Weight management and obesity measurement

We will talk first about BMI (body mass index), which can be calculated dividing weight in kg by height in meters squared. If i have 84 kg and 1.78 m, then my BMI will be 84/(1.78 x 1.78) = 26.51. Between 20-25 you are good, 25-30 overweight, 30-40 obese, over 40 morbidly obese. The only problem is that this method will not take in account your body composition, so if you are an athlete with well developed muscles, according to this method you can be classified overweight. BMI disregards the body fat distribution, which can be useful to show heart risks.

Other more accurate method is the Fatfold or Skinfold method. This one measure the thickness of the skin (including the underlying subcutaneous fat layer) using a Skinfold caliper. We measure front tight, upper back, waist and upper arm. There is a strong correlation between the thickness of the subcutaneous fat layer and the total amount of the body fat.

There are some exotic techniques such as Hydrodensiometry (underwater weighting) and Air Displacement Plethysmography, which both measure the density of the body, dividing body mass by body volume. Body density can be used to compute body fat percentage using a mathematical formula (assuming fat density is 0.9 and lean body mass is 1.1 as constants). The idea behind this is that obese people with low body density will float in water and lean people with high body density will sink more in water.

Other way is by using Bio-electrical Impedance, sending tiny electrical impulses through the body and measuring the return of impulses, as lean tissue conducts electrical impulses more easily than fat tissue. This method is now affordable, easy to use, but not very accurate.

DEXA (Dual Engage X-ray Absorptiometry) is a method that uses x-ray to determine body composition (including details like done density). It is very accurate, very expensive and has small risks due to irradiation.

Non-obese males value is 10-20% body fat, non-obese females 20-30% body fat. As we age the body fat increases as we replace muscle with fat.