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What is BMI?

Body mass index (BMI) is a measure based on two variants: height and weight. The standard formula applies to adult men and women. 

The formula is simple: BMI = weight (kg) / height (m2).

These are the different BMI categories accepted by official health authorities:

  • Underweight = <18.5
  • Normal weight = 18.5-24.9
  • Overweight = 25-29.9
  • Obesity = 30 or more

If your BMI is less than 18.5, you are considered underweight. Health risks associated with low weight include osteoporosis, infertility, and a weakened immune system. Low weight can also be a sign of an eating disorder or other underlying disease.

If your BMI is between 18.5 and 24.9, this is the level of weight considered "healthy." This can reduce your risk of developing weight-related health problems.

However, if your BMI is 25 or higher, you are at increased risk for diabetes, cardiovascular disease, and some types of cancer.

BMI: Body Mass Index

The National Heart, Lung, and Blood Institute presents BMI as a useful calculation for :

  1. Assessing overweight or obesity,
  2. Obtain a better estimate of body fat,
  3. Estimate the risk of diseases that increase with obesity: heart disease, high blood pressure, type 2 diabetes, gallstones, respiratory problems, and some cancers.

The higher the BMI, the higher the risk of these chronic diseases. 

A brief history of BMI

BMI was created in the 19th century when Belgian statistician Adolphe Quetelet, a pioneer in the study of human development, developed the Quetelet Obesity Index in 1832. He observed that weight increases with the square of height. His index measured obesity by dividing a person's weight in kilograms by the square of his height in meters, the same calculation we know today.

Obesity as a "disease" with clearly defined complications has existed for a hundred years. For much of human history, body fat was considered positive, a sign of wealth, good health, and an abundance of food.

But in the early 20th century, the insurance industry documented the link between insured weight and cardiovascular disease and mortality. Researchers followed suit and conducted epidemiological studies.

Life insurance companies then compiled tables with desired weights. Obesity was then considered 20-25% above this desired range and morbid obesity 70-100% above the desired range.

In the 1960s, the Quetelet index was even used as an indicator of weight in clinical trials. The 1970 Framingham study was one of the first to confirm its validity. Researcher Ancel Keys also confirmed the validity of the Quetelet index and renamed it BMI in 1972. Subsequently, BMI became the international standard for measuring obesity in the 1980s and became part of the terminology in the late 1990s when the U.S. government began developing public health initiatives. Today, the National Institutes of Health recommends that physicians include BMI in the routine evaluation of patients.

The benefits of BMI calculation

According to the Centers for Disease Control and Prevention (CDC), BMI calculation is one of the best methods for determining overweight and obesity in the population. Because only height and weight are required, it is inexpensive and easy for physicians and the general public to use. It also allows people to compare their weight status with that of the general population.

The National Obesity Watch reports that BMI correlates with body fat in most people. In addition, excess body fat is associated with current and future morbidity.  The CDC also agrees with the association between BMI and body fat, but that may vary by sex, age, and race.

If you are not a bodybuilder, a BMI of 30 or higher is an indication of high fat. Some people do not even know they are overweight. This can be an incentive to make lifestyle changes. With the help of BMI, these at-risk individuals can be easily identified and helped further. For those who are in the process of losing weight, BMI reduction can also be used as motivation.

For BMIs between 25 and 30, there is a gray area that is more difficult to assess. 

BMI and health problems

The World Health Organization reports that a high BMI is associated with 21 percent of ischemic heart disease cases, 23 percent of strokes, 58 percent of type 2 diabetes, and 39 percent of hypertension. A BMI above 30 is also associated with an increased risk of cancer, joint problems, infertility, and mortality.

Data from the Nurses' Health Study and the Health Professionals Follow-Up Study found that middle-aged women and men who gained 5-10 pounds after age 20 were up to three times more likely to develop heart disease, type 2 diabetes, hypertension, and gallstones than those who gained 2.5 pounds or less. Those who had gained more than 10 pounds were even more at risk.

A study at King's College London found that people with a higher BMI were more likely to suffer from various chronic diseases. In addition, obese young people have a similar profile to obese older people. The Prospective Studies Collaboration concluded that mortality was lower with a BMI of 22.5-25. A BMI of 30-35 reduces life expectancy by two to four years, and a BMI of 40-45 reduces life expectancy by eight to 10 years, which is comparable to the effects of smoking.

In contrast, a systematic review of cohort studies found that people with a low BMI had a higher risk of all-cause and cardiovascular mortality.  Overweight subjects had the lowest risk of all-cause and cardiovascular mortality. Subjects with obesity had no higher risk of all-cause or cardiovascular mortality than subjects with a normal BMI. Finally, subjects with a BMI of 35 or higher did not have a higher risk of all-cause mortality, although they had the highest risk of cardiovascular mortality. Therefore, the researchers concluded that these conflicting results could be explained by the inability of BMI to distinguish between fat mass and lean mass.

In a cross-sectional study that assessed BMI and bone density by bone densitometry (DEXA), it was found that 39% of subjects were not obese according to BMI, but obese according to bone densitometry. In a recent systematic review and meta-analysis of total mortality about BMI, overweight was associated with significantly lower total mortality, and grade 1 obesity (BMI 30-35) was not associated with increased mortality. However, grade 2 and 3 obesity (BMI 35 or higher) was associated with higher all-cause mortality.

Thus, there are conflicting studies on whether BMI is a sufficiently sensitive tool to effectively detect excess body fat and predict the risk of health problems.

The limitations of BMI

Many argue that BMI is simply a ratio of weight to length and is not a good indicator of body fat at all. BMI does not distinguish between fat, muscle, and bone mass. It also gives no indication of body fat distribution.

However, it is important to note that obesity is primarily concerned with excess body fat, not body weight. The World Health Organization defines obesity as significant excess body fat that poses health risks.

For the same BMI, the amount of body fat can vary. At the same BMI, women tend to have more body fat than men. Therefore, this tool does not distinguish between the two sexes.

This can also lead to a false sense of security. People with a normal BMI may still be at risk for diabetes or cardiovascular disease, depending on the distribution of body fat (abdominal fat or not). Similarly, people with normal BMI but who smoke, sit a lot, and/or have poor eating habits may have the same or higher risk of health problems than people with higher BMI but healthy lifestyle habits. The conclusion is that BMI alone does not identify all people at risk.  

Finally, it is not uncommon for people who are making lifestyle changes, with changes in diet and exercise, to find that their weight does not move much on the scale and therefore their BMI score does not change either. On the other hand, many changes may have occurred: an increase in muscle mass, a decrease in fat mass, and a better distribution of body fat, for example. Therefore, BMI may not reflect all these health-promoting changes in the body.


BMI incorrectly classifies athletes with normal or lower fat percentages as overweight. Separate standards should be established for sports teams. Athletes with strong muscle mass and BMI over 25 need not worry so much about this classification because it is body fat that is detrimental to health.

For people over 65, it has been shown that a BMI between 23 and 27 is more beneficial for survival than the usual BMI between 18.5 and 25.

BMI is not recommended for pregnant or breastfeeding women. However, the desired weight gain during pregnancy is based on the woman's pre-pregnancy BMI. The lower the pre-pregnancy weight, the more weight the pregnant woman should gain. On the other hand, it is not desirable for an overweight woman to gain too much weight before pregnancy, as her reserves are already optimal.

The table below shows the recommended total weight gain during pregnancy based on the mother's pre-pregnancy BMI.

BMI before pregnancy

Recommended weight gain (kg)Recommended weight gain (lbs)
BMI < 18,512,5 - 1828 - 40
BMI of 18.5 to 24.911,5 - 1625 - 35
BMI of 25.0 to 29.97 - 11,515 - 25
BMI ≥ 30.0c5 - 911 - 20

Source: Institute of medicine, 2009

What about children?

For children between the ages of 2 and 19, the determination of BMI is different from that of adults. First, the calculation is different.

It is calculated as follows: weight (pounds) ÷ height (inches) x 703

Second, there is one table for boys and another for girls, since boys and girls have different levels of body fat. Once the child's BMI is calculated, it is entered into the table along with age as the second factor.  The percentile is then entered and the child can be classified.

Children aged 2-5 years

Below the 3rd percentile

At the risk of underweight
Between the 3rd and 85th percentileHealthy weight
85th to 97th percentileAt the risk of being overweight
Between the 97th and 99.9th percentileOverweight
Above the 99.9th percentileObese
Children aged 5 to 19 years old

Below the 3rd percentile

At the risk of underweight
Between the 3rd and 85th percentileHealthy weight
85th to 97th percentileOverweight
Between the 97th and 99.9th percentileObese
Above the 99.9th percentileSeverely obese
Source: Child Growth Standards, World Health Organization, 2007

Beyond BMI

Other tools provide a better assessment of body fat, such as waist circumference and waist-to-hip ratio. The National Institutes of Health suggests including waist circumference in the assessment because BMI may not provide enough information about body fat status. Waist circumference is a better indicator of overall health than BMI.

Other body fat tests include skinfold measurement, in which a skin fold is pinched to measure the subcutaneous fat layer, bone density measurement, or bioelectrical impedance, which measures resistance to an electric current flowing through the body. However, these methods are more expensive, difficult to perform on large populations, and are not supported in clinical research.

Advice from a nutritionist

If you think your BMI does not classify you correctly, consult a health professional. For example, if you think you are underweight but have always been, ask your doctor to assess your health and do a blood test to detect any deficiencies. If you are overweight but exercise five days a week, you may be in good health because your fat content should be relatively low. Remember that muscle tissue takes up much less space than fat tissue, as muscle is 18 percent denser than fat tissue.

In conclusion, I think the use of BMI is still important and reasonable. Although it is not perfect, it is a simple and very efficient method for assessing health risks in individuals and populations. BMI calculation should be part of a comprehensive assessment that includes the measurement of waist circumference.


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