To get a concept of our weight gain due to the laws of physics, here is how you must reason.
1. To run you are an object that moves in a medium, which in this case is air.
2. To run at a relativistic speed, you must run faster than the speed of the molecules of gas that constitute the medium of air. So, you must run at about 1195 km/hr, reaching Mach 1 and breaking the sound barrier. This may lead you into free space where Aether and vacuum interact and the speed of aether particles (arguable since many physicists do not accept Faraday's theory of aether) is the speed of light in vacuum at 3e8 m/s. As you approach this speed, relativistic effects appear and you could gain mass.
Pertinent answers include
1. @poldi2 - You can't run fast enough to change your weight enough to detect with any ordinary scale @Morningfox, an energy-mass equivalence of 8.7 x 10^-16 (100 kg person) = 0.087 nanograms, @Billy-Bob unless you're capable of running close to the speed of light @Dick where you gain equivalence mass that is not that the object in motion actually becomes heavier..it just has the energy from a reference frame, @Andrew Smith due to the stress-energy tensor, @ Joseph (myself).
You would find stress-energy tensor useful in astronomy where a comet in motion at high speed is found to shrink when its length decreases and the relativistic mass increases to infinity - formula the total energy of a particle moving at speeds close to the speed of light (relativistic speeds) is given as mc^2/((1-(v^2/c^2)^1/2).?
But, strictly if you do take supplements (vitamins and minerals) and go running at least for half an hour each morning, you'd gain muscle mass and be healthy.