The most abundant elements by mass in the body of a healthy human adult are:
Oxygen (61.4%); Carbon (22.9%), Hydrogen (10.0%); and Nitrogen (2.6%). The weight which a 75 kg person would gain if all 1H atoms are replaced by 2H atoms is...
Since H is 10% and 1H atoms are replaced by 2Hatoms then weight of hydrogen is just doubled. Weight gain will be 75 x .1 = 7.5 kg