Depression is one of the main causes of disability worldwide and estimates of its prevalence in people over 60 range from 8.8% to 18.3%. High levels of a substance called homocysteine have been linked not only with an increased risk of depression but also with physical health problems. A study of 3,752 men aged 70 and over by researchers in Australia found that the odds of depression rose by 4% with every unit increase of homocysteine (micromoles per litre). Homocysteine was higher in people with a certain variation in the MTHFR C677T gene. Older adults with higher levels of homocysteine were 1.7 times more likely to suffer from depression. Those who had the variation in the gene were 22% more likely to have current depression or a history of depression.
Almeida, Osvaldo P. ... [et al] - Homocysteine and depression in later life Archives of General Psychiatry November 2008, 65(11), 1286-1294