Relation of plasma homocysteine to plasma amyloid beta levels

Background: Elevated plasma homocysteine and amyloid beta (A beta) have been associated with Alzheimer's disease (AD). We investigated the cross-sectional association between these biomarkers. Methods: We used linear regression to relate plasma homocysteine and A beta adjusting for age, gender, creatinine, APOE-epsilon 4, and ethnic group in 327 persons aged 78 +/- 6.6 years. Results: Plasma homocysteine correlated with age, serum creatinine, plasma A beta 40 and A beta 42, and was inversely correlated with serum vitamin B12, and folate. A beta 42, but not A beta 40, was related to later development of dementia. Homocysteine was related to higher A beta 40 levels (coefficient = 2.0; P < 0.0001) and this association was attenuated after adjustment for creatinine (coefficient = 1.0; P < 0.0001). The crude association between homocysteine and A beta 42 was weaker (coefficient = 0.5; P = 0.01) and became non-significant after adjustment for creatinine (coefficient = 0.4; P = 0.06). These associations were unrelated to ethnicity, the presence of APOE-epsilon 4 or dementia. Analyses by quartiles of homocysteine showed that these association were driven primarily by the fourth quartile. Conclusions: Plasma homocysteine is directly related to A beta 40. The association with A beta 42 is not significant. These results seem to indicate that homocysteine is related to aging but not specifically to AD.