Written by Christian Nordqvist
Reviewed by University of
Illinois-Chicago, School of Medicine
People often compare their
weight and shape with those around them, concerned about their weight being too
high or too low, but a healthy weight is not the same for each individual.
It depends on a range of factors, including age, sex, body type,
bone density, muscle-fat-ratio, overall general health and height.
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The idea of a healthy
weight may also differ between countries or cultures. In the United Kingdom and
the United States, a healthy BMI is slightly
higher than, for example, in The Netherlands. If a person in the Netherlands
were aiming to reach a "healthy weight," that weight would lower
target weight than for a person in the U.S.
Contents of this article:
§
Body mass index
§
Waist-hip ratio
§
Waist-Hip Ratio versus Body Mass Index
§
Body-fat percentage
§
Measuring body fat
§
In search of a healthy body
Body mass index (BMI)
was invented by a Belgian mathematician, Adolphe Quetelet, who was born in 1796.
It is the number given if a person's weight in kilograms is
divided by the square of their height in meters. It has been seen as a useful
guideline for approximating a healthy weight and for carrying out surveys of
populations.
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To calculate a person's BMI, they need to know their weight and
height.
In metric measurements, if a person weighs 80 kilos and is 1.8
meters tall, their height squared will be 3.24. Eighty divided by 3.24 gives a
BMI of 24.69.
A number of websites
provide an online tool to
calculate BMI.
According to the World
Health Organization (WHO), BMI can be classified as follows:
§ 15.9 and under is severely thin
§ 25 to 29.9 is overweight
§ 30 to 39.9 is obese
§ 40 and above is morbidly obese.
The Centers for
Disease Control and Prevention (CDC) note that BMI can be useful as a screening tool, but
it does not diagnose the body fatness or health of an individual.
BMI does not take into account a person's measurements, so that
an Olympic athlete may have the same height and weight as a person who
undertakes no physical activity.
They would have the same BMI, but their measurements would be
different. For example, muscle weighs more than fat. This means that an athlete
whose weight is composed mainly of muscle may have the same BMI as a person
whose weight is composed of fat.
Two people may also have different bone densities and body-fat
ratios, but BMI does not take these factors into account.
A person with osteoporosis may have a lower BMI, due to a lower
bone density, but they may not be healthier.
The disadvantages of using BMI are:
§ It underestimates body fat content in obese or
overweight people
§ It overestimates body fat content in muscular
or lean individuals.
The implications of
BMI may be different for people of different ages, gender and ethnicity. For
example, women have more body fat than
men.
Waist-hip ratio
Waist-hip ratio, or WHR, looks at the ratio of waist circumference
to hip circumference. This is an important factor for health. A higher
proportion of fat collected around the waist has been linked with a higher risk
of cardiovascular problems. In other words, it is healthier to be
"pear-shaped" than "apple-shaped."
The waist is measured at its narrowest point, and if there is no
narrow point, it is measured approximately 1 inch above the belly button. Hips
are measured at the widest point of the buttocks. The waist measurement is
divided by the hip measurement.
If an adult female has a 27-inch waist and 36-inch hips, her WHR
is 27 divided by 36, giving her a WHR of 0.75.
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For men:
§ Below 0.9 indicates a very low risk of
cardiovascular problems
§ From 0.9 to 0.99 suggests a moderate risk
§ Above 1 implies a high risk.
For women:
§ Below 0.8 means a very low risk
§ From 0.8 to 0.89 indicates a moderate risk
§ 0.9 or above suggests a high risk of
cardiovascular problems.
For men and women, WHR has different implications for
cardiovascular problems.
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WHR will give a better idea of the risk of cardiovascular
disease, compared with BMI.
Women with a WHR below
0.8 have been found to be healthier, with a lower risk of different types of cancer,
cardiovascular disorders, diabetes, and infertility.
Men whose WHR is 9 or less have
the same benefits as women with a low WHR, and men with a higher WHR, have
similar health risks to those of their female counterparts.
However, WHR does not measure the total body fat percentage of
the individual, or their muscle-to-fat ratio.
Body-fat percentage is calculated by weighing a person's total
fat, and dividing it by their weight.
Essential fat is the fat we need for survival. In adult women
the total proportion of essential fat is between 10 percent and 13 percent. In
adult men, essential fat makes up between 2 percent and 5 percent of the body.
Storage fat is the accumulation of fat in adipose tissue. Some
storage fat is needed to protect the internal organs in the abdomen and the
chest. Adipose tissue is a type of tissue that contains stored fat.
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The total body fat percentage includes both storage fat and
essential fat.
According to the
American Council on Exercise, male and female fat percentages should
be as follows:
§ Essential fat in men: 2 percent to 4 percent
§ Essential fat in women: 10 percent to 13
percent
A woman with 32 percent or
more body fat is considered obese, as is a man with 25 percent or more body
fat.
The advantage of measuring a person's body fat percentage is
that is it assesses the person's body composition, which is an important factor
in the development of some diseases.
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However, getting an accurate measure of body fat is not easy.
Measuring
body fat
Calipers are often used to measure body fat, but they are not
always accurate.
For an approximate
body fat measurement, an online tool can
give an idea.
Air displacement
plethysmography (ADP) is sometimes used to measure body fat. In ADP, The person
goes into a sealed chamber that measures their total body volume by calculating
how much air is displaced. Body density is determined by combining mass, or body weight, with
body volume.
However, studies have
suggested that ADP, too, may not give an accurate measure.
In near-infrared interactance, an infra-red light beam is sent
into the person's biceps. It is reflected from the muscle inside and absorbed
by the fat.
However, this method assumes that the fat in the arm reflects
fat composition in the rest of the body, which is not necessarily true. It may
not give an accurate reading.
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Dual energy X-ray absorptiometry (DXA) involves X-rays of two
separate energies scanning the body, one of which is absorbed more strongly by
fat. The technique can measure relative amount of fat at several points, and
provides a reading of a person's overall fat composition.
A recent study of 2,689 people of
different ages, genders and backgrounds has suggested that this method provides
an accurate measure of visceral adipose tissue (VAT), or body fat.
Finding an accurate way of measuring body fat is important,
because inaccurate readings may suggest a change in composition that has not
happened.
In the same way, finding an accurate way of assessing a healthy
weight is necessary, because weight does not always reflect health, and a
healthy body is not only a question of pounds.
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In search of a healthy body
To become healthier, people are urged to reduce body fat. To do
this, the American Council on Exercise (ACE) suggests eating healthily and
exercising, including resistance training, to build up lean body composition.
When striving for a healthy body, it is important to factor in
body composition and shape, gender and activity levels, and to use accurate
tools to evaluate each aspect.
There are tools and
calculators available online for home use.
Anyone who is seriously concerned about their body shape and
weight should seek medical advice.
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A doctor will take into account a person's diet, physical
activity, and family history of conditions that can be prevented by attaining a
healthy weight and healthy lifestyle.
Written by Christian Nordqvist
