Overweight is having more body fat than is
optimally healthy. Being overweight is
common especially where food supplies are
plentiful and lifestyles are sedentary.
Excess weight has reached epidemic
proportions globally, with more than 1
billion adults being either overweight or
obese in 2003.[1] In 2013 this increased to
more than 2 billion.[2] Increases have been
observed across all age groups.
A healthy body requires a minimum amount
of fat for proper functioning of the
hormonal, reproductive, and immune
systems, as thermal insulation, as shock
absorption for sensitive areas, and as energy
for future use. But the accumulation of too
much storage fat can impair movement,
flexibility, and alter appearance of the body.
Classification
See also: Body fat percentage
The degree to which a person is overweight
is generally described by body mass index
(BMI). Overweight is defined as a BMI of 25
or more, thus it includes pre-obesity defined
as a BMI between 25 and 30 and obesity as
defined by a BMI of 30 or more.[3][4] Pre
obese and overweight however are often
used interchangeably thus giving overweight
a common definition of a BMI of between 25
-30. There are however several other
common ways to measure the amount of
adiposity or fat present in an individual's
body.
Body mass index
The body mass index (BMI) is a measure
of a person's weight taking into account
their height. It is given by the formula:
BMI equals a person's weight (mass) in
kilograms divided by the square of the
person's height in metres. The units
therefore are kg/m2 but BMI measures
are typically used and written without
units.
BMI provides a significantly more accurate
representation of body fat content than
simply measuring a person's weight. It is
only moderately correlated with both
body fat percentage and body fat mass (R2
of 0.68).[5] It does not take into account
certain factors such as pregnancy or
bodybuilding; however, the BMI is an
accurate reflection of fat percentage in
the majority of the adult population.
Body volume index
The body volume index (BVI) was devised
in 2000 as a computer, rather than
manual, measurement of the human body
for obesity and an alternative to the BMI
Body volume index uses 3D software to
create an accurate 3D image of a person
so BVI can differentiate between people
with the same BMI rating, but who have a
different shape and different weight
distribution.
An obese man on a motorcycle
Belly of an overweight teenager.
Children with varying levels of
body fat
BVI measures where a person's weight
and the fat are located on the body, rather
than total weight or total fat content and
places emphasis on the weight carried
around the abdomen, commonly known as
central obesity. There has been an
acceptance in recent years that abdominal
fat and weight around the abdomen
constitute a greater health risk.[6]
Simple weighing
The person's weight is measured and
compared to an estimated ideal weight.
This is the easiest and most common
method, but by far the least accurate, as it
only measures one quantity (weight) and
often does not take into account many
factors such as height, body type, and
relative amount of muscle mass.
Skinfold calipers or "pinch test"
The skin at several specific points on the
body is pinched and the thickness of the
resulting fold is measured. This measures
the thickness of the layers of fat located
under the skin, from which a general
measurement of total amount of fat in the
body is calculated. This method can be
reasonably accurate for many people, but
it assumes particular fat distribution
patterns over the body—which may not
apply to all individuals, and does not
account for fat deposits not directly under
the skin. Also, as the measurement and
analysis generally involves a high degree
of practice and interpretation, an accurate
result requires that a professional
perform it. It cannot generally be done by
patients themselves.
Bioelectrical impedance analysis
A small electric current is passed through
the body to measure its electrical
resistance. As fat and muscle conduct
electricity differently, this method can
provide a direct measurement of the body
fat percentage, in relation to muscle mass.
In the past, this technique could only be
performed reliably by trained
professionals with specialized equipment,
but it is now possible to buy home testing
kits that let people do this themselves
with a minimum of training. Despite the
improved simplicity of this process over
the years, however, a number of factors
can affect the results, including hydration
and body temperature, so it still needs
some care when taking the test to ensure
that the results are accurate.
Hydrostatic weighing
Considered one of the more accurate
methods of measuring body fat, this
technique involves complete submersion
of a person in water, with special
equipment to measure the person's
weight while submerged. This weight is
then compared with "dry weight" as
recorded outside the water to determine
overall body density. As fat is less dense
than muscle, careful application of this
technique can provide a reasonably close
estimate of fat content in the body. This
technique does, however, require
expensive specialized equipment and
trained professionals to administer it
properly.
Dual-energy X-ray absorptiometry (DEXA)
Originally developed to measure bone
density, DEXA imaging is also used to
precisely determine body fat content by
using the density of various body tissues
to identify which portions of the body are
fat. This test is generally considered very
accurate, but requires a great deal of
expensive medical equipment and trained
professionals to perform.
The most common method for discussing
this subject and the one used primarily by
researchers and advisory institutions is BMI.
Definitions of what is considered overweight
vary by ethnicity. The current definition
proposed by the US National Institutes of
Health (NIH) and the World Health
Organization (WHO) designates whites,
Hispanics and blacks with a BMI of 25 or
more as overweight. For Asians, overweight
is a BMI between 23 and 29.9 and obesity
for all groups is a BMI of 30 or more.
BMI, however, does not account extremes of
muscle mass, some rare genetic factors, the
very young, and a few other individual
variations. Thus it is possible for an
individuals with a BMI of less than 25 to have
excess body fat, while others may have a
BMI that is significantly higher without falling
into this category.[7] Some of the above
methods for determining body fat are more
accurate than BMI but come with added
complexity.
If an individual is overweight and has excess
body fat it could, but won't always, create or
lead to health risks. Reports are surfacing,
however, that being mildly overweight to
slightly obese – BMI being between 24 and
31.9 – may be actually beneficial and that
people with BMI between 24 and 31.9 could
actually live longer than normal weight or
underweight persons.