More About Phosphate Metabolism

This is a great article [Heaney, 2004: (http://www.mayoclinicproceedings.com/content/79/1/91.full.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/14708952)], and Heaney (2004) made the argument that the supposed dangers of increases in the dietary phosphate (Pi)/calcium (Ca2+) ratio have been overstated. He also discussed the fact that increases in phosphate availability tend to decrease the bone resorption induced by a given steady-state concentration of parathyroid hormone (PTH). Heaney (2004) made the interesting point that the phosphate-induced decreases in serum calcium (that can occur at excessive intakes of dietary phosphate) could result from the phosphate-mediated suppression of the ongoing, PTH-mediated bone resorption. That would mean that the steady-state contribution of the PTH-mediated bone resorption to serum calcium could be decreased by the extra phosphate, and that, as Heaney (2004) argued, could decrease serum calcium and cause an increase in PTH secretion. So the decrease in serum calcium could be only an indirect result of an increase in dietary phosphate.

Heaney (2004) also made the argument that even moderate dosages of supplemental calcium salts, such as calcium carbonate, could bind most of the utilizable dietary phosphate in the GI tract. He also noted that many animal studies that have shown soft tissue calcification in response to large phosphate intakes have used supraphysiological dosages of phosphate. Apparently, the dietary phosphate concentrations in the diets of most laboratory animals, expressed as mmol of phosphate per kcal diet, are considerably larger than the phosphate contents of human diets. I think the units of mmol/kcal diet are sort of "scaled," but I can try to apply some of the allometric scaling calculations to those numbers that Heaney (2004) provided. Some of the arguments about safety that Heaney (2004) used were based on the use of calcium phosphate salts, but calcium phosphates tend to be very low in solubility. I don't know if that would have influenced the responses, and it looks like some of the calcium and phosphate must have been absorbed.

I also found out that the "phosphorus content" in a food refers, apparently, to the phosphate content, and it's really not a good "convention." There are other problems with the ways in which serum phosphorus vs. serum inorganic phosphate numbers are interconverted. I've seen articles in which mM values of serum phosphate have been converted into mg/dL using the molar mass of phosphorus (30.97 g/mol) and not the molar mass of inorganic phosphate (94.973 g/mol). That's not something that should be going on, and it's a vestige of some kind of outdated, bizarre convention that must have been developed a long time ago. But apparently the upper limit or approximate NOAEL for dietary phosphate is 5,000 mg/day, and Heaney (2004) discussed research, such as the article by Spencer et al. (reference 20, cited on p. 94), showing that major increases in the dietary phosphate/calcium ratio had failed to disturb calcium metabolism in the ways people claim such an increase would disturb calcium metabolism. I think it's very important for a person to discuss his or her phosphate considerations with his or her doctor before doing anything, because there are risks. But the phosphate phobia reminds me of the irrationality that used to surround the use of vitamin D supplements. I have an old book that came out about 12 years ago and that had been really thoroughly and carefully researched, and the author essentially says that dosages of vitamin D higher than 1000 IU could cause soft-tissue calcification all over the body and may be fatal. I'm not exaggerating that. The reason the author wrote that is that, before 1999, no one had looked carefully at the research on vitamin D. Vieth (1999) [Vieth, 1999: (http://www.ncbi.nlm.nih.gov/pubmed/10232622)] looked at the research carefully and identified all of the flaws in the reasoning, etc. That's a great paper, and the attention to details in Heaney's articles reminds me of the attention to details in the article by Vieth (1999). In that article by Vieth (1999), for example, there's research in which a group of authors wrote a case report of a person who had experienced symptoms and signs of vitamin D toxicity from relatively low dosages of vitamin D. A year after the first letter, the authors wrote to say that the supplement had actually contained vitamin D at concentrations "an order of magnitude" (i.e. ten times) higher than the labeled concentrations. Nonetheless, it's important to be more careful with phosphate than with many other nutrients, in my opinion. One thing Heaney (2004) mentioned is that taking calcium supplements even 2 hours after one has taken a source of dietary phosphate can improve the absorption of the phosphate and calcium, because they bind and form insoluble precipitates when given together, at the same time, etc.