How reliable is a Body Mass Index?

The Body Mass Index (BMI) is also called Quetelet Index (QI). For grown-ups, body mass index is defined as the individual's body weight divided by the square of their height. A normal BMI is between 18.5 and 25, underweight is less than 18.5 and overweight is above 25, which may be harmful to the heart and blood vessels.

Though the BMI is frequently used as a standard in medicine, it is not altogether reliable, because build is not taken into account in measuring. Consequently, a BMI measuring is not fit as a standard for a healthy weight for children, Asians and athletes, because they have a different build than the norm.

In a biometry, statistical data are recorded. In measuring BMI, biometry in men and women gives the same results, which means that the same formula can be used for both sexes. However, a BMI measuring does not involve body fat or muscle mass percentages. For this reason, other measurements are also carried out, such as measurements of the body fat percentage and the waist circumference.

BMI categories

• Under 18.5: underweight

• From 18.5 to 25:normal

• From 25 to 30: overweight

• From 30 to 40: obesity

• Over 40: morbid obesity

In the Netherlands there are about six million people with overweight, most of them over 45. Though underweight is far less common, i.e. in 334,000 people or 2 percent of the Dutch population, the risks are not to be underestimated. Every week a person dies of underweight, most of them girls with anorexia nervosa, who have the ambition to look like a model, but cannot cope with it psychologically.

A few examples of BMI calculations

There are two formulas for BMI calculations, viz the International System of Units, which is the common standard, and the Imperial System of Units (British), giving the same outcome, as shown by the formulas:

BMI=mass (kg)/(height(m))² or BMI=mass (lb)x4.88/(height (ft))²

Example I

According to these formulas, a person whose weight is 57 kilos (125.65 lbs) and height 1.63 metres (5.35 feet) has a BMI=57/1.63²=21.45 or, equally, BMI=125.65x4.88/5.35²=21.45

Example II

As you can see in in the BMI categories mentioned above, a BMI of 18.5 is the lower limit of normal weight. Consequently, a person whose height is 1.63 metres (5.35 feet) must not have a minimum weight (M) under 49.14 kilos (108 lbs), as the formula shows:

if BMI=M/1.63²=18.5, M=1.63²x18.5=49.15 kilos (108 lbs) or, equally, BMI=Mx4.88/5.35²=18.5, M=5.35²x18.5/4.88=108 lbs.

Example III

As you can see in in the BMI categories mentioned above, a BMI of 25 is the upper limit of normal weight. Consequently, a person whose height is 1.63 metres (5.35 feet) must not have a maximum weight (M) over 66.42 kilos (146 lbs), as the formula shows:

if BMI=M/1.63²=25, M=1.63²x25=66.42 kilos (146 lbs) or, equally, BMI=(Mx4.88)/5.35²=25, M=5.35²x25/4.88=146 lbs.

Underweight: disorders and symptoms

• Weak immune system, low resistance to infection, poor physical recuperation and stamina

• A condition in which core temperature drops below the required temperature for normal metabolism and body functions, for instance, due to lack of body fat in case of anorexia

• Fatigue and lack of energy

• Loss of muscle mass because the body starts using muscles as fuel

• Irregular menstrual cycle

• Anorexia or Bulimia

Obesity: disorders and symptoms

• Type 2 diabetes

• Cardiovascular diseases

• High blood pressure and cholesterol level

• Arthritis

• Difficulty breathing

• Varicose veins

• Gallstones

• Various forms of cancer

Losing or putting on weight in a safe way

Apart from the fact that underweight and overweight can make a person physically ill, it is also true that some people start suffering from low self-esteem, because they are ridiculed for their looks.

Overweight may be caused by medication, overeating, ageing and taking less exercise, underweight by lack of appetite, intestinal trouble, hyperthyroidism and anorexia.

For losing or gaining weight in a safe way, it is advisable to call in the help of a dietician, who can draw up a balanced diet. Losing weight in a safe way means eating 500 to 1000 fewer calories a day at most and taking enough exercise. Losing more weight than one kilo a week affects the muscle mass and the energy production of the body. The secret to losing fat without losing muscle involves not being too aggressive or extreme with the reduction of carbohydrates, which will prevent the body from consuming muscle mass for the production of energy.

In some cases it is necessary to call in the help of a psychologist to break through fixed patterns.

Usually, the fees for treatment charged by a dietician or psychologist are refunded by health insurance.

Sources: www.todio.nl