Bioavailability of phosphorus from various foods

[RightSide]

[I'm still relatively new here.  If the following stuff is all well known to the IHD community, I apologize in advance.]

In the past, dialysis patients were advised to eat meats and fish to get essential amino acids, whereas beans and other phosphorus-rich plant foods were limited.

But in the last few years, nutritional thinking has changed, as my nutritionist has now advised me:

Phytate, found in nuts, seeds, grains, and beans, is an effective chelator of phosphorus.   Since the human body lacks the enzyme phytase to break down phytate, none of that bound phosphorus (about 75% of the total) is absorbed.

For example, one cup of black beans contains 15 gm protein and 241 mg phosphorus.  But only about 60 mg of that phosphorus is absorbed by the human body, giving a ratio of 4 mg phosphorus absorbed for every gram of protein.

In contrast, the phosphorus in meats is mostly absorbed by the body.  The same amount of tuna fish would have 41 gm protein and 373 mg phosphorus, almost all of which is absorbed.  That gives a ratio of 9 mg phosphorus absorbed for every gram of protein.

So getting at least some of our essential amino acids from plants and whole grains is now increasingly recommended.  Most of the phosphorus is not absorbed, so you don't have to worry about it as much.  My nutritionist has told me that whole grain rice and broccoli are two protein-rich possibilities.

BTW, one factor that works against this is the popular dietary supplement, acidophilus (also found in yogurt's "active cultures").  Acidophilus bacteria provide phytase, which breaks down the phytate, liberating the phosphorus from plants and whole grains.  This will cause much more of that phosphorus to be absorbed by the body.

Of course, you should always discuss your personal diet plan with your dietitian.  But you might want to ask him if he's familiar with the phosphorus bioavailability issue.

[BigSky]

It wasnt the Phosphorus that I was told to watch with beans, it was the very high potassium to protein ratio.

That one cup of black beans has nearly 600 mg Potassium with only 15 grams protein.

[*kana*]

Oh, that is fantastic news!  I'm going to ask my dietician about this.  I am on PD and my potassium is always super low and I so miss my beans and soy nuts!

[Zach]

Part of the issue with beans and protein from other plant sources is that they are "incomplete proteins" meaning they need to be combined with other foods to make up the full "essential amino acids." 

And their waste product--nitrogen--is higher than animal sources of protein (which are considered high quality protein--containing all the essential amino acids).

But of course protein from plant sources can be included in your total daily protein consumption, just less than perhaps 50%.

Moderation is key ... as always.

[Restorer]

I did some quick search through studies, and what I found seems to confirm the 75% number, but not in the same way. From what I've been able to determine, 25% of the phosphorus from phytate is absorbed. But only 52% of "normal" (non-phytate) phosphorus is absorbed anyway. So, if I'm reading this right, phytate phosphorus is absorbed 50% as readily as non-phytate phosphorus.

My source:
http://journals.cambridge.org/download.php?...

But, I'm fairly sure not all of the phosphorus in vegetable sources is bound up in phytate. So it's hard to really know how much bioavailable phosphorus you'll be getting in any given food.

Anyone else have any other good sources for this kind of information? Useful numbers would be things like the amount of phosphorus absorbed from phytate, and how much phosphorus is bound in phytate in certain food items.