The present invention provides a method for diagnosing or determining the risk of developing Alzheimers disease and for treating Alzheimers disease with S-equol. An aspect of the present invention includes the use of a direct mitochondrial target engagement biomarker to diagnose or assess the risk of developing Alzheimers disease. Another aspect of the present invention includes the use of a pharmaceutically effective amount of S-equol to treat or prevent Alzheimers disease in a subject diagnosed with or determined to be at risk of developing Alzheimers disease.
