Healthy weight standards for teenagers

Use BMI (weight in kilograms divided by height in m squared) to match the percentile range of people of the same age and gender. The range between P10 and P85 is a healthy weight, below P10 is underweight, between P85 and P95 is overweight, and above P95 is obese.

Last week, I was sitting in the clinical nutrition department. I met a mother dragging her 14-year-old son in. The child is 165cm and 58kg. The mother opened her mouth and asked if she could prescribe a weight loss diet. She said that the highest normal adult BMI found online was 23.9. Her child was already 21.3, and he was about to be overweight. This is actually the most common misunderstanding that ordinary people have about adolescent weight standards: taking adult fixed values ​​and applying them to children who are still growing and developing. You must know that teenagers are in a critical period for growing taller and building muscle. The proportions of bones, muscles, and fat are changing at every age. It is impossible to use a fixed value to block children of all ages.

Many people will ask, which of the two sets of standards is the right one to choose? In fact, there is no absolute right or wrong, but the applicable scenarios are different. The value of our national standard is calculated based on the growth data of nearly 200,000 domestic adolescents. Since the body fat rate of East Asians is generally 2-3 percentage points higher than that of Europeans and Americans with the same BMI, the overweight threshold of the national standard is 0.5-1 BMI unit lower than the WHO standard. For example, the overweight line for a 12-year-old boy is the national standard of BMI 21.5 and the WHO standard of 22. When conducting school group screening for domestic children, the accuracy of using the national standard will be about 30% higher. However, if the child grew up abroad or is of mixed race, the WHO standard will be more suitable.

I have been working as a nutrition guide for teenagers for almost 8 years. To be honest, I would never label a child as "fat" or "thin" just by using a BMI value. I met a 13-year-old track and field boy on the school team before. His BMI was right at the overweight line of P87, his waist circumference was only 68, and his body fat rate was only 14%. All of them were muscles accumulated through long-term training. If he was forced to lose weight according to the standard, wouldn't it be a joke? When evaluating children, I usually add three additional judgment dimensions: First, look at the waist circumference. For boys over 10 years old, the waist circumference should not exceed half of the height, and for girls, the waist circumference should not exceed 48% of the height. Nowadays, many children are "invisibly obese". Their BMI looks normal but their belly is round, and their body fat rate has long exceeded the standard. ； Secondly, look at the trend of the growth curve. If a child's weight percentile has been stable at P70 for the past three or four years, and suddenly rushes to P90 in half a year, he must be vigilant even if he has not reached the overweight line. Such large fluctuations are much higher than the risk of slow weight gain. ； Third, look at athletic ability. A 13-year-old boy can run 800 meters continuously without panting, and can do more than 3 pull-ups. A 12-year-old girl can jump rope for 1 minute without getting stuck. Even if the BMI touches the P85 line, there is no need to deliberately lose weight. Instead, you need to add more protein to help grow muscles.

In fact, in the past two years, the industry has been quite fiercely arguing about whether to set a unified weight standard for teenagers. There is a group of scholars who work on adolescent psychology and believe that the specific standards should not be made public on a large scale. Nowadays, many children watch short videos and have a deformed "white and thin" aesthetic. They are originally normal weight, but when they meet the standards, they feel fat and are on desperate diets. I treated a girl who was a freshman in high school last year. Her BMI happened to be in the healthy range of P12. She was starved for half a year and dropped to P3. She ended up with amenorrhea for half a year. She also developed mild fatty liver due to malnutrition. The gain outweighed the gain. But another group of public health experts believe that there must be clear and unified standards. The overweight and obesity rate among adolescents aged 7-18 in China has reached 19%. Many parents are paranoid and their children are so fat that their necks are black (acanthosis nigricans sign, a typical manifestation of insulin resistance). They still think it is a "normal phenomenon of growing bodies." Without a unified screening scale, there is no way to intervene in advance.

My own experience is that standards are for parents and doctors as a reference, not for children to create body anxiety. Last year, there was an 11-year-old girl who lay at home for three months with a broken bone. Her BMI rose from 20.1 to 22, just on the Chinese standard for overweight. Her mother was so anxious that she had to stop eating rice. I checked her growth curve for the previous three years, and it has been stable between P75-P80. Her waist circumference is only 62. She is not really fat at all, but has lost muscles and accumulated fat after lying down for a long time. Later, the plan given to her was to jump rope for 20 minutes every day, add a boiled egg in the morning and evening, and there was no need to eat or go hungry. After three months, her weight did not change much, her height increased by 3cm, and her BMI dropped directly to 20.2, smoothly returning to the healthy range.

In the final analysis, no standard is as reliable as the child's usual state - he can run and jump without panting, he can concentrate in class, and his six-month-to-year growth curve does not show a large jump or drop, so there is no need to worry about the numbers. If you are really unsure, it is much better to go to the community pediatric care department or the nutrition department of the hospital to see a doctor than to search for half-baked information on the Internet.