types of b12 injections for those with methylation issues Vitamin B-12 Injection - Mobile and Clinic IV Therapy — ASAP IVs Comparative Bioavailability and Utilization of
Description
In Figure 4, vitamin B12 level was significantly decreased in rats treated with PTU (297.52 12.51) compared with the control group (443.11 22.54)

This is another aspect of the catch-22 backlash: without appropriate levels of B12, the effects of FIP may worsen, and because of FIP, your cat's ability to retain adequate reserves of B12 is diminished
For instance, deficiencies in both could cause fatigue, but nerve pain was unique to B12, and bone pain was associated with a D deficiency

Insulin-like growth factor I and impaired glucose tolerance
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