In another thread Rob hinted that phytic acid might not be an issue, simply because the amount of phytic acid present in soylent might not be enough to do much damage. I decided to follow up on this idea. The tl;dr of what follows is that it appears, based on the data I looked at, that a typical soylent recipe doesn’t contain enough phytic acid to create problems, provided that there is a sufficiently generous quantity of minerals (particularly Zinc) in the soylent.
For starters, I calculated the following things. How much phytic acid would a typical soylent recipe contain? And, how much of each mineral could that amount of phytic acid bind? My hypothesis was that it might be the case that we could safely add enough of a given mineral that even if all of the phytic acid bound to that mineral, there would still be more than the RDA of that mineral not bound to phytic acid. This turned out to be the case for some minerals.
Let us assume a prototypical soylent recipe: 200g maltodextrin, 100g oat powder, 85g whey protein, 30g psyllium husk powder, 50g oil, and micronutrients. Here the phytate-containing ingredients are oat powder and psyllium husks. The oats will contain 0.89-2.4% phytic acid by mass (source). I couldn’t find any information about the phytic acid content of psyllium husks, so let’s say 0-3.5%. (3.5% is higher than any food listed in the source). Using these amounts, we obtain a range of 0.89-3.45g of phytic acid.
Phytic acid has a molar mass of about 660g/mol. I’ll use the molar mass of the major minerals to calculate how much of each these minerals could be tied up by 3.45g of phytic acid. Again I am being very pessimistic here, making the totally unrealistic assumption that all of the phytic acid (in my maximally pessimistic quantity estimate) binds to a single mineral.
To calculate this, first note that since I have 3.45g of phytic acid and the molar mass is 660g/mol, I have 3.45/660 = 0.00522 mol of phytic acid. That amount can bind 0.00522 mol of calcium, or of magnesium, etc. If I multiply by the molar mass of the mineral of interest I get the amount of mineral, by mass, that might be bound. The results are:
As long as I have an excess of at least this amount of each mineral above the RDA, my phytic acid can’t possibly create shortages of these minerals. In the cases of calcium, phosphorus, and magnesium, it would be easy to add sufficient amounts to make this happen, while remaining well below the upper limits.
In the remaining cases – iron, zinc, copper, and manganese – the phytic acid could theoretically bind the entire supply of the mineral in question, even if we had the maximum safe amount of the mineral. This means that in these cases the issue becomes subtler. But it doesn’t necessarily mean that phytic acid is going to create a problem for these minerals.
Heuristically, we expect that a larger proportion of the phytic acid will bind to the minerals of which there is a lot: phosphorus, magnesium, etc. Only a small amount of phytic acid will bind to the minerals of which there is a small amount.
The reason the phytate isn’t an issue for phosphorus, calcium, and magnesium, is that in these cases there just isn’t much phytate relative to how much mineral there is. Since the phytate is shared by all of the minerals it binds, what is really true is that there isn’t much phytate relative to the overall quantity of minerals. When it comes to a specific mineral, we expect that the amount of phytate binding to that mineral is a proportion of the total phytate which is not too far away from the proportion of the mineral of interest out of the total quantity of phytate-binding minerals. Given that, what we can theoretically expect to see is that no mineral gets knocked down to a really problematic extent by phytic acid, since the overall amount of phytic acid is small relative to the overall amount of mineral.
Does this idea bear out in practice? Let’s see if we can find some empirical data.
Zinc is supposed to have an especially high affinity for phytic acid. According to source, “molar ratios [of phytic acid to Zinc] in excess of 15:1 progressively inhibit Zn absorption and have been associated with suboptimal Zn status in humans.” I have 0.00522mol of phytic acid at worst. Say I include 30mg of Zinc: just a ways below the DRI’s upper limit of 40mg. Then I have 0.0004587mol of Zinc, for a molar ratio of 11.38. According to our source, this is safe! However, there isn’t a ton of error margin here, which shows that the balancing act of Zinc and phytic acid is a little bit delicate in our situation.
We haven’t taken into account other factors affecting Zinc absorption. The presence of high amounts of calcium hurts it; the presence of protein helps it. (Source.) Hopefully we are close enough in our estimate anyway.
Let us try to estimate whether phytic acid will interfere with our iron absorption. According to the same source, vitamin C and protein both reduce the extent to which phytic acid interferes with the absorption of iron. This is good news for us.
One study found that when people ate wheat rolls containing 2mg phytic acid (added artificially after the natural phytate was artificially removed), their iron absorption was inhibited by 18%, relative to their iron absorption when eating rolls with no phytic acid. Subjects ate two rolls, each prepared using 40g wheat flour, and we can estimate that the meal contained roughly 400 calories. A 400-calorie portion of our soylent should contain about 3.45g * (400/2,000) = 690mg of phytic acid, worst case: 35% the phytate content of the mentioned rolls. Moreover, we may conjecture that the absorption of iron from soylent would be ceteris paribus better than it would be from rolls, because soylent has high levels of vitamin C and protein. So, with much margin for error, the phytic acid in our soylent should inhibit iron absorption by less than 18%. This clearly poses no issue.
For copper and manganese, from what I saw there isn’t enough data to draw a lot of meaningful conclusions. We have contradictory data concerning the question of whether phytic acid hurts, helps, or doesn’t affect copper absorption. As far as I can tell there is exactly one study on how phytic acid affects manganese absorption. At least one study found that phytic acid improves selenium absorption. I found nothing about chromium or molybdenum.
The big picture
Overall, I have tried to argue that there is not enough phytic acid in soylent to create major problems, provided that sufficiently generous quantities of minerals are included. Since phytic acid has a large molar mass, the number of phytic acid molecules is much smaller than the number of minerals in soylent. This suggests a priori that the phytic acid shouldn’t create a major issue. For the minerals where empirical data exists, the data supports this conclusion; it suggests that the amount of phytic acid in soylent won’t inhibit the absorption of the minerals in question to a problematic extent.
I don’t consider all of this to close the book on the phytic acid question. I am certain that there are plenty of holes that can be picked in my science, and I didn’t use nearly as much data as might be desired. But I think the data is pointing in a good direction. Please let me know if you have any comments, criticisms, things to add, etc.!