To answer this, please read the following quote from Professor George Birkmayer, who is the world expert on the subject:

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The most promising natural substance in our body is NADH, which stands for nicotinamide adenine dinucleotide hydride. NADH is the biological form of hydrogen. It reacts with the oxygen present in every living cell, thus producing energy and water. The more NADH a cell has available, the more energy it can produce, the better it functions, and the longer the cell (and the entire organism) lives.
Is it possible to increase the amount of NADH in the cell by adding NADH from outside? The answer is: yes. This implies that we can increase the energy level in our cells. Due to this, the cells can produce more of all the components essential for life, and thus they will function better and live longer. This is feasible by supplementation with NADH in order to boost the hydrogen taken up by the human body.

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The amount of NADH a cell contains depends on the amount of energy it requires. The heart and the brain need the most energy of all our organs. Hence, these organs benefit the most from an external supply of NADH. All other organs, particularly the lungs, the liver, and the kidneys, also get more energy from NADH and function better. The biological hydrogen is the fuel for cellular energy production, and nutritional supplementation can provide our body with more NADH.