The human heart of song and story is a changeable, if not fickle, thing. But in one crucial sense, the organ has been seen as immutable. Scientists long thought that mammalian hearts stop producing most of their new cells shortly after birth, and that when they grow bigger, they do so primarily because the size of their existing cells increases. A recent study in The Journal of Physiology, however, confirms that exercise can substantially increase the number of cells in the hearts of young lab rats — and it also indicates, for the first time, that these additional cells are still present in mature hearts.
Researchers in Australia took young, healthy male rats and kept some of them sedentary while making others run on treadmills. This active cohort was divided into three groups, each of which began exercising at a different stage of life. In human terms, these starting points corresponded to childhood, adolescence and adulthood. After the rats were put through a month of daily hourlong workouts at a moderate pace, the hearts of some of them were examined microscopically. Exercise was then curtailed for the rest of the rats, which spent the next
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