Are BMI values ​​reliable? What are the standards?

Body mass index (BMI) is a commonly used indicator for health checks. It can be calculated as long as you know your height and weight. The formula is as follows:

BMI = weight (kg) / height2 (m2)

According to Taiwan's official recommendations, the normal range of BMI is between 18.5-24. A weight below this range is considered underweight, while a weight above this range is considered overweight or obese.

Where is the optimal BMI value? Is it necessarily bad to be too high? The lower the better?

BMI and mortality

The famous medical journal The Lancet published a paper this year, which collected data from more than 10 million people from all over the world to study what kind of BMI can make people live the longest.

The researchers found that 20-25 seemed to be the best BMI value, and this group of people had the lowest mortality rate. This trend is the same all over the world. Neither too low nor too high is good.

Correlation does not equal causation. Although too low or too high a BMI is related to mortality, it cannot be directly inferred that abnormal BMI causes increased mortality. For example:

●The BMI of smokers is lower than that of non-smokers. Readers who have quit smoking should have a special understanding of this (people will gain weight). But despite your lower weight, the health risks of smoking are still very high. Please don't use cigarettes to lose weight.

● Certain diseases can cause water retention, resulting in a false increase in BMI; many diseases can cause patients to lose weight, resulting in an abnormally low BMI. Disease, rather than BMI, may be the culprit behind the increased mortality rate.

In response to the above two points, The Lancet specifically excluded smokers and patients with chronic diseases, and also excluded cases of death within five years of the start of the study. This ensured that the subjects were generally healthy, did not smoke, and had no "undiagnosed" serious illnesses (because they were still alive and well in five years...)

The health hazards of obesity are quite clear. Combined with this study, we can "almost" say without any suspense: a BMI over 25 or below 20 will shorten your life.

Almost...

BMI greater than 25? Not necessarily a bad thing

Once the above research was released, the first person to bear the brunt was Scott himself. My current weight is 87 kg and my height is 183 cm, so my BMI is about 26, which is considered overweight.

Does this mean I should lose weight or increase my life insurance coverage?

That’s not necessarily the case!

A 2007 American study examined the BMI and body fat percentage of athletes and the general population. The study used precision instruments to measure body fat, defining excessive body fat as greater than 20% for men and greater than 33% for women, to see if BMI could accurately indicate those with excessive body fat.

The results showed that many male and female athletes had normal body fat but were mistakenly classified as "overweight" by BMI; many ordinary women had too high body fat but were mistakenly classified as "normal" by BMI.

If BMI is to be used to determine whether an athlete is overweight, the standards will have to be adjusted. The following are new standards developed based on the research results:

BMI is used as an "alternative" to body fat percentage because it is simple and easy to calculate, but BMI cannot reflect the actual body composition.

An American football lineman might have a BMI of 30 but only 12% body fat; an inactive middle-aged man might have a BMI of 24 but 25% body fat.

Considering the harmful effects of excess body fat on health and the protective effect of muscle mass, will middle-aged men who do not exercise really live longer?

Past studies have also shown that people with high BMI but high muscle strength do not have an increased mortality rate. People who have stronger grip in middle age tend to live longer.

Based on the above data, Scott decided not to lose weight for the time being.

BMI less than 20? Don't be happy too soon

One of the less discussed points in the Lancet study mentioned earlier is that people with a BMI between 18.5 and 20 have an increased mortality rate, which contradicts the traditional view that the ideal BMI is between 18.5 and 24.0.

Why is this so? Isn't it true that the lighter the weight, the better? (Of course not)

Scott's hypothesis is that low BMI is associated with higher mortality because it identifies people with low muscle mass.

Muscles are essential to health for several reasons:

Muscles are large warehouses for storing glucose, so people with higher muscle mass tend to have better insulin sensitivity and better tolerance to carbohydrates. Scott believes that insulin resistance is the root cause of many diseases of civilization, so muscle mass may have a disease-preventive effect.

Muscle is lost with age, and insufficient muscle mass in the elderly will increase the risk of falls and bedriddenness. If one has a weak foundation when young, he or she will be more likely to die from frailty and related complications when older.

Low muscle mass may reflect a person's lack of exercise habits, and activity level is a major killer of health, so low muscle mass is associated with high mortality.

Past studies have found that low arm circumference and leg circumference are independent risk factors for mortality, that is, people with thick arms and thighs tend to live longer, which may reflect the impact of muscle mass on health.

A slim figure is considered by many to be a sign of health, but if your BMI is less than 20, don't be too happy too soon, as you may have a problem with insufficient muscle mass.

Conclusion

Summary of this article:

According to observational studies, people with a BMI between 20 and 25 live the longest.

The above standards are not applicable to athletes, especially those who do weight training. Body fat percentage may be a better indicator.

Although a BMI between 18.5-20 meets the official recommended standards, there is a concern that muscle mass is too low, which is a major health concern.

This article comes from: Dr. Scott's One-Minute Fitness Class

※For more information, please see "Dr. Scott's One-Minute Fitness Class"