Most people are familiar with the concept of fat oxidation, although it's more frequently referred to by its colloquial name, "Fat burning." You don't actually burn fat in the same way that a fire burns, but rather the fat undergoes a few chemical changes that result in its breakdown. In the body, the oxidation of fat requires a certain amount of oxygen, which explains why aerobic exercise is usually considered the superior form of exercise for fat reduction. Of course, you can still lose excess body fat by following a strict diet that entails consuming fewer calories than you expend as energy. Then again, while lowering calories is the cornerstone of any diet, where those calories come from does make a difference in the rate and extent of body fat loss. Although it's a debatable issue among researchers, maintaining a lower output of insulin usually opens the metabolic door to greater rates of fat loss. This is why low carbohydrate and ketogenic or ultra-low-carb diets containing 20 grams of carbohydrates a day or less are probably the most popular and effective fat-loss diets. But fat oxidation can also refer to another process, whereby fat becomes oxidized due to the presence of excessive oxygen. In this case, the fat isn't "burned" but rather goes rancid. The usual culprit in this scenario is free radicals and reactive oxygen species (ROS). Free radicals are simply unpaired electrons. Electrons normally occur in pairs, but when an electron breaks free, it becomes a free radical. The free radical lives to combine with other electrons, including those that are already paired. When this happens, the normal electron pair undergoes chemical changes that result in oxidation.
This process of free radical production goes on constantly since it's a product of normal oxygen metabolism. Since the greatest level of oxygen metabolism occurs in the portion of the cell called the mitochondria, this is also the site of the greatest free radical and ROS production. The mitochondria are cigar-shaped organelles in the cell that are thought to have evolved from an ancient partnership with commensal bacteria, which enjoyed the internal environment of cells. Precisely how these bacteria turned into cellular structures remains a mystery as do many other aspects of the evolution of the human body. In any case, most of the free radicals that cause the bad type of fat oxidation come from the mitochondrial metabolism of oxygen. The free radicals cause havoc by attaching to electrons in structures that are vulnerable to oxidation, such as cellular membranes. By doing so, the free radical-induced oxidation of the polyunsaturated fat found in those cell membranes compromises the integrity of the cell, allowing potential toxic substances to enter the cell, such as mutagens that can alter nuclear material in the cell, such as DNA and thus cause cancer. Free radicals and ROS . . .