Homocysteine, folate, methylation, and monoamine metabolism in depression. Teodoro Bottiglieri et al. British Medical Jl This is an old study. It's been cited over 500 times. Here's why. they took severely ( hospitalized) depression patients and controls: and 1. it looked at methylation indicators in serum, red blood cells, and even cerebrospinal fluid; 2. documented an inverse relationship between serum homocysteine and all other indicators for products of methylation, INCLUDING metabolite indicators of monoamine neurotransmitter metabolism. In other words, there was strong evidence that the methylation metabolism failure was linked to depression.