I Paid $300 for DNA-based fitness advice and all I got was junk science

In January of this year, I decided I would complete a half-marathon. I would push my body to the limit and be reborn as my best self. For months, I woke up at 6 to run before work, motivated by the promise of sweet endorphins and even-sweeter self-righteousness. But then the winter wore on, and I gained some weight, and now I’m back to 5Ks instead.

In my quest to return to former glory, I redownloaded the “couch to half-marathon” app and spent $100 on a pair of New Balance shoes. I became interested in a genetic-testing kit that promised to provide the information I needed to complete 13.1 miles once and for all.

Genetic testing for wellness seems like a natural choice for someone like me. I still wear my Fitbit every day two years after getting it, making me one of the few and the proud that have stuck with their device for longer than six months. I own a smart scale and a running watch; I track calories and read training forums. But the forum...