As far as HowStuffWorks.com can tell, no one's ever put a centenarian on a near-starvation diet. That kind of research has been performed instead on worms, yeast and rats. That last example has yielded some particularly important findings, as what we see in rats (as fellow mammals) often translates to humans as well. Lab tests on rats found that those that eat 40 percent fewer calories than their counterparts -- called caloric restriction in the field of longevity research -- tend to live longer, healthier lives. The same goes for worms and yeast. Why would this be?
It turns out that for such an elegant system, the human body can be fairly rough on itself. Our bodies crave certain foods but doesn't appear to take into account how the process of eating affects our cells -- specifically, how insulin can damage them. Say, for example, a person develops an insatiable desire for the chocolate bar that he or she craves, blood sugar levels spike, spurring insulin production. Normally, this is a good thing because insulin governs glucose concentration in blood. However, it can also damage or even kill cells. Cellular death is what causes death by old age; as we age, cells stop multiplying and when enough do, our bodies simply can't function properly anymore.
While insulin has been shown to have devastating effects on cellular health, it's just one of many potentially harmful hormones that our bodies use to carry out cellular processes. Adrenaline's another good example; we use it to spur the fight-or-flight response that allows us to escape or ward off danger, but it can also create cardiac bands -- blown-out strips of cells -- on heart tissue.
The body may be overly harsh on itself when carrying out these processes, but it's also aces at protecting itself from external conditions. This is where the starvation diet comes in. One hypothesis states that a calorie-restricted diet causes the body to enter survival mode. In this state, the body's survival mechanisms -- and one gene in particular -- kick in. In fact, two wholly independent studies (one out of Harvard, one from Massachusetts Institute of Technology -- published within 16 days of each other in June 2004) found that the SIRT1 gene begins to both combat cellular destruction and cause the body to burn fat to make up for the carbohydrates and proteins that the body doesn't receive through diet. This combination of shedding fat and protecting cells creates a perfect storm of longevity, according to the Harvard and MIT researchers.