Phytates (Inositol Hexaphosphate and Related Compounds), in Plant-Based Foods, as Poor Sources of Utilizable Phosphate

The authors of this article [Johnson and Tate, 1969: (http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1480-3291&volume=47&issue=1&startPage=63)], a poorly-chosen but adequate one, discuss the fact that phytates in grains and plants and plant proteins are inositol polyphosphates, and they're not digested effectively in non-ruminants (i.e. humans and non-cow mammals, etc.). Phytic acid is the major one, and it's just myo-inositol hexaphosphate, a.k.a. inositol hexakisphosphate. But some of the other "phytates" are just inositol phosphorylated at one or more different oxygens, etc. It's really astonishing to me that this issue isn't discussed more than it is (I've never seen discussions of the extremely low digestibility of phytates), even apart from any consideration of the importance of phosphate. Iqbal et al. (1994) [Iqbal et al., 1994: (http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1375699&blobtype=pdf)(http://www.ncbi.nlm.nih.gov/pubmed/7959229)] discussed the fact that the phytase (inositol polyphopsphate phosphohydrolase) activity in the human stomach and small intestine is, as is known, not produced by any human enzyme with phytase activity but by yeast or bacterial phytases found in foods. This is a great article, in which Letcher et al. (2008) [Letcher et al., 2008: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=18684107)(http://www.ncbi.nlm.nih.gov/pubmed/18684107)] discussed the fact that phytates are unlikely to be transported into cells to a significant extent, because, upon their absorption, the compounds would form complexes with calcium and other divalent cations that are very low in solubility, etc. Human cells can hydrolyze inositol polyphosphates, but maybe there's something about the orientations/conformations of the phosphate linkages that precludes their hydrolysis by human enzymes (inositol triphosphate and other inositol phosphates act as second messengers and exert other signalling functions, etc.). But Letcher et al. (2008) make it sound as if dietary phytates don't even yield much phosphate, assuming they're absorbed at all, because of poor uptake into cells. I found some articles showing that meat essentially provides much more nutritionally-available phosphate than plant protein sources (plant foods that provide phosphate in phytate compounds) do, and that's the sense I'm also getting from these articles. So the nutrition information on the "phoshorus" content (from phosphate) in a plant-protein-based food is potentially going to be essentially incorrect, given that little utilizable phosphate can be provided by phytate compounds. Lopez et al. (2002) [Lopez et al., 2002: (http://cat.inist.fr/?aModele=afficheN&cpsidt=13944404)] discussed research showing that 80 percent of the phosphorus in plants or seeds exists in phytate compounds. The authors of this article [Sandberg et al., 1987: (http://jn.nutrition.org/cgi/reprint/117/12/2061.pdf)(http://www.ncbi.nlm.nih.gov/pubmed/2826727)] found that 58 percent of phytate(s) were hydrolyzed to inositol pentaphosphates (IP5's) and IP4's and IP3's, and that makes it sound as if microbial phytase activity can release variable amounts of phosphate. But one has to consider the fact that the IP5's or IP4's might just bind to divalent cations and precipitate, etc. And even the phytates that are absorbed by paracellular diffusion (many compounds can be absorbed by passive diffusion, to a larger extent than most articles imply or state) are not likely to yield much utilizable phosphate and may very well just be excreted in the urine (Letcher et al., 2008), etc.

That research is important, given that phosphate depletion could contribute to osteomalacia (see article, in the previous posting, on the capacity of calcium supplementation to bind phosphate in the GI tract and contribute to osteomalacia by inducing phosphate depletion) and all sorts of other problems, over time (see previous postings). I'm not even suggesting that eating meat protein and obtaining phosphate from meats is desirable or undesirable. I'm just trying to get a handle on what the actual amounts of utilizable phosphate are in different food groups.