Cao JJ, Nielsen FH. Acid diet (high-meat protein) effects on calcium metabolism and bone health. Curr Opin Clin Nutr Metab Care. 2010 Aug 16. [Epub ahead of print]
PURPOSE OF REVIEW: Update recent advancements regarding the effect of high-animal protein intakes on calcium utilization and bone health.
RECENT FINDINGS: Increased potential renal acid load resulting from a high protein (intake above the current Recommended Dietary Allowance of 0.8 g protein/kg body weight) intake has been closely associated with increased urinary calcium excretion. However, recent findings do not support the assumption that bone is lost to provide the extra calcium found in urine. Neither whole body calcium balance nor bone status indicators, negatively affected by the increased acid load. Contrary to the supposed detrimental effect of protein, the majority of epidemiological studies have shown that long-term high-protein intake increases bone mineral density and reduces bone fracture incidence. The beneficial effects of protein such as increasing intestinal calcium absorption and circulating IGF-I whereas lowering serum parathyroid hormone sufficiently offset any negative effects of the acid load of protein on bone health.
SUMMARY: On the basis of recent findings, consuming protein (including that from meat) higher than current Recommended Dietary Allowance for protein is beneficial to calcium utilization and bone health, especially in the elderly. A high-protein diet with adequate calcium and fruits and vegetables is important for bone health and osteoporosis prevention.
For decades now, it’s often been thought, felt or claimed that a high dietary protein intake had a detrimental effect on calcium metabolism and bone health; certainly many groups promoting low-protein dietary approaches tend to echo/parrot this idea.
This idea came around in the mid-20th century but was based on some, shall we say, questionable research. In it, totally purified proteins were given (that is, no other nutrients were present) and a loss of calcium in the body (in the urine) was documented. It was simply assumed that this had a negative impact on bone health.
Despite later research showing that it was much more complicated than this (i.e. that proteins containing other nutrients had different effects and that other parts of the diet played a major role in the overall effect), this idea is simply repeated as if it were still unquestionably true. I dealt with this issue to some degree in The Protein Book, in a chapter called Protein Controversies, which is reproduced here on the main site.
As well, there has long been a secondary data set (seemingly ignored by anti-protein folks) showing that higher protein diets actually IMPROVE bone healing following things such as breaks or fractures. Clearly the idea that ‘protein is bad for bone’ is a bit more complicated than just a soundbite. The review paper I want to look at today examines the topic in some detail.
The paper begins by pointing out that bone is over 50% protein to begin with and that there has long been concern that the modern Western diet is detrimental to bone health due to the production of acids within the body. This is something I imagine readers have at least seen mentioned in recent years (I get the occasional question about it) with some going so far as to claim that the body’s pH is THE KEY to all health (some even claim that a drop in cellular pH is the cause of cancer).
While it’s not quite that cut and dry, clearly the modern Western diet tends to promote the production of metabolic acids and at least some degree of metabolic acidosis. This is due to a number of factors including a high protein intake (proteins are acid promoting), insufficient fruit and vegetable intake (both of which are net base producing for the most part), along with other factors such as sodium and potassium balance (excessive sodium intake relative to potassium can increase the acid load of the body). You can find long lists of foods online in terms of their net acid or base producing potential.
And certainly, as discussed briefly in Protein Controversies, acidosis can cause problems in the body. It’s relevant to today’s paper in that the body appears to buffer this acid load by releasing calcium, presumably from bone. In that current research is suggesting that the RDA for protein is actually too low for some populations (notably older individuals) and with the current interest in high-protein diets for weight/fat loss and maintenance, it’s important to know whether or not these dietary approaches are having negative impacts on bone health.
The paper looks in some detail at the issue of acid/base balance and calcium metabolism. As noted above, the generation of metabolic acids causes a number of effects in the body, all of which could potentially impact negatively on calcium metabolism and bone health. As well, studies clearly show both that:
- The generation of metabolic acids causes increased calcium loss in the urine
- Counteracting acidosis with base-forming minerals (e.g. potassium bicarbonate) decreases calcium excretion
While the above is clear, the direct impact of dietary protein on bone health is a bit less clear with the results of more direct epidemiological data showing mixed results in terms of the actual impact on bone health. As well, citing a review by Fenton, the paper points out that:
…neither calcium balance nor the bone resorption marker, N-telopeptides, was affected by diet-induced changes in net renal acid excretion despite a significant linear relationship between an increase in renal net acid excretion and urinary calcium.
That is, while it’s clear that increased dietary acid load causes increased urinary calcium excretion, it’s less clear if this has any real direct impact on the body’s net calcium balance or overall bone health.
Moving on to more direct effects, the paper looks at the very old data (using primarily purified proteins) showing that for every increase in dietary protein by 1 gram, there was a 1 mg increase in urinary calcium loss (raising the question of why not simply scale calcium intake to protein intake to offset this); this led to the assumption that bone health was being compromised.
However, in direct contrast to this, the majority of epidemiological studies find that a higher protein intake is associated with increased bone mineral density with only a few finding a negative impact. As well, while weight loss per se tends to cause a decline in bone health, some research has found that high-protein weight loss diets reduce the loss of bone mineral content; that is, high-protein intakes on a diet are beneficial.
The primary acid formation from protein comes from the sulfur containing amino acids (cysteine and methionine) and these are found in higher amounts in animal vs. vegetable proteins; it’s often been assumed that a higher vegetable protein intake would therefore have less of an impact on bone health.
However, this also turns out to be incorrect; the paper points out that studies of high-meat protein intakes either show no overall effect on net calcium balance and a higher animal protein intake is actually associated with increased bone mineral density; as well studies show a negative association between vegetable protein and bone mineral density.
It’s worth noting that strength/power athletes, who have traditionally consumed a high-protein diet are typically found to have higher bone densities compared to sedentary individuals. As the paper points out:
Changes in bone mass, muscle mass and strength track together; thus maintenance or an increase in muscle mass and function maintains or enhances bone strength and mineral density.
And while the increase in urinary calcium excretion with increasing protein cannot be simply ignored, current data suggest that this isn’t actually due to a loss of bone mass. Rather, increased protein intake leads to increased calcium absorption from the gut; the loss in the urine is simply due to more calcium being absorbed. The increased loss is simply due to more being absorbed from the diet; interestingly, this effect is more pronounced when calcium intake is low to begin with.
In terms of mechanism, higher protein intakes raise levels of the hormone IGF-1, which stimulates bone formation; this probably explains the benefits of a high-protein intake on bone healing. As well, high protein intakes have been shown to decrease levels of parathyroid hormone (PTH), a hormone that is involved in the loss of bone mass. Low protein intakes are associated with increased PTH and lowered bone mineral density.
Finally, as I mentioned in the introduction, you can’t simply look at protein intake outside of the rest of the diet and there are clear interactions with other nutrients. I mentioned above that protein intake interacts with calcium intake, increased absorption. As well, a high protein intake has been shown to increase bone health in older individuals when calcium and Vitamin D are supplemented. Finally, ensuring a sufficient intake of fruits and vegetables (which neutralize the acid load of protein) should help to ensure the impact of dietary protein on bone health is positive rather than negative.
Summing up, the researchers conclude thus:
Although a high meat or protein intake increases renal acid load and urinary calcium excretion, recent findings do not support the claim that bone is the source of the extra calcium lost in the urine. In addition, evidence is lacking that shows high-protein intakes, including that from animal sources, affect whole body calcium balance or contribute to osteoporosis development and fracture risk.
I don’t have a whole lot to add to the above conclusion. Clearly the negative impact of dietary protein on bone health would appear to be overstated to some degree. Under certain circumstances (low calcium/Vitamin D intake, insufficient intake of fruits and vegetables), it’s certainly possible that a high-protein intake could have negative impacts. But again this comes down to an issue of context. And in the context of sufficient net acid neutralizing foods (fruits, vegetables, sufficient potassium intake) along with sufficient calcium/Vitamin D intake, the impact of protein on bone health would appear to be positive overall.