Deuterium is like thick, gluggy oil - when you put thick oil into an engine, the engine sputters, makes strange noises, and eventually breaks.
Nature has put systems in place to deplete deuterium and protect the nanomotors, or "little engines," in our cells’ mitochondria from coming into contact with this thick oil.
However, the side effects of a modern life - pollution, global warming, processed foods, less healthy lifestyles, etc. - have resulted in many people having way too much deuterium inside their cells. This results in an inability to effectively deplete deuterium and the destruction of our nanomotors.
This starts a vicious cycle of deuterium building up and breaking more of our nanomotors. Fewer nanomotors means less energy and more sickness and disease.