Abstract: A composition effective for restoring mitochondrial and other cellular function and/or increasing mitochondrial energy in one or more cells contains adenosylcobalamin. Other aspects are directed to method of improving in a physiological state linked to metabolic fatigue in one or more cells and/or reducing fatigue in an individual; method of treating, reducing an incidence of, and/or reducing a severity of a chronic illness and/or of a mitochondria-related disease or condition associated with altered mitochondrial function or a reduced mitochondrial density.