A 78-year-old doctor says he's reversed his age by 20 years. Here are the 4 diet principles he follows to stay young.

• Dr. Michael Roizen is 78, but says his "biological age" is 57.6.
• He's the chief wellness officer at Cleveland Clinic, and takes his own advice on staying healthy.
• Dr. Roizen shared his diet principles with Business Insider, including sticking to the Mediterranean diet.

An expert in healthy aging who claims to have reversed his biological age by 20 years shared his diet principles with Business Insider.