Carbon has several different isotopes. Carbon 12 (6 protons and 6 neutrons) is the most common but Carbon 13 (6 protons and 7 neutrons) is also present in the human body. Both isotopes metabolize the same way so both can be used by the human body in the same way. If all the Carbon 12 atoms in the human body were converted to Carbon 13 (one extra neutron in each nucleus) then a 150 pound human being would gain about 2 pounds.

(The Great Courses – Origins of Life)