The most frequent BMI values* of men (blue) and women (red) from birth to old age. The green shaded area represents the "normal weight" according to the WHO, from BMI 18.5 to 25, beginning at the age of 20 years. © SBMIC
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Four phases of BMI development
Phase 1: Baby fat
Newborn body mass must be low enough to pass through the birth canal. After birth, babies accumulate fat mass as an energy reserve for growth and as a protection against impacts in cases of falling and other accidents. The BMI reaches a peak at about the age of seven months.
Phase 2: Initial lankiness
Toddlers use up most of their baby fat, and the shape of the body begins to change. The growth in length exceeds the growth in mass, thereby resulting in a decreasing body mass index. Its minimum is reached at the age of about six years.
Phase 3: Growing up
In childhood and adolescence the body grows both in length and in mass. The latter increases to a faster extent, resulting in an increasing body mass index. The body gradually develops into an adult shape.
Phase 4: Adulthood
After the body has reached its final length, the BMI curves flatten, but continue to increase at a slower speed because a certain amount of body mass is still accumulated with age.
Difference between men and women
Adolescence
The BMI increase in boys and men is sharper than in girls and women, as the boys and men are generally more heavily built.
Early adulthood
The muscle mass reaches its peak at around 25 to 30 years of age. This is the point where both curves take a distinct downward turn. The BMI is higher in men than in women due to their higher muscle mass which is also accompanied by a higher mass of bone, skin, and fat.
Middle to old age
In men, the BMI curve takes a continuous downward turn, then later forming a very slowly increasing plateau at old age.
In women, the BMI curve retains a continuous upwards trend, reapproaching the male curve to some extent until old age. This is most likely due to the fact that women retain a certain gain in weight after pregnancies. In addition, their fat mass is generally higher which may result in a more pronounced fat accumulation with age.
*Sources: Kromeyer-Hauschild K, Wabitsch M, Kunze D et al. Monatsschr Kinderheilkd 2001, 149:807 (children and juveniles); unpublished user data of a life-expectancy calculator 1999–2013 (1.1 million adults 20–65 years).
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Fat-free mass
Fat or non-fat, that is the BMI question. But a BMI as high as this is far more likely to be caused by fat than by muscles.
