The High Protein and Bone Health Debate Laid to Rest Posted by evolutionaryhealthperspective on June 21, 2013 · Leave a Comment

One of the common arguments I hear against high-protein diets, specifically those high in animal protein, is that they are detrimental for bone health. In fact, many nutritional textbooks published in the 1990’s and early 2000’s adopt this stance. Unfortunately, this viewpoint is based upon a handful of research studies with 1) an inadequate understanding of the relationship between food intake, absorption, and urinary levels of minerals, specifically calcium, 2) not addressing the complex hormonal regulation of bone remodeling and calcium homeostasis, 3) and a failure to examine populations who consumed extremely high protein diets. To conclude this article, we will examine more methodologically sound research has demonstrated the opposite, indicating that high protein diets may be beneficial for bone health.

Calcium Homeostasis

Calcium is responsible not only for structural functions (our skeleton), it is also critical in signaling functions, enzymatic functions (clotting factors), maintains membrane potential alongside Na+ and K+, and muscle contraction. The serum level of calcium is tightly regulated to maintain normal levels of 9-10.5 mg/dL (2.2-2.6 mmol/L). The homeostatic range of calcium is one of the most tightly regulated molecules in the body.

High-Protein Diets and Bone Health: Anecdotal Evidence

One of the largest pieces of evidence to suggest that high-protein diets are not detrimental to bone health is that of the aboriginal Eskimo. The diet of the eskimo was almost void of plant material and they subsisted mainly on animal meat and fish. The fact that the Eskimo maintained healthy skeletal structure despite the low carbohydrate and high protein content of their diet and their high latitude (affecting vitamin D production), this group of people were view as a paradox among nutritional scientists. As I have mentioned in previous posts, in science, a paradox simply means your hypothesis is inaccurate; there must be a different explanation.

A famous, landmark paper (1) explored the Eskimo’s diet in-depth and examined the affect of acculturation and adopting westernized dietary habits had on their health. Needless the say the westernization of their diet was not beneficial; in fact, it had quite the opposite effect. Here is a direct quote from the paper regarding the effect westernization had on their health.

“His nutritional status is deteriorating, in terms of both undernutrition and overnutrition, in direct relation to the proportion of processed foods in his total diet. In the subarctic, where dietary acculturation is extensive, the Eskimo has the full complement of diet-related diseases that are characteristic of other segments of the U.S. population of low socioeconomic status: obesity, cardiovascular disease, hypertension, and tooth decay”.

High-Protein Diets and Urinary Calcium: The Simple Model of Calcium Regulation

Anecdotal and epidemiological evidence is a place to start, and that is precisely what we have done here. We have used it as a starting point to examine mechanisms and causality. That being said, let’s look at the research

In the 1970’s, a slew of researchers investigated the role of high-protein diets on bone health by examining the levels of urinary calcium (2, 3). This research was based upon the notion that calcium is “leached” from the bones to maintain normal pH levels from the increased acid load of high-protein diets. For now, let us assume the theory that high protein diets require additional calcium to buffer out an acidic internal milieu (a point I will make null-in-void in a moment). These researchers did indeed find increased urinary calcium levels in those with high-protein diets. Unfortunately, the researchers did not seek to investigate further or examine other mechanisms of why the blood levels of calcium may be increased.

What other mechanism may be responsible for increases in urinary calcium besides drawing calcium from the bones? Recent, and not so recent evidence has shown that increasing protein intake increases calcium absorption from the digestive tract (4,5,6,7). Increased calcium absorption from dietary sources requires the kidneys to filter out the increased serum calcium in the urine to maintain the homeostatic level (9-10.5 mg/dL). Figure 1. depicts this mechanism quite nicely. This research confirms this notion, showing that the increase in blood calcium is indeed due to increased absorption of calcium rather than from bone sources.

Before we move onto the next section I want to revisit the acid/base balance argument for this is key in the argument against high-protein diets. As previously mentioned, proponents of this theory suggest that our bones “give up” calcium in order to maintain homeostatic pH levels by balancing out the increased acid load due to high-protein intake. Unfortunately for this theory, our acid/base is extremely robust and relies on a whole slew of mechanisms to maintain our pH level. In fact, I am currently looking through the newest edition of Guyton and Hall’s Textbook of Medical Physiology and there is an entire chapter (chapter 30) on acid-base regulation. Nowhere in this chapter do they discuss the role of bone resportion for calcium to buffer decrease pH. That would be extremely inefficient compared to all of the other mechanisms we have in place

In regards to the hypothesis of high-protein diets being detrimental to bone health in regards to acid/base balance, it appears that diets high in protein are NOT detrimental to bone health.

Figure 1. Calcium regulation through diet, calcium resorption, and calcium deposition (image c/o docstoc.com).

Hormones: Complex Regulation of Calcium Levels

Before we move on to examine populations who have subsisted on high-protein diets and maintained bone integrity, we must address the fact that calcium homeostasis is more complex than intestinal absorption and kidney filtration.

As with the majority of our regulation systems in our body, calcium homeostasis is maintain through a feedback system that ensures calcium levels are never too high (hypercalcaemia) or too low (hypocalcaemia).

When calcium levels are increased our thyroid releases calcitonin to increases deposition of calcium in tissue and decreasing renal absorption of calcium.

When calcium levels fall below normal physiological your parathyroid releases parathyroid hormone (PTH) to increase calcium levels by increasing bone demineralization to release calcium ions, increases calcium absorption in the kidney, and PTH enhances intestinal uptake of calcium indirectly through the biosynthesis of vitamin D.

Figure 2. Hormonal Regulation of Blood Calcium

What is not pictured is the interaction of vitamin D directly at the bone tissue. This interaction is quite complex in and of itself and is dependent on numerous factors including the status of the other fat-soluble vitamins, A, K, and E. For an in depth look at this issue please refer to Chris Masterjohn’s work on Vitamin D toxicity and Nutrient Synergy

High Protein Diets Improve Bone Health

High-protein diets increase calcium absorption in the digestive tract and increased blood calcium elicits calcitonin release from the thyroid and calcium deposition in bone tissue, therefore, we have a mechanism through which high-protein diets increase bone density. This mechanism has been studied and recent research has provided evidence in support of this proposed mechanism.

An excellent study published in 2011 by Cao et al. (8) directly supports the hypothesis that high protein diets increase urinary calcium yet have no effect on bone health. In this study the researchers found “dietary treatments did not affect other markers of bone metabolism. In summary, a diet high in protein and PRAL increases the fractional absorption of dietary Ca, which partially compensates for increased urinary Ca, in postmenopausal women. The increased IGF-I and decreased PTH concentrations in serum, with no change in biomarkers of bone resorption or formation, indicate a high-protein diet has no adverse effects on bone health” (8). This evidence supports the notion that high protein diets are not detrimental to bone health.

A large body of evidence is surfacing that supports the hypothesis that the increased intestinal calcium absorption due to high protein diets may actually increase bone health (9,10,11,). While one of these studies did indeed include calcium supplementation, specifically the Framingham, not all of them did. Furthermore, recent meta-analysis that have done excellent an excellent job critical analyzing and evaluating the literature and they also find that high-protein diets are beneficial to bone health. In their review in 2011, Kerstetter et al., stated, “Recent epidemiological, isotopic and meta-analysis studies suggest that dietary protein works synergistically with calcium to improve calcium retention and bone metabolism. The recommendation to intentionally restrict dietary protein to improve bone health is unwarranted, and potentially even dangerous to those individuals who consume inadequate protein”. (12)

From my opinion, I think the issue of high-protein diets having a negative impact on bone health due to an increase in the net acid load of the body has been put to rest. High-protein diets are not only safe for bone health, they may in-fact be highly beneficial, especially in older individuals and those at high risk for osteoporosis.