One of the common questions I see, both around the web and on the Trying Low Oxalates (TLO) Yahoo Group is about whether or not cooking foods can lower their oxalate value. As a general rule, the answer is a resounding NO, but read on to learn why and how cooking DOES alter oxalate values.
Note: All oxalate values given in this article are from the spreadsheet available on the TLO group, and are current as of the date of publication of this article. If you’d like to see what cooking does to your favorite veggies, visit this page to find out how to get the whole list for yourself – it’s free!
Heat Does Not Destroy Oxalate
Let me clear up one thing right away: oxalates are heat stable (reference here). So simply heating a food, via any method – frying, microwaving, whatever – will not “destroy the oxalates,” or otherwise alter the amount of oxalates present. In fact, any heating method which causes a food to lose water will tend to raise the oxalate content of food. This is a simple matter of concentration; there is less weight of food present after cooking, but the same amount of oxalate. Hence the amount of oxalate per gram of food goes up.
But there are many who claim that other coooking methods, such as steaming or boiling, will lower the amount of oxalates. How true are these claims?
Serving Size – Possibly the Most Important Factor
One thing to remember about cooking foods is that it can drastically change the amount of food in a single serving. For example, a half cup of raw spinach weighs less than seven grams. A half cup of steamed spinach, on the other hand, weighs over six times as much – more than 45 grams! And while you may not think that a half cup serving of any vegetable is enough to make a difference, I guarantee it can. That half cup of steamed spinach contains 365 milligrams of oxalate – almost five times as much as the half cup of raw spinach.
Those of you paying close attention will note that the rise in oxalate is not proportional – that is, a sevenfold increase in weight did not correlate to a sevenfold increase in oxalate. There are several factors which may account for this discrepancy. For one thing, we don’t know the source of the two spinach samples. Were they from the same bag? And even if they were from the same bag, did they both come from the same plant? It’s been shown that differing soil conditions, plant varieties, and plant maturities can all affect the level of oxalate in a food. Simply put, no two leaves of spinach will be exactly the same in oxalate content, even if they weigh the same.
In addition, there’s probably some water left clinging to the steamed spinach; even when squeezed, steamed spinach will likely retain some of the cooking water. This water quickly adds weight to a sample and subsequently dilutes the oxalate content. There is also the fact that some oxalates are water-soluble, while others are not. This means that it is likely that some of the oxalates from the steamed spinach wound up in the steaming water (see my article on soluble vs. insoluble oxalates for more elaboration).
So if some oxalates can be removed to steaming water, it stands to reason that boiling your food would reduce oxalates, right?
Since spinach is one of the few foods which has been tested boiled, steamed and raw, I will return to it for my illustration. A half-cup serving of spinach which has been boiled for eight minutes clocked in at 130mg of oxalate. Not so bad…until you look at the weight. For some reason, that boiled half cup only weighed roughly 17 grams – far less than the steamed half cup. If we were to compare their numbers on a per-gram basis (a much better apples-to-apples comparison), we see that the steamed spinach contains 7.97 milligrams of oxalate per gram, and the boiled spinach contains 7.74mg per gram. A reduction? Yes. A noticeable one as far as your body is concerned? Nope. And boiling for less time would undoubtedly have even less of an oxalate reducing effect. This makes the recommendations like boiling “just for a minute!” to neutralize oxalate completely useless.
(By the way, this is also an excellent argument for weighing your food rather than measuring it by cups. Just sayin’.)
Since not all foods which have been tested have weights, it’s sometimes impossible to see these types of discrepancy in the TLO spreadsheet. But if a food has been tested both raw and boiled, and weights are given, then you can get a pretty good handle on whether or not boiling will reduce its oxalates. For example, carrots are a food which can have their oxalate levels reduced significantly by boiling – the spreadsheets show a 73% reduction (on a mg/g basis) for boiled carrots as compared to raw.
Another practice which is sometimes recommended as a way to reduce toxins and other anti-nutrients in foods is soaking. But does it have any affect on oxalates?
The short answer here is we don’t really know, but probably not much. There have only been a few foods which have been tested both before and after soaking and drying. One of these (pumpkin seeds), actually shows an increase in amount of oxalate on a mg/g basis. However, the testing of the two samples was done at two different times, so the seeds were almost definitely from two different batches, which all but negates the comparison.
Another example is macadamia nuts, which showed essentially no change after having been soaked for eight hours and then dried. But the reality is that most foods have not been tested in this way, so the safest route is to assume that soaking produces no change in the oxalate value of a given food.
Don’t Give Up Hope!
Just because cooking or soaking a food rarely lowers its oxalate value in any meaningful way, is no reason to give up hope on following a low oxalate diet! Besides the many tools available on this site, the TLO group is chock full of helpful people and resources. Almost everything I know about oxalates, I learned there! So don’t be afraid to dig in, and get your own copy of the list, so that you know which foods are safe and which you’ll want to consume in moderation (or not at all).
Also bear in mind that I’m not trying to address any of the other aspects of nutrition which may be altered by cooking – many say that raw foods are better for you because they contain enzymes or “are live.” Others say soaking, boiling and such remove phytates, goitrogens or other toxins. I claim no special knowledge about these factors, but they are definitely other things to take into consideration when preparing your foods.
Most important is to remember that What is right for you isn’t the same as what is right for anyone else. The only way to determine the foods your body responds to most favorably is by careful observation and constantly listening to your body’s own signals. There are loads of tasty, low-oxalate food combinations out there (the resources on my low oxalate recipes page are the most reliably low ox), and they’re almost invariably packed with nutrition as well.