The MitochondriaThe mitochondria are interesting structures. For one, they aren't original parts of our cellular machinery. The current theory is that they were a bacteria that was engulfed by a eukaryote, a type of cell that contains organelles as well as a nucleus that separates the cell's DNA from the rest of the cell. Eukaryotes are the cells that make up most of the life on the planet. In exchange for protection within their predator, the mitochondria can provide energy via aerobic pathways of energy that the eukaryotes cannot use. Without the mitochondria, life on the planet would be quite different, if it existed at all.
Also, mitochondria have their own DNA which mostly produces structures responsible for a form of energy metabolism called the electron transport chain. This DNA is similar in structure to a bacteria's DNA and is not protected by it's own membrane. For these reasons, the amount of energy produced by the mitochondria can have major effects on your risk for diseases such as Alzheimer's Disease, Parkinson's Disease, Type 2 Diabetes, Cancer, and Cardiovascular Disease.
Free radicals and diseaseA common thread among many of these diseases that are prevalent today is high levels of free radicals. Free radicals are unstable molecules that contain unpaired electrons. Since these molecules strive for stability, they steal electrons from other molecules making them free radicals in the process and causing them to become unstable. This can cause your cells to malfunction. Reducing free radicals is fairly important for health, but taking antioxidants doesn't typically work because antioxidants work by donating electrons, which makes the antioxidant a free radical itself. The best way to reduce free radicals is to not make many in the first place.
How does this relate to your mitochondria? Another common thread in all of these diseases is mitochondrial dysfunction. Recall from above that one of the ways the mitochondria generate energy is via the electron transport chain. The electron train is a form of producing energy where complexes in the mitochondria pass around electron to generate energy. Do you see a common thread between free radicals and the electron transport chain? As a natural byproduct of energy production via the electron transport chain, some electrons leak out and react with oxygen forming the free radical superoxide, which is very reactive.
While the mitochondrial DNA is contained within the mitochondrial membrane, it is not separated from the electron transport chain by a membrane. This means that free radicals can react with and damage the mitochondrial DNA. Since the mitochondrial DNA contains the instructions for building the electron transport chain, this can cause the electron transport chain to malfunction, potentially generating even more free radicals that can damage the cell. A relatively recent study showed that cells with damaged mitochondrial DNA or a malfunctioning electron transport chain generate more free radicals than healthy cells(1).
So why exercise at all? Why not just sit on your butt all day long? At the proper dosage, exercise will activate antioxidant systems that will neutralize these free radicals. In fact, one of the worst things you can do is just sit around as sitting will reduce the expression of genes that make your cells' internal antioxidant system, the NRF-2 pathway.
Breaking up prolonged periods of sitting has been shown to increase expression of these genes which increases the amount of glutathione your cells make, their master antioxidant(2). Overeating while being sedentary most of the day will generate free radicals in the cell because they have to process that energy, and sitting down will reduce the amount of antioxidants that will take care of those free radicals. Cruciferous vegetables such as broccoli, the omega 3 fatty acid DHA, blueberries, turmeric, and most sulfur containing vegetables will activate this system as well. This is most likely the mechanism by which broccoli and other cruciferous vegetables exert their anti-cancer effect.
ConclusionSo how does all of this relate to eating fewer calories and moderate exercise being healthy? Obviously you will generate fewer free radicals by generating less energy through your mitochondria because you will be passing fewer electrons around inside of them. As such, you will cause less free radical damage to your mitochondrial DNA which will prevent the excess free radical production associated with mitochondrial dysfunction and a poorly functioning electron transport chain.
When you overeat, you are generating more free radicals because you are forcing more energy through the electron transport chain. The same goes for excessive exercise. Moderate exercise is the goal because some exercise is necessary for other parts of your body to function properly, and just sitting all day creates free radicals without generating antioxidants to take care of them. It is also necessary to increase the number of mitochondria you have within your cells. A higher number of mitochondria is ideal because it allows you to generate more energy while generating fewer free radicals within each one. This causes less damage to each mitochondria and decreases the likelihood that they will malfunction.
So what should you do if you are just looking to be healthy? Basically what I've recommended in nearly every blog I've written. Eat a Paleo Diet relatively low in carbohdyrate(150g per day or so), get 10,000 steps per day, stay off your butt, and strength train a couple of days per week to maintain muscle mass. Avoid things like bootcamps, high volume/high intensity exercise, extremely long duration cardio, and worst of all sitting.