothesis versus scientific evidence.” After conducting 4 meta-analyses concluded:
- Measurement of an acidic pH urine does not reflect metabolic acidosis or an adverse health condition.
- The modern diet, and dairy product consumption, does not make the body acidic.
- Alkaline diets alter urine pH but do not change systemic pH.
- Net acid excretion is not an important influence of calcium metabolism.
- Milk is not acid producing.
- Dietary phosphate does not have a negative impact on calcium metabolism, which is contrary to the acid-ash hypothesis.
It’s time to stop people who say that protein promotes poor bone health and that milk is especially bad in this regard. It’s not true, not evidence-based, and just plain wrong! Dismount from soap-box...