How is Obesity Determined?
Body Mass Index (BMI)
The most commonly used method to determine obesity is the Body Mass Index (BMI). It's a simple calculation that uses your height and weight to figure out if you're at a healthy weight.
The formula for BMI is: $$ BMI = \frac{weight(kg)}{height(m)^2} $$
For those using imperial measurements, multiply your BMI result by 703 when using pounds and inches.
BMI Categories
- Underweight:
< 18.5
- Normal weight: 18.5 - 24.9
- Overweight: 25 - 29.9
- Obese (Class I): 30 - 34.9
- Severely Obese (Class II): 35 - 39.9
- Morbidly Obese (Class III): ≥ 40
While BMI is widely used, it doesn't distinguish between weight from muscle and weight from fat. Athletes with high muscle mass might be classified as overweight or obese despite being healthy.
Body Fat Percentage
A more accurate way to determine obesity is by measuring body fat percentage. Several methods exist:
1. Skinfold Measurements
Using calipers to measure fat thickness at specific body sites:
- Men: chest, abdomen, and thigh
- Women: triceps, suprailium, and thigh
Multiple sites are measured to account for different fat distribution patterns across the body.
2. Bioelectrical Impedance Analysis (BIA)
This method sends a small electrical current through the body:
- Fat tissue conducts less electricity than muscle tissue
- The device measures the resistance to calculate body fat percentage
For most accurate BIA results, measure at the same time of day and hydration level.
3. Hydrostatic Weighing
Considered the "gold standard" for body fat measurement:
- Subject is fully submerged in water
- Based on the principle that fat floats while muscle sinks
- Very accurate but requires specialized equipment
Waist Circumference
A simple but effective indicator of obesity, particularly abdominal obesity:
Risk Categories (Waist Circumference)
Men:
- Low Risk:
< 94cm (37 inches)
- High Risk: 94-102cm (37-40 inches)
- Very High Risk: >
102cm (40 inches)
Women:
- Low Risk:
< 80cm (31.5 inches)
- High Risk: 80-88cm (31.5-34.5 inches)
- Very High Risk: >
88cm (34.5 inches)
A person with:
- Weight: 85kg
- Height: 1.75m Would have a BMI of: $$ BMI = \frac{85}{1.75^2} = 27.8 $$ This falls in the "overweight" category.
When determining obesity, it's best to use multiple methods rather than relying on just one measurement, as each method has its limitations.
