Dr. Francisco J. Villarreal of University of California, San Diego, said epicatechin is a flavonoid found in dark chocolate.
The researchers examined five profoundly ill patients with major damage to skeletal muscle mitochondria -- structures responsible for most of the energy produced in cells. These "fuel cells" are dysfunctional as a result of both type 2 diabetes and heart failure, leading to abnormalities in skeletal muscle, Villarreal said.
Patients with heart failure and diabetes experience abnormalities in both the heart and skeletal muscle that can result in impaired functional capacity. They often complain of shortness of breath, lack of energy and have difficulty walking even short distances.
Trial participants consumed dark chocolate bars and a beverage with a total epicatechin content of approximately 100 milligram per day for three months. Biopsies of skeletal muscle were conducted before and after treatment.
After three months, the researchers looked at changes in mitochondria volume and the abundance of cristae, are internal compartments of mitochondria necessary for efficient function of the mitochondria.
"The cristae had been severely damaged and decreased in quantity in these patients," Villarreal said in a statement. "After three months, we saw recovery -- cristae numbers back toward normal levels, and increases in several molecular indicators involved in new mitochondria production."
The findings were published in the journal Clinical and Translational Science.