The fat mass (or FM, from the English Fat Mass) represents the totality of the lipids present in the human body. Usually expressed as a percentage of the total body mass, it consists of two components: the primary fat and the storage fat.
Essential fat, or primary fat, means the amount of fat contained in the central nervous system, bone marrow, mammary glands, kidneys, spleen and other tissues. Given this particular anatomical location, essential fat has a physiological role of primary importance, to the point of being considered:
the minimum percentage of fat mass compatible with a state of good health.
For males, this value should not fall below 3-5% (already at these levels there is a greater susceptibility to infections), while in females the primary fat should be above 12% (already at levels less than 16 percentage points some athletes become amenorrheic, with a significant loss of bone minerals).
The fat deposited in the adipose tissue is the body's main energy reserve; it is found above all in the subcutaneous level, but also in the visceral area (the proportion between the two varies according to age, sex, ethnicity and state of physical fitness). Normal values are around 12%.
For what has been said so far, if mathematics is not an opinion, in the reference man and woman the fat mass represents, respectively, 15 and 24% of the total body mass. These values are much lower in athletes, where they reach levels close to the percentage of primary fat, and definitely higher in the obese.
The determination of fat mass in vivo can take place according to different methodologies, which differ in practicality, accuracy and costs (plicometry, bioimpedance analysis, body circumferences, Dexa, creatinine, magnetic resonance, CT, K40 and ultrasound). An even simpler and more immediate method consists in calculating the lean mass starting from the stature and from some body circumferences, according to the formula developed by Wilmore and Behnke:
Men FM (%): 495 / {1.0324 - 0.19077 [log (waist-neck)] + 0.15456 [log (stature)] - 450
Women FM (%): 495 / {1.29579 - 0.35004 [log (waist + hip-neck)] + 0.22100 [log (stature)]} - 450
By comparing the result obtained with the values in the table, you can get an idea of your physical fitness level.
| Rating | Men | Women |
| Minimum weight, danger to health | 2% - 4% | 10% - 12% |
| Athletic form | 6% - 13% | 14% - 20% |
| Good state of fitness | 14% - 17% | 21% - 24% |
| Above average | 18% - 25% | 25% - 31% |
| Obesity | ≥ 26% | ≥ 32% |
