 Why do centenarians escape cancer? People who live to be over 100 years old. As we get older, our risk of getting and dying from cancer grows year by year, but once you hit around 85-90, your cancer risk starts to drop. That kind of makes sense. I mean, if you didn't get it by then, maybe you're never going to get it. If you live that long, maybe it's a sign there's something special about you. It seems that centenarians are endowed with a particular resistance to cancer. So, what's their secret? Well, every day, 50 billion of ourselves die, and every day 50 billion new ones are born. There's a balance, otherwise our body would atrophy, shrink, or get too big and crowded. Now, sometimes we need to grow, like when we're a baby, or that growth spurred around puberty. Our cells don't get larger when we grow up, they get more numerous. So, a child's hand may only be made up about 50 billion cells, and may have to add a half trillion or so growing up. Once we've already grown up, we don't want a lot of extra cells hanging around, and we still need our cells to grow and divide, out with the old and with the new. We just don't want to be making more cells than we're putting out to pasture. When you're a kid, extra growth is good, but when you're an adult, extra growth can mean a tumor. How do our cells know when to tip the balance in favor of more dividing and less dying, and when to come back into balance? Well, a key signal is IGF-1, a growth hormone called insulin-like growth factor number one. Levels go up when you're a kid, so you grow, and then come back down. Should your level stay a bit too high as an adult, though? There's this constant message to our cells, grow, grow, grow, divide, don't die, keep going, keep growing. And so, not surprisingly, the more IGF-1 we have in our bloodstream, the higher our risk for cancer. More IGF-1, more prostate cancer. More IGF-1, more breast cancer. Of course, it's not the original tumor that tends to kill you, it's the metastases. IGF-1 is a growth factor. It helps things grow, so it helps cancer cells break off from the main tumor, migrate into surrounding tissues, and invade the bloodstream. What do you think helps breast cancer get into the bone? IGF-1 and the liver, IGF-1. Lung, brain, lymph nodes, IGF-1 helps transform normal cells into cancer cells in the first place, then helps them survive, proliferate, self-renew, grow, migrate, invade, stabilize the new tumors, and even helps hook the blood supply up to the new tumor. IGF-1 is a growth hormone that makes things grow. That's what it does. But too much growth when we're all grown up can mean cancer. Next, we'll turn to how we can prevent all this.