100 g raw spinach contains 3,900 micromol of soluble oxalate and 3,560 micromol of insoluble oxalate. Forty-seven percent of the soluble oxalate is lost into the cooking water when spinach is boiled for 1 minute, whereas only 10% of the insoluble oxalate is lost.

A study found that in 12 men consuming a 1,970 micromol/day oxalate diet without spinach, fecal oxalate excretion was 1,200 micromol, about 60% of the dietary intake, whereas urinary oxalate excretion was 375 micromol, about 10%; these results imply that 30% (600 micromol) was degraded. During the fourth week of consuming an additional 102 g spinach, which contained about 3,000 micromol added oxalate, fecal oxalate excretion increased to 4,000 micromol--80% of total dietary intake. Again, about 600 micromol oxalate could not be accounted for, which suggests that degradation had occurred.
Another study found that, on average, 68% of a very high intake of oxalate (29,000 micromol oxalate as spinach) was excreted in feces during the last 2 weeks of a 3-week study. However, excretions were very high (near intake) in three subjects but only 10% to 20% of intake in the remaining four subjects. These data suggest that healthy individuals vary widely in their microbial degradation of oxalate.

The majority of dietary oxalate that is excreted in the urine is excreted within 1 to 8 hours after ingestion. Increased excretion is still detectable between 8 and 24 hours after consumption of oxalate-containing meals or loads. Dietary oxalate can be assumed to be absorbed and excreted within 24 hours after ingestion in people with normal gastrointestinal function.

I had four kidney stones when I was 20. I captured the last one (peed into a sieve) and it was analyzed as calcium oxalate. Each of them occurred during or after periods of long-distance travel. The Dr. told me to avoid spinach and chocolate. I did for a while. But I think the contributing factor was more likely dehydration from the travel than overindulgence in chocolate. At that time I certainly wasn't OD'ing on spinach!

I'd say unless you have a history of kidney stones, there's no reason to avoid spinach at all. I eat plenty of it and plenty of chocolate now and I've never had a recurrence. But I am careful to hydrate well, particularly when traveling.

The connection to oxalate consumption is speculative - from what I understand, the two haven't been linked yet. It is, however, true that oxalates can precipitate out some nutrients but if you're eating a diverse diet you won't get enough to care about. Eat varying kinds of leafy greens, drink lots of water, and you don't have to be anal about it. Leeks are also high as are a number of other foods btw in case you need more to worry about :)

I hope this helps others: Oxalobacter Formigenes: The Oxalate Eater. Another cause of high oxalate levels is due to missing bacteria called oxalobacter formigenes. Oxalobacter formigenes is a bacterium (probiotic) found in the gut of most animals. Almost all 6 to 8 year olds have oxalobacter formigenes in their intestines, but only 60 to 80% of adults. Oxalobacter formigenes degrades oxalate by eating it. In fact, oxalate is its only source of food. Unfortunately, it has been shown that oxalobacter formigenes is frequently decreased or missing in people with inflammatory bowel diseases such as celiac disease...............source:http://glutenfreeworks.com/blog/2010/11/17/kidney-stones-renal-calculus-causes-treatment-in-celiac-disease/#.UPrZnGIRbeU................ I did research in local health food store and I wasn't able to find this bacteria. It is a medical shame that the doctors and urologist are hiding this simple solution from us.

I wouldn't worry about it too much. Cooking breaks down some of the oxalate. To cut down the oxalate content even further, you can include vitamin C to your meal - tomatoes are a nice accompaniment, but citrus works too.