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Metabolism doesn't slow with age
Spontaneous movement is a big lever. Researchers named Rothwell and Stock studied people who ate far more than seemed necessary and stayed lean, and found they moved constantly, burning up to eighteen hundred calories a day. For people who simply move a bit more, the difference is about a thousand calories a day.
Very active older people fit the pattern. Choreographer Twyla Tharp deadlifts, boxes, and works out for two hours a day at five in the morning, and Stanford's Mike Snyder, in his seventies, is moving, moving, moving. They are mentally sharp and physically lean.
Movement also keeps cells from carrying too much fuel. Overloaded with calories, the mitochondria and the cell's whole energy-management system get sick, and metabolism is the combined output of trillions of such cells.
How do researchers measure calories burned?
Researchers use a method called doubly labeled water. Volunteers drink water made with heavy isotopes of hydrogen and oxygen. Hydrogen leaves the body only as water, while oxygen leaves as water and carbon dioxide. The difference in the two rates shows how much carbon dioxide the body produced, and that figure converts to calories burned over one to two weeks. Nathan Lifson developed the technique in the 1950s.
How much does fidgeting vary between people?
In a 1999 study in Science, James Levine and colleagues at the Mayo Clinic overfed sixteen volunteers by 1,000 calories a day for eight weeks. Fat gain varied tenfold between people. The difference came mostly from changes in non-exercise activity, such as standing and fidgeting, which ranged from a decrease of about 98 to an increase of about 692 calories a day. Levine called this category non-exercise activity thermogenesis.
How can excess fuel damage mitochondria?
When more fuel arrives than a cell can burn, the mitochondrial electron transport chain backs up. Electrons leak out and form reactive oxygen molecules that damage proteins and membranes. In muscle, fat overload has been linked to impaired insulin signaling. A single cell can contain hundreds to thousands of mitochondria.
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