For today’s edition of Dear Mark, I’m answering three questions. First, are the amino acid supplements sarcosine, D-serine, and D-cycloserine worth taking? What do they do and who can they help? Next, what should you do to prevent an infection? Besides the regular stuff like vitamin C and vitamin D, is there anything else to take to mitigate the damage and prevent pathogenic incursions? And finally, I field a question from a fencer and martial artist who’s feeling the beginning twinges of knee pain. He leads a lunge-heavy life—it’s an inescapable part of his training—and I offer a few suggestions for limiting the damage.