Extra fat poses particular problems for woman in sports. In addition to aggravating rheumatoid arthritis of the knees, encouraging high blood pressure, making you more resistant to your own insulin (encouraging diabetes if you have that tendency), and setting you up for kidney trouble, cirrhosis of the liver, and gall-bladder disease, fat makes you much less efficient at using oxygen, so it slows you down and decreases your endurance.
Fat also slows you down because you are carrying around dead weight – as if you were exercising while carrying a 15- or 30-pound suitcase. According to Kenneth J. Cureton and colleagues at the University of Georgia’s Human Performance Laboratory, 5 percent extra fat can make you 3.9 percent slower, 10 percent extra fat can make you 5.8 percent slower, and 15 percent extra fat can make you 8.6 percent slower.
Fat rubs between the muscle fibers, acting as a friction brake on the smooth motion of your muscles and thus the speed and power of your movement. Finally, a thick layer of fat insulates you. In cold weather that’s fine, but in hot weather you can’t cool yourself, and you risk heat exhaustion or heatstroke.