It depends on a range of factors, including age, sex, body type, bone density, muscle-fat-ratio, overall general health, and height.
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.
Body mass index
BMI alone will not give an overall picture of a healthy weight.
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.
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
- 18.5 is underweight
- 18.5 to 24.9 is normal healthy weight
- 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.
Shortcomings of the BMI
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, 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."
Comparing hip and waist measurements may be a better predictor of health.
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.
- 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.
- 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.
Waist-hip ratio versus BMI
WHR will give a better idea of the risk of cardiovascular disease, compared with BMI.
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.
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.
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.
Calipers can be used to measure body fat.
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.
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.
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.
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.