obese. In 1984 the national percentage of overweight
individuals was reported in a major assessment of the
nation’s health. Men having a BMI of 28 or greater
were considered overweight. This BMI number was
chosen to define overweight because 85% of American
men ages 20–29 fell below it. A different calculation,
not BMI, was used for women in the report.
In 1985, the term overweight was redefined as a
BMI equal to or greater than 27.8 for men and equal to
or greater than 27.3 for women. No BMI was selected
to define underweight individuals. This definition of
overweight was used in reports on obesity until 1998.
In 1998, the United States National Institutes of
Health revised its weight definitions to bring them in
line with the definitions used by the World Health
Organization. Overnight 30 million Americans went
from being classified as normal weight to being classi-
fied as overweight. Overweight is now defined for both
men and women as a BMI of 25 or less. At the same
time, an underweight classification was added, as was
the classification of obese for individuals with a BMI
greater than or equal to 30.
Interpreting BMI calculations for adults
All adults age 20 and older are evaluated on the
same BMI scale as follows:
BMI below 18.5: Underweight
BMI 18.5–24.9: Normal weight
BMI 25.0–29.9: Overweight
BMI 30 and above: Obese
Some researchers consider a BMI of 17 or below
an indication of serious, health-threatening malnour-
ishment. In developed countries, a BMI this low in the
absence of disease is often an indication anorexia
nervosa. At the other end of the scale, a BMI of 40
or greater indicates morbid obesity that carries a very
high risk of developing obesity-related diseases such as
stroke, heart attack, and type 2 diabetes.Interpreting BMI calculations for children
and teens
The formula for calculating the BMI of children
ages 2–20 is the same as the formula used in calculating
adult BMIs, but the results are interpreted differently.
Interpretation of BMI for children takes into consid-
eration that the amount of body fat changes as chil-
dren grow and that the amount of body fat is different
in boys and girls of the same age and weight.
Instead of assigning a child to a specific weight
category based on their BMI, a child’s BMI is com-
pared to other children of the same age and sex. Chil-
dren are then assigned a percentile based on their
BMI. The percentile provides a comparison between
their weight and that of other children the same age
and gender. For example, if a girl is in the 75th per-
centile for her age group, 75 of every 100 children who
are her age weigh less than she does and 25 of every 100
weigh more than she does. The weight categories for
children are:
Below the 5th percentile: Underweight
5th percentile to less than the 85th percentile:
Healthy weight
85th percentile to less than the 95th percentile: At
risk of overweight
95th percentile and above: OverweightApplication of BMI information
The BMI was originally designed to observe
groups of people. It is still used to spot trends, such
as increasing weight in a particular age group over
time. It is also a valuable tool for comparing body
mass among different ethnic or cultural groups, and
can indicate to what degree populations are under-
nourished or overnourished.
When applied to individuals, the BMI is not a
diagnostic tool. Although there is an established link
between BMI and the prevalence of certain diseases
such as type 2 diabetes, some cancers, and cardiovas-
cular disease, BMI alone is not intended to predict the
likelihood of an individual developing these diseases.
The National Heart, Lung, and Blood Institute rec-
ommends that the following measures be used to
assess the impact of weight on health:
BMI
Waist circumference (an alternate measure of body
fat)KEY TERMS
Anorexia nervosa—A psychiatric disorder signified
by obsession with weight loss and voluntary self-
starvation accompanied by serious, potentially
fatal health problems.
Morbid obesity—A term used to describe individ-
uals 100 lb (45 kg) or more than 50% overweight
and/or who have a body mass index above 40.
Triglycerides—A type of fat found in the blood.
High levels of triglycerides can increase the risk of
coronary artery disease.Body mass index