Most people and their doctors recommend BMI, which measures weight against height, to assess whether they are overweight and at risk of diseases and obesity.
BMI stands for Body Mass index, which is defined as the body mass divided by the square of the body height. It is generally expressed in units of weight in kg/ height in meters (squared).
If inches and pounds are used, the results can be converted to the standard BMI by multiplying by 703, which is the conversion factor [(kg/m2)/(lb/in2)].
BMI has been criticised recently for being inaccurate and inappropriate and various modifications have been applied. The index is confusing and very hard to understand.
Recently a simple rule of thumb has been suggested as an alternative. People should simply checking that their waist circumference is less than half their height. You don't even need a tape measure for this, just a piece of string.
► Put a piece of string under your foot and extend it to the top of your head.
► Grab the top mark with your fingers and then fold the string in half.
► If the folded string does not reach all the way around your waist you are overweight and at risk.
Critics of BMI, and there are many, say that it can overestimate the dangers of excess weight for people who have large muscle mass or heavy bone structures. More importantly BMI can often miss people with ‘apple shapes’, 'pear shapes' or who have big bellies in men, and excess fat on the hips of women. BMI may also be unsuitable for children and teenagers.
Excess levels of fat around their waist and hips has been recognised as a significant health risk for men and women.
A recent research study confirmed that the 'piece of string' method was more reliable.
About 3000 people were measured by the two methods and their health status checked. The study found that 30% classified as ‘normal’ using the BMI method were classified as 'at risk' by the 'piece of string' method.
The benefits of the 'piece of string' method are:
► The string method is much simpler and easier for people to understand.
► The string method focuses people's attention on the right area of the body that causes the risk.
► Muscle weighs more than fat. So people who have more muscles, genetically or through working out, will have BMI scores higher than they really are. Athletes and muscular people are not overweight or fat.
► An Olympic 100 meter sprinter, who is 6 ft (2m) tall and weighs 200 lbs (90 kg) may have the same BMI score of 26 as an overweight person with the same height and weight who does no exercise. BMI confuses muscles and bone mass in the athlete, with fat in the couch potato.
► The 'piece of string' method is more accurate, easier to understand is ideally suited for mass screenings through the population.
The authors say that waist height is far more accurate and easier for screening large populations that BMI.
Studies have shown that the waist-to-height ratio was a better predictor of lifespan and the risk of disease and death. It is easier to calculate and understand.
It can be used with any unit of measurement from centimetres to inches, metres to feet.
It is just a simple ratio or waist circumference to body height, and there is no squaring of the height as as is required for the BMI.
This article discusses the new waist-to-height ratio (WHtR) and the evidence that it is better than the BMI and the suggestion that it should replace it.
For overweight and obese people the waist-to-height ratio (WHtR) appears to be better predictor of mortality risk than BMI. A study of 300,000 people in Britain, found the (WHtR) was a better predictor of high blood pressure, diabetes and cardiovascular events like strokes and heart attacks than body mass index.
A Taiwanese study, that involved more than 20,000 men and 15,00 women, found that WHtR was significantly better BMI or Waist Circumference in predicting diabetes, hypertension, high total cholesterol, high triglycerides, and low HDL-cholesterol in the people studied.
One way of explaining the extra risk that occurs by being over weight is by calculating the potential average loss years lost at various ages such as 30, 50, and 70 years.
In the simplified charts shown below the explanation for the boundaries and colors is:
Waist-to-height ratio less 0.5 – Low Risk Level. The boundary at a ratio of 0.4 represents low risks for ‘normal’ body shape (green) and potentially slight risks from being underweight for height (brown).
Waist-to-height ratio above 0.5 – Moderate to High Risk Level. The boundary at a ratio of 0.6 represents moderate risks (yellow) and high risks (red).
Some examples for the threshold ratio of 0.5 are:
Because it is much easier to calculate and understand, the Waist-to-Height Ratio (WHtR) would probably be better for individuals and
health authorities to use in trying to encourage people to lose weight and understand what extra weight means for their health and life expectancy.
The charts are shown below