Here's a most interesting summation of recent scientific findings concerning "Normal" Aging by Dr. Henry Lodge, Columbia University
You replace about 1% of your cells every day. That means 1% of your body is brand-new today, and you will get another 1% tomorrow. Think of it as getting a whole new body every three months. It’s not entirely accurate, but it’s pretty close.
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You choose whether those new cells come in stronger or weaker. You choose whether they grow or decay each day from then on. Your cells don’t care which choice you make. They just follow the directions you send. Exercise, and your cells get stronger; sit down, and they decay.
This whole system evolved over billions of years out in nature, where all animals face two great cellular challenges: The first is to grow strong, fast and fit in the spring, when food abounds and there are calories to fuel hungry muscles, bones and brains. The second is to decay as fast as possible in the winter, when calories disappear and surviving starvation is the key to life. You would think that food is the controlling signal for this, but it’s not. Motion controls your system.
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The other master signal to our cells—equal and, in some respects, even more important than exercise—is emotion. One of the most fascinating revelations of the last decade is that emotions change our cells through the same molecular pathways as exercise. Anger, stress and loneliness are signals for “starvation” and chronic danger. They “melt” our bodies as surely as sedentary living. Optimism, love and community trigger the process of growth, building our bodies, hearts and minds.
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Most aging is just the dry rot we program into our cells by sedentary living, junk food and stress.