Experts have expressed uncertainty about relying too heavily on BMI, stressing that it is not an accurate measure of body fat or health.8
BMI fails to take age and sex into account. Women naturally tend to have more body fat than men of equal BMI, while older people tend to have more body fat than younger people with the same BMI.
Furthermore, BMI measurements have no way of measuring where body fat is located in the body. Studies have indicated that belly fat - the fat surrounding abdominal organs - is more dangerous than peripheral fat beneath the skin in other body areas.
If you are obese according to BMI, you are most likely obese according to body fat percentage also. When sampling from the general population, over 95% of men and 99% of women identified as obese by BMI were also obese via body fat levels.10
Individuals who are very muscular such as bodybuilders or those that have very little muscle definition may not receive an accurate BMI reading by using height and weight measurements alone. Muscle weighs more than fat. Hence a muscular person may appear to have a higher BMI and be perfectly healthy, or a frail, inactive person may appear to have a lower BMI and in reality have more body fat than is healthy.7,12
Those who have enough lean mass to be classified as obese by BMI but not by body fat percentage, are far and few in society. These persons would normally be highly active athletes, and it is unlikely sedentary persons or those with infrequent exercise habits would fall into this category.10
If you are normal weight or overweight according to BMI (18.5-29.9) there is still a chance you are actually obese, which is primarily due to low levels of lean mass (muscle, water and glycogen).
BMI also does not account for lactating or pregnant women, children and teenagers who have not reached physical maturity and are still growing, and a tendency for natural differences in height and weight ratios between races.
The National Heart, Lung, and Blood Institute suggests that an assessment of weight and health risks involves using three key measures:9
- Waist circumference
- Risk factors for diseases and conditions associated with obesity.
When measuring waist circumference, if fat is predominantly around your waist rather than at your hips, you are at a higher risk of heart disease and type 2 diabetes.9
This risk increases with a waist size greater than 35 inches for women or greater than 40 inches for men.
Correctly measuring your waist involves placing a tape measure around your middle, just above your hipbones and completing the measure just after you breathe out.
The following chart adapted from the National Heart, Lung and Blood Institute may be helpful in determining the potential risks associated with your BMI and waist circumference.11
Classification of overweight and obesity
Classification of overweight and obesity by BMI, waist circumference and associated disease risks for type 2 diabetes, hypertension and CVD relative to normal weight and waist circumference
|Classification||BMI (kg/m2)||Obesity class||Men 102 cm (40") or less
Women 88 cm (35") or less
|Men > 102 cm (40")
Women > 88 cm (35")
|35.0-39.9||II||Very high||Very high|
|Extreme obesity||40.0+||III||Extremely high||Extremely high|
Risk factors for conditions associated with obesity
In addition to being overweight or obese, the following conditions put you at a greater risk for heart disease and other conditions:9
There are many methods to measure body fat including with calipers.
- High blood pressure (hypertension)
- High LDL cholesterol ("bad" cholesterol)
- Low HDL cholesterol ("good" cholesterol)
- High triglycerides
- High blood glucose (sugar)
- Family history of premature heart disease
- Physical inactivity
- Cigarette smoking.
People who are considered obese and have a BMI of 30 or greater or those who are overweight with a BMI of 25-29.9 and have two or more risk factors are recommended to lose weight. Losing between 5-10% of your current weight will assist in lowering your risk of developing diseases associated with obesity.9
According to health experts, people who are overweight but have no other health risk factors (such as high cholesterol or high blood pressure) should eat healthier and exercise to keep from gaining additional weight.11
Your doctor can advise you and will evaluate your BMI, waist measurement and other risk factors for heart disease.
Other methods to measure body fatness include skinfold thickness measurements (with calipers), underwater weighing, bioelectrical impedance, DXA and isotope dilution.
However, these methods are not always readily available, and they are either expensive or need highly trained personnel. Furthermore, many of these methods can be difficult to standardize across observers or machines, complicating comparisons across studies and time periods.2
As BMI is based on weight and height measurements, by losing weight you will reduce your BMI and put yourself into a lower risk group. A healthy diet, including a balance of food groups, vitamins and minerals, is essential for a long and active life. Body weight and shape are a balance of energy intake (dietary calorific content) against output (calorific burn from activity and exercise).5
Many studies have shown that, to lose weight slowly and steadily, any diet that includes a healthy balance will work if you are motivated. Ideally, a balanced eating plan is always best to lose weight healthily.