Association of Vitamin B12, Vitamin D, and Thyroid-Stimulating Hormone With Fatigue and Neurologic Symptoms in ... Fibromyalgia, 2022, Munipalli et al

Interesting that they've used <400 ng/L as a threshold for deficiency, that is much higher than the lower reference points seen in Norway at least (I've seen the lowest points lie between 140-170 pmol/L, equivalent to 190-230 ng/L). I wonder how the results had been if these lower thresholds had been used (they also looked at symptoms when deficiency was set to <350 ng/L, but that is still higher than the lowest point of the reference ranges I'm familiar with).

My guess is that the differences in symptoms would have been less noticeable, as there would be more patients defined as not deficient that also had fatigue. The threshold value of 400 ng is also above what I've seen argued as a safe point for subclinical B12 deficiency, so I wonder if there might be something else going on than lack of B12 being the problem. B12 is found in animal sourced foods that contribute a lot of other important nutrients to the body, those with a lower B12 concentration could be getting less of these nutrients.

And obviously also a lot of other things

 

The paper says: The values most closely associated with the date of the documentation of fibromyalgia were used in this study.

So it is possible, depending on how often these tests are run where the study was performed. They are using their own facility's cutoff for deficiency, which may be different than whatever institution that actually analysed the patient's blood, that might also give some weird results as reference ranges differ between labs for a reason.

 

B12 levels were a good thing to look at in a big fibromyalgia population, but the results aren't very impressive. The symptoms of fatigue, memory loss, brain fog, cognitive impairment numbness and neuropathy were not very well correlated with B12 levels, if at all. The 350 ng/L level was specifically identified as giving the best prediction of fatigue (i.e. it wasn't decided before), and even then only 19.8% of people with less than 350 had fatigue, while 15.7% of people with more than 350 had fatigue. That's a result that sounds a lot like chance. The results suggests that, especially considering the potential selection bias for testing that @shak8 mentioned, fatigue associated with fibromyalgia isn't primarily caused by low B12.

They didn't provide a straightforward scatter graph of B12 levels; I wish investigators would just present their base data in that way before adjusting for this and that.