The faster you move, the
heavier you get
This mass-energy equivalence was developed by Albert Einstein in
1905 in one of his Annus Mirabilis papers entitled "Does the
inertia of an object depend upon its energy content?” 
http://www.goodreads.com/book/show/643669.E_mc2
Energy, E, is proportional to mass, m, and the speed of light, c: E=mc2. What this formula describes is that energy and mass are two forms of the same things at the speed of the light. At the speed of the light mass can turn into energy and energy can turn into mass. The faster you move the more energy you are putting in, and the greater the mass you become.
http://www.eastfieldcollege.edu/smpe/Physics/
However, this is only applies for objects moving near the speed of the light. This is difficult for humans to perceive because we are moving too slow compare to the speed of light (186,000 miles/sec). Therefore, the phenomenon is negligible at human speeds.