In addition to lactate, exercise intensity also affects the muscle's ability to produce other compounds known as myokines. So these are molecules released from muscle cells that signal to non-muscle tissues that the body is physically active. So again, it's similar to what we've been talking about with lactate. Myokines have anti-inflammatory, they have anti-cancer effects. They also participate in metabolic pathways involved in fat oxidation, glucose uptake. They play a role in, again, cancer biology as well. So generally speaking, the greater the intensity of exercise, the greater the myokine release. Again, it's one of those, you're putting stress on the muscles and the muscles are then forced to adapt. And one of the adaptations is releasing myokines. Again, duration also matters.

So the harder and the longer the muscles work, the greater the myokine release. Some myokines are a little more sensitive to exercise intensity. So IL-6 is probably one of the most well-known myokines. It was initially thought to be a pro-inflammatory cytokine. So a cytokine that plays a role in instigating inflammation. When it is produced from muscle, it acts as a myokine. So it does play a role in inflammation. But when it's produced from muscle during exercise, it's signaling to other tissues to have an anti-inflammatory response. And so you'll often find— an even larger production of anti-inflammatory cytokines such as IL-10 in response to IL-6 produced during exercise. Irisin is another myokine. It's involved in cancer protection. It's involved in bone health, metabolism, and more.

And then there's also another well-known myokine known as oncostatin M. And that myokine sort of plays a little more prominent role in, you know, anti-cancer effects. The key here is I think that exercise intensity and/or duration are really what increase myokine levels. So you're going for a 2 or 3-mile run, probably crank it up a little more intense, right? You want to be at, you know, at least 85% your max heart rate. So you're not going to be really talking during that run. Are you going on an 8-mile run? Maybe duration is on your side, right? And intensity can go down a notch. Since we were just talking about cancer, keep in mind that any level of physical activity is better than none.

So a study investigating physical activity in breast cancer and colorectal cancer found women who were more physically active before being diagnosed with breast cancer had about a 23% reduced risk of dying from any cause and a 23% reduced risk of dying from breast cancer compared to those who were less active. Those who were more active before being diagnosed with colorectal cancer had a 26% reduced risk of dying from any cause and about a 25% reduced risk of dying from colorectal cancer. So being active after diagnosis had even stronger benefits for both cancer types. So these women had a 48% reduced risk of dying from any cause and a 28% reduced risk of dying from breast cancer compared to those that were less active after diagnosis.

And the individuals with colorectal cancer had a 42% reduced risk of dying from any cause and a 39% reduced risk of dying from colorectal cancer, again, after being diagnosed with the cancer. So I do want to talk a little bit about, you know, there's a lot of mechanisms by which exercise can, you know, dampen cancer metastasis, can improve cancer outcomes. But there's one specific mechanism that involves tumor cells escaping from the original tumor site and then traveling into circulation to other locations, other organs and other tissues. They take camp there, they grow and divide, and they form a secondary tumor. Those cancer cells are called circulating tumor cells, and that process I just described is often called cancer metastasis.

Those circulating tumor cells are in circulation for a period of time, and these circulating tumor cells are very sensitive to the shearing forces of blood flow. So when you exercise and blood flow increases, those circulating tumor cells, they actually die. And this happens in a dose-dependent manner. So the more intense the exercise, the more sensitive they are to cell death. Duration's also a key, right? So, I mean, you're talking about increasing the shear forces. Both intensity and duration are key here. And so it's really just a matter of getting that blood flow up, intensity, duration, and that has been shown to kill circulating tumor cells. And again, this is associated with more positive outcomes with respect to cancer survival.

Not only— we talked a lot about muscles being little chemical-producing factories. They're producing chemicals like lactate. They're producing myokines. But they also act like sponges to soak up compounds that can be harmful to the brain. So skeletal muscle has the ability to take up a compound known as kynurenine and convert it into kynurenic acid, which is a nontoxic metabolite. And it does this by increasing an enzyme on the muscle called kynurenine aminotransferase. So this essentially reduces the amount of kynurenine available to then be transformed into other harmful metabolites, such as quinolinic acid in the brain. So quinolinic acid is a neurotoxin that plays a role in depression. It plays a role in schizophrenia and neurodegenerative disease.

And so this is just one other mechanism by which exercise also seems to improve mental health. It's also another way that, again, exercise intensity is important here. That is what is increasing that enzyme, that kynurenine aminotransferase on the muscle cells to then transport kynurenine into the muscle so that it's not converted into the quinolinic acid. So just another mechanism that I kind of wanted to point out because, again, intensity plays a role here with those aminotransferase enzymes processing more of that kynurenine.