Please recommend a nail strengthener.

My fingernails break and snap easily. I went to the hospital, and they said they break because they lack nutrition. So...

Even though I eat well, my nails break every two weeks. I mentioned this to a personal trainer, and they advised me to apply a nail strengthener or clear nail polish. They said that would prevent them from breaking. Since my nails are naturally a bit weak, I tried applying colored nail polish in double layers, but it felt too heavy and tight, so I removed it. Both cream-type and topcoat forms are fine. Please recommend a nail strengthener or serum.