Dieting - Setting Goals for Weight and Shape
What are Body Mass Index, Percent Body Fat, and Waist to Hips Ratio
There is no single number that can adequately summarise the health of a person. In the same way that there are different measures of blood pressure, there are also different ways to indicate how “correct” a person's weight is, or how well a person adheres to the “correct” diet. This article explains what some of the measures are, what they mean, and what are the limitations of their use.
Body Mass Index (BMI)
This is a the most common measure of weight. It is defined as a person's weight (in kilograms), divided by his height (in metres) squared. It is often stated that the Body Mass Index, or BMI, should be in the range 18 to 25. For many people, this is a good general rule.
Example: Tiger Woods is 6 feet and 1 inch tall, and he weighs 185 pounds. Converted to metric, he is 184 cm (1.84 metres) tall, and weighs 84 kg.
His BMI is therefore 84 / (1.84 x 1.84) = 24.8
This is within the “normal” range.
BMI is not an appropriate measure for everyone. Athletes often show high BMI values, because they may have a high percentage of muscle. At an inch (2 cm) taller that Tiger, Arnold Schwarzenegger weighs 235 pounds, or 107 kg. His BMI is over 30.
Percentage Body Fat
This equals the amount of body fat, divided by the total body weight. The “correct” amount of percentage body fat depends on sex and age, and there are several ways to measure it. A reasonable guide is that for men, 10 to 20 percent body fat is good, whereas women should be in the range 15 to 25 percent. Professional advice should always be sought before deciding on a target value, as individual circumstances are important.
Body fat measurement was once only available from specialists, but can now be estimated electrically, using callipers to measure body girth at several points, or by immersing in a tank of water on weigh scales (Hydrostatic Weighing).
One difficulty with using percentage body fat as a measure of health, is that it gives no information about where on the body the fat is distributed.
Waist to Hips Ratio
This is a simple measure to take, and is simply the waist measurement in inches divided by the hip measurement in inches. Hip measurements are taken at the widest point. Ideally, men should have a ratio less than 0.95, and women should have a ratio less than 0.8
Although this is a simple measure to take, it may be inconsistent.
Summary of Dietary Health Indicators
There is no measurement for “correct” weight that takes into account all of the health factors. There are three main figures used – Body Mass Index, Percentage Body fat, and Waist to Hips Ratio. Used together, they can be used as a guide to achieve better health.
References for Dietary Health Indicators
There are many websites available that will provide more information about each of these Health Indicators.
Low Fat, Low BMI
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Low Fat, High BMI
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