In the D + T fusion reaction, the mass lost is approximately 0.0283 atomic mass units, equivalent to about 4.7 x 10^-29 kg. This mass loss is related to the energy produced, calculated using Einstein's equation E=mc², with the reaction yielding 17.6 MeV. All of the mass is not converted to energy, as a helium nucleus is formed in the process. The mass defect can be calculated using the formula m(D) + m(T) - m(4He) - m(n). Accurate experimental values can be found on reliable physics resources for verification.