Oxalate In Food

[tielfan]

Sorry if this is in the wrong place. 

The internet says that oxalate in food can bind with a variety of minerals (calcium, iron, magnesium, etc), but calcium oxalate gets most of the attention.  I would like to know whether oxalate has a preference for binding with calcium, or if it is equally likely to bind with any other suitable mineral that happens to be present. 

Thank you.

[LaurieAG]

Hi tielfan, welcome to Hypography.

 

The internet says that oxalate in food can bind with a variety of minerals (calcium, iron, magnesium, etc), but calcium oxalate gets most of the attention.  I would like to know whether oxalate has a preference for binding with calcium, or if it is equally likely to bind with any other suitable mineral that happens to be present.

 

If you google oxalate binding calcium you'll find the US National Kidney Foundation says the following.

 

https://www.kidney.org/atoz/content/calcium-oxalate-stone

When fat is not absorbed the right way, the fat binds to calcium and leaves oxalate behind. The oxalate is then absorbed and taken to the kidney, where it can form stones.

 

[exchemist]

Sorry if this is in the wrong place. 

The internet says that oxalate in food can bind with a variety of minerals (calcium, iron, magnesium, etc), but calcium oxalate gets most of the attention.  I would like to know whether oxalate has a preference for binding with calcium, or if it is equally likely to bind with any other suitable mineral that happens to be present. 

Thank you.

The oxalate ion is a bidentate ligand and forms 5 membered rings with a wide variety of divalent metal cations. The solubility of these compounds varies however. According to the Wiki article on oxalate, Mg oxalate is more soluble than Ca oxalate, which is in turn more soluble than Fe oxalate. I presume the prevalence of Ca oxalate in kidney stones is due to the prevalence of Ca++ ions in the body. Oxalates of alkali metals (Na, K), which are monovalent, are more soluble than those of the divalent cations, presumably because their structure involves 2 cations to each oxalate anion, and the binding in the crystal structure is not as strong.   

[tielfan]

Thank you everyone!

The National Kidney Foundation link is talking about the formation of kidney stones related to inflammatory bowel disease. But I'm interested in what happened in the plant that is being consumed as food and/or what happens in the interior of the digestive tract as the food makes its way through the body in the normal way, not in relation to kidney stones. 

Suppose we have equal amounts of calcium and iron, with enough oxalate to form a compound with all of one mineral, or with half of each mineral.  Will we end up with all our calcium tied up as calcium oxalate while the iron remains free, because the oxalate likes calcium better? Or will we end up with half the calcium tied up in calcium oxalate, half the iron tied up in ferrous oxalate, and half of the calcium and iron still free, because the oxalate has no preference? 

 

My knowledge of chemistry is close to zero, but it seems to me that there shouldn’t be a preference for one specific mineral if all the minerals present have the right form of ion.  exchemist mentioned the prevalence of Ca++ ions in the body.  The internet says that iron exists in the body as Fe++ ions, so it seems like it should be just as likely to bind with oxalate.  Wikipedia says that iron oxalate can also end up in kidney stones, and it has been implicated in gout.  https://en.wikipedia.org/wiki/Oxalate  Magnesium occurs in the body as Mg++, so it should also be a viable partner.

 

The internet indicates that these minerals take the same form in plants, so they should all be equally likely to bind with oxalate there too.  It’s my understanding that most of the oxalate in plant foods formed a compound while it was still in the plant, before the plant was eaten, and the only reason to have free oxalate in the plant at the time it’s eaten would be if there weren’t enough minerals in the plant to bind with all the oxalate.

Edited March 4, 2018 by tielfan

[exchemist]

Thank you everyone!

 

The National Kidney Foundation link is talking about the formation of kidney stones related to inflammatory bowel disease. But I'm interested in what happened in the plant that is being consumed as food and/or what happens in the interior of the digestive tract as the food makes its way through the body in the normal way, not in relation to kidney stones. 

 

Suppose we have equal amounts of calcium and iron, with enough oxalate to form a compound with all of one mineral, or with half of each mineral.  Will we end up with all our calcium tied up as calcium oxalate while the iron remains free, because the oxalate likes calcium better? Or will we end up with half the calcium tied up in calcium oxalate, half the iron tied up in ferrous oxalate, and half of the calcium and iron still free, because the oxalate has no preference? 

 

My knowledge of chemistry is close to zero, but it seems to me that there shouldn’t be a preference for one specific mineral if all the minerals present have the right form of ion.  exchemist mentioned the prevalence of Ca++ ions in the body.  The internet says that iron exists in the body as Fe++ ions, so it seems like it should be just as likely to bind with oxalate.  Wikipedia says that iron oxalate can also end up in kidney stones, and it has been implicated in gout.  https://en.wikipedia.org/wiki/Oxalate  Magnesium occurs in the body as Mg++, so it should also be a viable partner.

 

The internet indicates that these minerals take the same form in plants, so they should all be equally likely to bind with oxalate there too.  It’s my understanding that most of the oxalate in plant foods formed a compound while it was still in the plant, before the plant was eaten, and the only reason to have free oxalate in the plant at the time it’s eaten would be if there weren’t enough minerals in the plant to bind with all the oxalate.

No, there will definitely be a relative preference for specific ions rather than others, even if all are divalent cations. 

 

From a quick internet search I get solubility product constants for FeC2O4 and CaC2O4 of about 2 x 10⁻⁷ and 2 x 10⁻⁸ respectively. This suggests the oxalate ion binds strongly to both, but more strongly to Ca than to Fe, giving a lower solubility. 

 

For Mg it is ~5 x 10⁻⁶, i.e. more soluble than either, so the binding is less strong for Mg than for either Ca or Fe. 

 

My understanding is that in many plants, oxalate is present in the form of oxalic acid itself, (H2C2O4). If it enters the body in this form it can find Fe or Ca and will then precipitate, due to the insolubility of these oxalates.

 

Regarding Fe 2+ in the body, my understanding is that almost all of this is already bound very strongly, in haemoglobin, i.e. you do not have very much free Fe2+ ions floating around.

 

But with all the above I stress I am not an expert on all this: I'm just going on my half-remembered chemistry and what I can easily find on the internet. 

[tielfan]

Your half-remembered chemistry is far better than anything I can manage on my own. So thank you very much for your help!

Edited March 4, 2018 by tielfan

[tielfan]

The information you provided gave me the tools to do some more googling.  I only understand a fraction of what I read, but this link:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3883082/   seems to say that oxalate prefers magnesium over calcium, because it fits together better.  There are other compounds that compete with the oxalate too, for example citrate. This is all talking about kidney stones, which may not be the same as what happens during ordinary digestion.

 

 

This link:  http://oxalicacidinfo.com/  says you are right that there is a lot of free oxalic acid in food.  Apparently the information that’s widely posted about high and low oxalate foods is based on their oxalic acid content, not the amount of oxalate that has already precipitated with a mineral. A lot of the oxalic acid info comes from the USDA, so it seems likely that the values they post for calcium and other minerals in the food also refers to minerals that are not insolubly bound to something else. The plant must have kept the minerals and oxalic acid apart somehow, but once the digestive tract sets them free they would be able to bind with each other.  So it still seems reasonable to look at the net difference between the oxalic acid and the minerals in the food as the amount of mineral that could potentially be bound by the oxalic acid.

 

But at the same time, this link:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5626694/  says that “Calcium oxalate may represent up to 80% of the dry weight of some plants”.  So there must also be a lot of precipitation going on in the plant itself.     

 

There must be other factors that influence what happens to the minerals in food.  The figures for spinach indicate that it has more than enough oxalic acid to bind with all the relevant minerals it contains, but it’s still reported that 5% of the calcium in spinach is bioavailable.  Some of it manages to escape from the antinutrients.

[exchemist]

The information you provided gave me the tools to do some more googling.  I only understand a fraction of what I read, but this link:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3883082/   seems to say that oxalate prefers magnesium over calcium, because it fits together better.  There are other compounds that compete with the oxalate too, for example citrate. This is all talking about kidney stones, which may not be the same as what happens during ordinary digestion.

 

 

This link:  http://oxalicacidinfo.com/  says you are right that there is a lot of free oxalic acid in food.  Apparently the information that’s widely posted about high and low oxalate foods is based on their oxalic acid content, not the amount of oxalate that has already precipitated with a mineral. A lot of the oxalic acid info comes from the USDA, so it seems likely that the values they post for calcium and other minerals in the food also refers to minerals that are not insolubly bound to something else. The plant must have kept the minerals and oxalic acid apart somehow, but once the digestive tract sets them free they would be able to bind with each other.  So it still seems reasonable to look at the net difference between the oxalic acid and the minerals in the food as the amount of mineral that could potentially be bound by the oxalic acid.

 

But at the same time, this link:  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5626694/  says that “Calcium oxalate may represent up to 80% of the dry weight of some plants”.  So there must also be a lot of precipitation going on in the plant itself.     

 

There must be other factors that influence what happens to the minerals in food.  The figures for spinach indicate that it has more than enough oxalic acid to bind with all the relevant minerals it contains, but it’s still reported that 5% of the calcium in spinach is bioavailable.  Some of it manages to escape from the antinutrients.

Yes the paper you refer to is a bit curious - and in my view not very clearly written. I think they are not saying that oxalate prefers Mg to Ca exactly, but that Mg2+ ions can interfere with the formation of the extended "aggregates" of Ca oxalate that are found in kidney stones. These "aggregates" are clearly not just simple crystals of pure Ca oxalate, or they would not (presumably) use the term "aggregate" to describe them.

 

I found your other paper, about the role of citrate, interesting. This says that is is common for urine to be supersaturated with Ca oxalate but that the formation of aggregates is inhibited by proteins and other species that cause the incident crystals to form colloidal dispersions that pass harmlessly through the system and out. Presumably then it is the failure of this colloidal dispersion mechanism (effectively a form of detergency) that allows kidney stones to form, rather than exceeding a solubility limit for Ca oxalate. I suppose it may be that Mg2+ ions can be attracted to the surface of tiny Ca oxalate crystals and thus form a surface layer than inhibits aggregation as well. I wonder if that is what the first paper is really saying.  

Edited March 4, 2018 by exchemist

[tielfan]

Maybe someday I'll find something that clearly explains it all.  But this is probably the best I can do for now.  Thank you for all your help, it was VERY helpful.