OK, oxalic acid is a huge debate right now. Some keepers avoid anything that has oxalic acid, others don't worry about it. I am wondering what we really KNOW about it as it applies to tortoises.
Like many of us, I have seen scary facts, secondhand facts, conflicting facts, and more. Hopefully, we can clear up some of it. Here are some of the things I have read (I do not know that they are all absolutely TRUE, however)...
1. Oxalic acid binds with the available calcium in a foodstuff. The more oxalic acid, the less available calcium. This is the foundational fact- we know this to be true.
1a. Vitamin A and good hydration offsets the effects of the acid. See also point 3.
2. Oxalic acid does not seem to interfere with calcium intake from other foods, which suggests that a dash of calcium can offset the effect.
3. Oxalic acid is one element in the formation of bladder stones. Other elements include too little vitamin A and D, too much calcium, too little water/humidity/hydration, or too much calcium. Many experts think that dehydration is the key element- concentrating the minerals in the blood and urine.
3a. Bladder stones are often found in wild tortoises. Wild torts also are known to eat plants that are considered dangerously high in oxalic acid (to the point of being considered toxic to humans). Is the oxalic acid responsible for the stones, or is dehydration? A lot of wild torts are also known to skirting the edge of dehydration for quite a bit of the year.
4. Lists of oxalic acid tend to be based on an old USDA report, other newer studies have found different results- but they sometimes disagree with each other.
4a. One issue is that the soil a plant is raised in makes a difference, the age and exact part of the plant makes a difference, and how the plant is treated, cooked, or processed makes a difference. To claim, for example, that spinach has 0.95mg per 100 g of food is an oversimplification- are we talking baby leaves, freshly picked stuff, 'older' leaves in a grocery store?
5. Some sites report that oxalic acid is not a problem unless it is 'activated' by cooking.
So....
There are two main things we blame oxalic acid for- stealing calcium and contributing to soft shell, and for bladder stones. Are there other concerns I am missing?
I have seen no reports for tortoises or humans that links oxalic acid to calcium deficiencies. I doubt this is a concern in a varied diet with a little calcium supplementation.
That mostly leaves the issue of stones. Dr. Mader ("Reptile Medicine and Surgery") thinks that hydration is the key factor here, and admits that we do not know exactly how stones are formed in tortoises. Oxalic acid and stone formation in humans seems tricky as well- some sites blaming it, other sites saying there are other issues that are more important. Our dietitian at work does not think it is a big enough deal to worry about for our clients.
My gut feeling is that we are making this a bigger deal than we need to. If we can eat it, the torts can eat it. If we don't eat much of it (as in parsley, watercress or rhubarb), they should not get much of it. A small pinch of calcium powder acts as insurance with most meals.
I think the bigger, more important issues are the basics (in no order):
- good hydration (water, water content in food, humidity, etc. depending on the species)
- vitamins A and D
- overall Ca: P ratio of about 2 or 3:1 over time
- right temps
- good diet with variety
Thoughts, corrections, clarifications, discussion?????
Like many of us, I have seen scary facts, secondhand facts, conflicting facts, and more. Hopefully, we can clear up some of it. Here are some of the things I have read (I do not know that they are all absolutely TRUE, however)...
1. Oxalic acid binds with the available calcium in a foodstuff. The more oxalic acid, the less available calcium. This is the foundational fact- we know this to be true.
1a. Vitamin A and good hydration offsets the effects of the acid. See also point 3.
2. Oxalic acid does not seem to interfere with calcium intake from other foods, which suggests that a dash of calcium can offset the effect.
3. Oxalic acid is one element in the formation of bladder stones. Other elements include too little vitamin A and D, too much calcium, too little water/humidity/hydration, or too much calcium. Many experts think that dehydration is the key element- concentrating the minerals in the blood and urine.
3a. Bladder stones are often found in wild tortoises. Wild torts also are known to eat plants that are considered dangerously high in oxalic acid (to the point of being considered toxic to humans). Is the oxalic acid responsible for the stones, or is dehydration? A lot of wild torts are also known to skirting the edge of dehydration for quite a bit of the year.
4. Lists of oxalic acid tend to be based on an old USDA report, other newer studies have found different results- but they sometimes disagree with each other.
4a. One issue is that the soil a plant is raised in makes a difference, the age and exact part of the plant makes a difference, and how the plant is treated, cooked, or processed makes a difference. To claim, for example, that spinach has 0.95mg per 100 g of food is an oversimplification- are we talking baby leaves, freshly picked stuff, 'older' leaves in a grocery store?
5. Some sites report that oxalic acid is not a problem unless it is 'activated' by cooking.
So....
There are two main things we blame oxalic acid for- stealing calcium and contributing to soft shell, and for bladder stones. Are there other concerns I am missing?
I have seen no reports for tortoises or humans that links oxalic acid to calcium deficiencies. I doubt this is a concern in a varied diet with a little calcium supplementation.
That mostly leaves the issue of stones. Dr. Mader ("Reptile Medicine and Surgery") thinks that hydration is the key factor here, and admits that we do not know exactly how stones are formed in tortoises. Oxalic acid and stone formation in humans seems tricky as well- some sites blaming it, other sites saying there are other issues that are more important. Our dietitian at work does not think it is a big enough deal to worry about for our clients.
My gut feeling is that we are making this a bigger deal than we need to. If we can eat it, the torts can eat it. If we don't eat much of it (as in parsley, watercress or rhubarb), they should not get much of it. A small pinch of calcium powder acts as insurance with most meals.
I think the bigger, more important issues are the basics (in no order):
- good hydration (water, water content in food, humidity, etc. depending on the species)
- vitamins A and D
- overall Ca: P ratio of about 2 or 3:1 over time
- right temps
- good diet with variety
Thoughts, corrections, clarifications, discussion?????