For example, if you're obese, it takes more energy for you to breathe because your heart has to work harder to pump blood to the lungs and to the excess fat throughout the body. This increased work load can cause your heart to become enlarged and can result in high blood pressure and life-threatening erratic heartbeats.

Obese people also tend to have high cholesterol levels, making them more prone to arteriosclerosis, a narrowing of the arteries by deposits of plaque. This becomes life-threatening when blood vessels become so narrow or blocked that vital organs like the brain, heart or kidneys are deprived of blood. Additionally, the narrowing of the blood vessels forces the heart to pump harder, and blood pressure rises. High blood pressure itself poses several health risks, including heart attack, kidney failure, and stroke. About 25 percent of all heart and blood vessel problems are associated with obesity.

Clinical studies have found a relationship between excess body fat and the incidence of cancer. By itself, body fat is thought to be a storage place for carcinogens (cancer-causing chemicals) in both men and women. In women, excess body fat has been linked to a higher rate of breast and uterine cancer; in men, the threat comes from colon and prostate cancer. 5:27 PM 25/07/2009

There is also a delicate balance between blood sugar, body fat, and the hormone insulin. Excess blood sugar is stored in the liver and other vital organs; when the organs are "full," the excess blood sugar is converted to fat. As fat cells themselves become full, they tend to take in less blood sugar. In some obese people, the pancreas produces more and more insulin, which the body can't use, to regulate blood sugar levels, and the whole system becomes overwhelmed. This poor regulation of blood sugar and insulin results in diabetes, a disease with long-term consequences, including heart disease, kidney failure, blindness, amputation, and death. Excess body fat is also linked to gall bladder disease, gastro-intestinal disease, sexual dysfunction, osteoarthritis, and stroke.

Adipose tissue or body fat or just fat is loose connective tissue composed of adipocytes. Adipose tissue is derived from lipoblasts. Its main role is to store energy in the form of fat, although it also cushions and insulates the body. Obesity or being overweight in humans and most animals does not depend on body weight but on the amount of body fat—specifically, adipose tissue. Two types of adipose tissue exist: white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue also serves as an important endocrine organ by producing hormones such as leptin, resistin and the cytokine TNFa. The formation of adipose tissue appears to be controlled by the adipose gene. Adipose tissue was first identified by the Swiss naturalist Conrad Gessner in 1551.

One's body tends to deposit fat according to your individual genetic code. In other words, hereditary characteristics dictate areas in your body that accumulate fat. If you are a typical female, you will accumulate fat around your thighs and hips. Typically, males accumulate fat around the midriff and lose it there last. The essential fat, required for normal physiological functioning, consists of fat stored in the marrow of bones, heart, lungs, liver, spleen, kidneys, intestines, muscles and lipid rich tissues of the central nervous system. In females, the extra 9% of sex specific fat is required for childbearing and other hormonal related functions. In addition to essential fat deposits, storage fat consists of fat accumulation in adipose tissue. Men and women have similar quantities of storage fat - on average 12% for men and 15% for women.

A person's total body fat percentage is the total weight of the person's fat divided by the person's weight. The resulting number reflects both essential fat and storage fat. A person's exact body fat percentage generally cannot be determined, but there are several techniques which can be used to estimate it accurately.

Ideal Body Weight and Body Fat Percent

The ideal weight and fat-lean ratio varies considerably for men and women and by age, but the minimum percent of body fat considered safe for good health is 5 percent for males and 12% for females. The average adult body fat is closer to 15 to 18% for men and 22 to 25% for women. Athletes tend to be at low end of this scale due to their increased lean weight (muscle mass). While low levels of body fat seem to be related to improved performance, body composition alone is not a great predictor of sports success. A linebacker needs to have enough body mass (lean and fat weight) to generate high forces and avoid injury. Body fat among elite athletes vary largely by sport. There is little evidence of any benefit when men drop under 8% and women drop under 14 percent body fat.

Essential fat is that amount of fat necessary for maintenance of life and reproductive functions. The percentage for women is greater than that for men, due to the demands of childbearing and other hormonal functions. Essential fat is 2–5% in men, and 10–13% in women. Storage fat consists of fat accumulation in adipose tissue, part of which protects internal organs in the chest and abdomen. The minimum recommended total body fat percentage exceeds the essential fat percentage value reported above. A number of online tools are available for calculating estimated body fat percentage.

Arguably, body fat percentage is the superior gauge of an individual's fitness level, as it is the only body measurement which directly calculates the particular individual's body composition without regard to the individual's height or weight. The widely-used body mass index (BMI), on the other hand, simply makes blanket assumptions as to what every individual of a certain height should ideally weigh, regardless of the body composition which makes up that weight. The BMI gives particularly inaccurate information with regard to individuals with above-average lean muscle mass, classifying such individuals as "overweight" or "obese" despite the fact that their body fat percentage would indicate they are in excellent physical condition

The American Council on Exercise has categorized ranges of body fat percentages as follows: