Everyone has fat and muscle in their body. Some people have more fat than others. In particular, the average female has a higher percentage of body fat than the average male of the same age. This makes a difference in how people perceive cold. The fat in your body serves to protect vital organs, and to store energy. But it doesn't do anything actively ... it just sort of sits there. Muscle, on the other hand, is made from fibres that are always contracting and expanding, even when you are standing still. This constant movement of the muscle tissue generates heat, and helps you keep warm when it's cold out. If it's really cold, this muscle movement becomes much larger, and you perceive it as shivering. You can hear this energy being produced by your muscles! Go to a quiet place and plug your ears with your fingers. You will hear a low rumbling sound. It's coming from the muscle fibres all over your body that are continually contracting and relaxing, and rubbing against each other as they move. The sound, at a frequency of 20-25 Hz, is transmitted down the bones in your arm and into your ear through your fingers. You can verify that it's happening by tensing the muscles in your arm and hand; the sound should increase in intensity. Women have more body fat than men, as a percentage of body weight, and less muscle. The average 30-year old woman's body is 26 percent fat, while a man of the same age has just 21 percent fat, with more muscle. As a result, the man's body will be generating more heat, all other factors being equal, than the woman's. This may be why women may often find a room unpleasantly cool, while men find it comfortable. Despite this warming effect of muscle, it is sometimes advatageous to have a higher percentage of body fat, if the environment demands it. Human subgroups which have evolved in cold climates in northern regions have a shorter, more rounded physique, resulting in a higher proportion of fat on their bodies, on average, and their diet (at least, in pre-industrial times) provided this fat. The extra fat served two purposes: Fat is an excellent insulator, and may help the body conserve heat in cold climates Fat stores energy, helping with survival in a hunting economy where there may be lean times In fact, fats are an important part of your diet. They are a concentrated source of energy, and fat in your body stores vitamins, including A, D, E, and K. Fat is necessary for healthy skin, to insulate the body, and to protect and support your internal organs. However, too much fat in your diet is not good for you. About half of all the people in North America are obese (at least 30% over their ideal body weight). Obesity (too much fat on the body) can lead to serious health problems including heart disease, diabetes, and high blood pressure. Because parts of the body have no fat covering them, the blood vessels are near the surface of the skin and can lose heat to the air very easily. This is why you should always wear a hat when it's cold out ... 30% of the heat loss you experience will be through the blood vessels in your head, if you don't wear a hat! Resources