It has been 125 years since Dr. Alzheimer of Germany first announced Alzheimer's disease to the world in 1900, yet the causative agent of this disease remains unidentified to this day. Through the efforts of many researchers, deposits of amyloid beta protein outside nerve cells and tau protein within nerve cells have been identified as the pathological hallmarks. These proteins have been suggested as the cause of Alzheimer's disease. However, even when amyloid is reduced through amyloid therapy, no recovery of cognitive function is observed. Currently, attention is focused on whether treatment targeting tau protein might lead to cognitive recovery. Yet, the reason why this tau protein accumulates within nerve cells remains unknown. We previously published in the 2010 issue of PLOS ONE that homocysteic acid (HCA), generated by the super-oxidation of the amino acid methionine via OH radicals, is a causative agent that produces amyloid and tau proteins and further induces cognitive decline. This time, we comprehensively reviewed how this HCA is related to various lifestyle-related diseases associated with aging, and is a factor determining lifespan beyond Alzheimer's disease, cancer, and aging itself. By reducing this HCA, we will explore the potential to extend healthy lifespan and curb the escalating costs of healthcare.