Exercise Prevents Spending Calories on Fat Storage and Other Deleterious Biological Processes

The reason for that is because exercise causes stress and that stress turns on a whole raft of beneficial effects. And there are really two kinds of this stress. Now, the first kind is energetic stress. Like I went for a run this morning, I spent probably about 500 calories running and I can only do so many things with the calories my body has. And if I spend 500 extra calories exercising, that means I'm not gonna spend 500 calories on other activities that my bother body might engage in.

And one of them is fat storage. Everybody knows, I hope they know that physical activity is one of the best ways, maybe the best way to prevent weight gain. So a lack of physical activity is one of the ways that encourages weight gain. And in a famous experiment, for example, Bente Peterson and Copenhagen took a whole bunch of young healthy Danes, right, who are normally very active and made them sit on a couch for two weeks and eat the same amount of food. And she scanned them before and after those two weeks, and just in two weeks, they gained 7% more body fat, including seven more percent in their bellies. And that's concerning because belly fat or visceral fat is highly inflammatory.

Now, another study also looked at another kind of energetic stress, which is how you allocate energy towards reproduction. Remember, I can only do so many things with my calories. So if I'm spending more energy and physical activity, I'm not gonna spend as much energy on reproduction. And in a famous study done here at Harvard, my colleague Peter Ellison looked at women who are running 20 kilometers a week, right? That's not a huge amount. So that's about 180 calories a day. And compared it to sedentary women and the sedentary women were taking that extra energy and they're plowing it into being into reproduction by increasing their levels of estrogen and progesterone. 50% higher levels in the second half of the menstrual cycle and higher levels of progesterone and estrogen increase rates of cancer. So the energetic stress from physical activity decreases inflammation, which is involved in everything from metabolic syndrome to cancer to heart disease. And it also decreases reproductive hormones which decrease cancer rates.