What are the limitations of BMI?
BMI alone cannot show whether a person's weight is healthful, but using it in combination with other indicators can provide a more complete picture.
Age and sex
BMI incorporates height and weight but not body composition, fitness, age, or sex.
For adults ages 20 years and older, BMI incorporates weight and height, but it does not take age or sex into account.
A woman tends to have more body fat than a man with the same BMI. Likewise, an older person tends to have more body fat than a younger person with an equal BMI.
For these reasons, BMI may not give the detail necessary to determine whether a person's weight is healthful.
BMI does not reflect the location or amount of body fat, and these factors can impact health.
For example, studies have indicated that people who have fat around the waist and surrounding the abdominal organs may be more at risk of health problems than those with fat in other areas.
A 5-year investigation of 1,964 people, published in Scientific Reports in 2017, was one study that confirmed these findings.
BMI and health
If a person has a high BMI, they are likely to have a high proportion of body fat, especially if their BMI falls in the obesity category.
However, it is possible to be "overweight" according to BMI, but have healthful levels of fat.
For extremely muscular people, such as athletes and bodybuilders, height and weight measurements alone may not accurately indicate health, because muscle weighs more than fat.
A healthy, muscular person may have a BMI in a very high range. Meanwhile, a frail, inactive person may have a low BMI, but more body fat and less lean tissue than is healthful.
Stages of development
Regular BMI cannot accurately indicate the state of a person's health at some stages in life.
- pregnancy and breastfeeding
- childhood and adolescence, while a person is still growing
For this reason, BMI calculations are different for children and teens. These measurements take age and sex into account.
Norm can also vary among people of certain races and ethnicities.
Making BMI accurate
The National Heart, Lung, and Blood Institute (NHLBI) suggest that an assessment of weight and health risks should incorporate three key measures:
- waist circumference
- risk factors for diseases and conditions associated with obesity
Waist size may be a useful indicator of future health risks.
This risk increases with a waist size greater than 35 inches for non-pregnant women or greater than 40 inches for men, according to the Centers for Disease Control and Prevention (CDC).
To measure their waist, a person should:
- Place a tape measure around their middle, just above the hip bones and the naval.
- Take the measurement just after breathing out.
Weight, obesity, and health risks
The following information, adapted from the NHLBI, may help indicate the risks associated with BMI and waist circumference.
The chart shows weight categories according to BMI, and the effects of higher waist circumference on the risks of type 2 diabetes, hypertension, and cardiovascular disease.
|Classification||BMI (kg/m2)||Obesity class||
Men 40 inches or less
Women 35 inches or less
Men: 40 inches or more
Women: 35 inches or more
|Underweight||18.4 or less|
|Overweight||25.0–29.9||Increased risk||High risk|
|Obesity||30.0–34.9||I||High risk||Very high risk|
|35.0–39.9||II||Very high risk||Very high risk|
|Extreme obesity||40.0+||III||Extremely high risk||Extremely high risk|
A doctor may also measure body fat composition.
Risk factors for obesity-related conditions
Being overweight or having obesity can increase the risk to the heart.
If a high BMI and large waist size pair with other risks, such as high blood pressure, it might be time to take some action to prevent future problems.
The following issues can also increase the risk of developing heart disease, for example.
- high blood pressure (hypertension)
- high levels of low-density lipoprotein ("bad") cholesterol
- low levels of high-density lipoprotein ("good") cholesterol
- high levels of triglycerides
- high blood sugar levels
- a family history of early heart disease
- physical inactivity
- cigarette smoking
- a high consumption of alcohol
A doctor will recommend that a person consider losing weight if they:
- have a BMI of 30 or greater
- have a BMI of 25–29.9 plus two or more risk factors
If a person has obesity or excess weight plus two or more risk factors, they may be at risk of a number of obesity-related health problems in the future.
Losing 5–10 percent of their current weight can reduce the risk of developing these health problems.
Some people are overweight but have no other risk factors, such as high cholesterol or high blood pressure. They should follow a healthful and varied diet, and get regular exercise to prevent additional weight gain.