“Exercise changes us. It increases the metabolic rate, increases the amount of muscle, raises the level of calorie-consuming enzymes inside the muscle, and increases the burning of fats. . . . It can also be shown that physically fit people have slightly elevated metabolism. Even when they are at rest fit people burn more calories than fat people do.”​—Fit or Fat?

A certain set of enzymes is needed for the burning of fats. If you do not have these fat-burning enzymes, “you are going to get fat. Enzymes will increase only if you stimulate the DNA by exercise and if you eat enough that there will be amino acids available for biosynthesis,” says Bailey.

At times muscles need sudden bursts of energy, increasing the demand fiftyfold in a split second. To get it, they must have enzymes capable of metabolizing the energy sources. Only in muscle cells are such enzymes to be found​—special enzymes with this capacity to burn calories so fast. Ninety percent of all the calories burned in the body are burned in muscles. These enzymes are found in the mitochondria scattered throughout muscle cells, and during exercise they promote the burning of fats in muscle tissue to supply energy.

Concerning these enzymes, Fit or Fat? says: “It has been shown repeatedly that steady aerobic exercise actually causes an increase in the number and size of mitochondria in each muscle cell. Further biochemical studies have confirmed that, with exercise, there is an increase in metabolizing enzymes inside those mitochondria.” Aerobics makes it happen; without it fat happens.