The physiologic reasons that elderly people burn less fat are not known. Klein's team did not find any defect in the ability to mobilize body fat. There was little resistance of body fat to catecholamines, the hormones used to mobilize fat tissue for conversion to energy, and fat breakdown produced plenty of fatty acids for muscle to oxidize. Klein believes the answer lies in the muscles themselves. Changes that occur in aging muscle may help explain why our muscles no longer oxidize fat as they did when we were younger.
Muscle cells lose some of their mitochondria, the cellular structures that produce energy.
Loss of mitochondria may contribute to a loss in the ability to oxidize fat. So with a decrease in mitochondrial capacity, elderly muscles may simply be unable to oxidize as much fat as younger muscles. But even before the precise mechanisms are understood, there's good news for elderly people. The paper mentions that an intensive 16-week exercise program helped the elderly subjects oxidize fat more like younger participants. It comes back to about where it is in younger adults.