Provided herein are methods of diagnosing Alzheimer's disease (“AD”) based on characteristic changes of the levels of certain free amino acids or dipeptides (collectively termed as “AD diagnosis markers”) in the body fluid sample of an individual, carnosine synthesis activities in the plasma, and dopamine synthesis activities in the plasma. Also provided are methods of simultaneously determining the levels of at least two free amino acids or dipeptides in the biological fluid sample of an individual.