Monday, November 1, 2010

An informal meta-study: oxalate

If you are a nutrition graduate student trying to think of what your dissertation should be on, please please please consider doing some work identifying how much oxalate is in foods. Oxalate is one of the key contributors to calcium-oxalate kidney stones, the type afflicting 85% of the people with kidney stones. The FDA doesn't require companies to label their products with oxalate, so there are relatively few studies assessing how much oxalate the average person (or even the average person susceptible to kidney stones, which is a different population) gets from various foods.

I went through 5 different diet recommendations, 1 from my urologist, 2 from my nutrition-clinician, and 2 from online, and came up with some serious disagreements. Just in the way the data is presented there are important discrepancies in terms of ranges used, categories, and recommendations based on those ranges. Typically there is nary a serving size comparison listed for a person to be able to say "okay, 1/2 cup of this has ~8mg, so if I have 1/4 cup that's only 4 and I'm fine." On top of it all, no questionable food is mentioned by all diets.

Some of the discrepancies:
at least 1 study says low, medium, and high: potatoes and strawberries
conventional wisdom (also known as the doctors of my friends with kidney stones) say to avoid dark drinks like colas and root beer. The only diet of the 5 with anything to say about it says colas are low oxalate.
3 studies say low and 1 medium: bananas, yogurt, brown rice, oatmeal, cucumbers
3 studies say chocolate is high, but one says white chocolate is low.
2 low, 2 medium: lettuce, broccoli, apple juice
2 low, 1 high: zucchini, yellow squash
1 low, 3 medium: apples, asparagus
2 high, 1 medium: eggplant
1 high,  1 medium: carrots. The one high study says carrots are medium when canned, which is particularly weird since canning makes everything else worse.

Informally a person might be tempted to try to average the studies. If one says low and one says medium, then it's probably around 2-3mg. But that's not how a meta study ought to be done. I don't even have access to the original studies, just the summaries of the studies, and I don't know the criteria used in the diets analyzing the studies.

All this has helped me realize that a diet recommendation is just a poorly designed meta-study.

You will notice a lot of very good fruits, vegetables, and healthier starches on that list. All diets agree that chocolate, nuts, processed soy (tofu, soy milk, etc), and wheat germ are very high, but they differ on white flour and some allow certain kinds of nuts and one lets white chocolate through. They all say animal proteins are fine (except sardines) but then they encourage you to eat a low protein diet??

There's a lot of room for some enterprising researchers to make a career clearing some of this up.


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