They have also evolved to require a molecule called nicotinamide adenine dinucleotide, or NAD. As we will see later, the loss of NAD as we age, and t… - David A. Sinclair

Here's the important point: there are plenty of stressors that will activate longevity genes without damaging the cell, including certain types of exercise, intermittent fasting, low-protein diets, and exposure to hot and cold temperatures (I discuss this in chapter 4). That's called hormesis.28 Hormesis is generally good for organisms, especially when it can be induced without causing any lasting damage. When hormesis happens, all is well. And, in fact, all is better than well, because the little bit of stress that occurs when the genes are activated prompts the rest of the system to hunker down, to conserve, to survive a little longer. That's the start of longevity.

The way doctors treat illness today "is simple," wrote S. Jay Olshansky, a demographer at the University of Illinois. "As soon as a disease appears, attack that disease as if nothing else is present; beat the disease down, and once you succeed, push the patient out the door until he or she faces the next challenge; then beat that one down. Repeat until failure.

A new scientific truth does not triumph by convincing its opponents and making them see the light, Planck wrote shortly before his death in 1947, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.