The research repeatedly shows apparent connections between lower levels of GABA & GABA receptors and OCD. See this, this, this, this, this, and this. The GABA(A) receptor is decreased when homocysteine is elevated (which fits with MTHFR defects being associated with autism, for MTHFR defects negatively impact conversion of homocysteine to methionine). Myo-inositol has been shown to help protect subunits of GABA(A) receptors in the hippocampus, and some people on Amazon are reporting that it helps them with OCD thoughts.
The research thus supports a two-pronged dietary approach to lessening fixations that consists of
- supporting homocysteine conversion by making sure to consume enough betaine, zinc, B12 (but not the cyanocobalamin form), and folate (definitely avoid folic acid, though; too much of it apparently causes pseudo-MTHFR deficiencies)
- protecting GABA(A) receptors from internal or external antagonists by consuming enough myo-inositol (it seems prudent to limit caffeine consumption, as caffeine appears to interfere with at least one inositol-related function)