The Facebook group is private, so they told me to ask Dr. Russell-Jones. I did, and here's his reply:
"Oxalates in food can contribute to levels of oxalate seen in urine, but the contribution is minor, further many people basically shovel oxalate foods into their heads and yet their oxalate levels are not raised. Thus, in our family, we eat lots of leafy green veggies and yet my oxalate level is on the low side at 46, which is about the lowest that I have in 1200 OATs. Elevated oxalate levels though are quite common in both chronic fatigue syndrome and also in autism. Hence if it is not diet, where does it come from?
Plotting all the variables that I have against each other, there is a correlation between levels of glutaric acid and oxalates, meaning that it is a product of reduced levels of functional B2. In functional B2 deficiency, you have a reduced ability to gain energy from fat or sugars and so you have to turn to amino acid breakdown for energy. Thus, in functional B2 deficiency, amino acids are broken down for energy, and glucogenic amino acids, which are broken down to glycolic acid are converted to oxalic acid. So the more B2 deficient you are the higher the oxalates. There are exceptions, which depend upon diet and energy intake"
It is not quite so simple because, glutaric acid metabolism only requires FMN, not FAD, so in Molybdenum deficiency, you can be replete in Iodine and Selenium, and so glutaric acid can be low, BUT, other measures of functional B2 deficiency may still be present."