Deuterium is actually a rare type of hydrogen. Instead of just one proton in its nucleus, deuterium also has an extra neutron.
This makes it twice as big, and twice as heavy.
It is important to know that deuterium is not a toxin - it is naturally occurring and has a purpose. In nature, deuterium helps things grow.
However, having too much deuterium in your cells can result in mitochondrial dysfunction and lead to premature aging, metabolic problems, and disease.