We generally believe that "obesity is the root of all diseases," but large-scale data studies actually show the surprising fact that "slightly overweight people" live the longest. When we fall ill, our bodies need energy to fight, and excessively thin people face a much higher mortality risk due to insufficient energy reserves and muscle mass. This video scientifically explains the "obesity paradox" -- that in the absence of diabetes or metabolic disease, a slightly plump body with a BMI of 23-27 is far more advantageous for longevity than the thin physique society demands.

1. The Definition of Obesity and the "Longevity Weight" We Did Not Know About

In this video, Professor Lee Seung-hoon of Seoul National University Hospital -- sometimes called "the Heo Jun of the 21st century" -- challenges conventional beliefs about obesity. We typically consider our "normal weight" to be our height minus 100 or 110, and we tend to prefer very lean physiques. But medically speaking, should not "healthy weight" be defined as the weight at which people live the longest?

Looking at the globally used Body Mass Index (BMI) standards, Korea classifies a BMI of 25-30 as "Stage 1 Obesity." However, Western countries call this range not obese but "Overweight." Remarkably, studies have begun to show that people in this range have the lowest mortality rates.

In Korea, a BMI of 25 to 30 is called Stage 1 Obesity. By Western standards, that is not considered obese. They call it overweight. (...) For the average Korean adult male height of 172.5cm, this corresponds to a weight range of 74.4kg to 89kg.

Starting from the 2010s, research results came out so differently from what was expected... I felt that without explaining the medical controversies and narrative behind it, people were getting unnecessarily stressed because society has set the "normal" weight standard far too low. Today's talk will be a liberating lecture for those who carry a few extra pounds.

2. The Obesity Paradox: Do Heavier People Live Longer?

In 2013, Dr. Flegal of the U.S. Centers for Disease Control and Prevention (CDC) published a meta-analysis of 97 studies worldwide involving approximately 28.8 million people. The results were shocking: the overweight group (Stage 1 Obesity by Korean standards) had a 6% lower mortality rate than the normal-weight group.

This phenomenon is called the "Obesity Paradox." The term was actually first coined in the early 2000s by doctors studying heart failure patients. Among patients with failing hearts who were short of breath and in critical condition, extremely thin patients died sooner while heavier patients survived much longer.

In heart failure patients, the mortality pattern is reversed... The thinnest people die the most, and the heaviest people survive the most. As weight increases, mortality steadily decreases. (...) We were taught that obesity primarily causes myocardial infarction. It does cause it more often, but then heavier people do not die from it? It was utterly baffling. Why is this happening?

Professor Lee's own research confirmed these findings. After tracking stroke and cerebral hemorrhage patients, those with a BMI around 26-27 had the highest survival rates, while underweight patients had the highest mortality risk.

The trend is even more pronounced with stroke than with cerebral hemorrhage, and again the longest-surviving group was the heavier people with a BMI around 27. And as previously shown, the thinnest people had the highest mortality, followed by normal weight, then overweight, then obese -- exactly replicating the myocardial infarction and heart failure data.

3. Why Do Slightly Overweight People Have a Survival Advantage?

So why does this happen? Professor Lee explains it through the lens of "energy reserves" and "immunity." In everyday life, a lean body may be more comfortable for moving around, but when the "wartime situation" of illness strikes, the story changes entirely. Fighting disease requires enormous energy, and thin people collapse easily because they lack fuel (fat and muscle) to burn.

Our bodies use energy in a priority order:

1. Maintaining body temperature: An essential condition for survival
2. Activity (exercise): Used for movement
3. Immunity: Only deployed when energy is left over

In other words, extremely thin people exhaust their energy on maintaining body temperature and minimal activity, leaving insufficient energy for their immune system to fight viruses and diseases.

Getting a disease is bad when you are obese, but once you are sick, you need stamina to fight it. So because the body has more fighting energy, mortality is lower. (...) When you are sick, the energy required is enormous. It is the reverse.

In what order does the body use energy? First, body temperature. If there is energy left after that, it allows you to exercise. If you ate a bit more than that, then it goes to immunity. Immunity only gets used when there is surplus energy. That is why people who eat only chicken breast and have chiseled abs actually get the flu more often and have a harder time recovering.

Additionally, visceral fat is not simply a harmful lump of grease -- it is an important organ that regulates metabolism and secretes hormones. A moderate amount of visceral fat is essential for survival.

4. Medical Debates and Data Pitfalls

Of course, there were strong counterarguments. In 2016, a British research team published a study in The Lancet claiming that "normal weight has the lowest mortality rate," directly contradicting Dr. Flegal's research. However, this study had blind spots: it excluded all smokers, people with pre-existing conditions, and those who died within five years from the data.

This study excluded all smokers. And even if a person had no current illness, they were excluded if they had underlying conditions like heart disease or cancer. (...) By cutting out more than half of the total population, the data could arguably be statistically skewed. It only included extremely healthy people. But in reality, among people we call "normal," there is virtually nobody without some condition...

An interesting observation is that the researchers' backgrounds and physiques curiously matched their findings. The heavier American Dr. Flegal argued for the benefits of being overweight, while the leaner British researchers championed the advantages of being thin. Furthermore, the coincidental timing of the emergence of obesity drugs like Saxenda around 2015 and the publication of research promoting thinness is an intriguingly conspiratorial detail.

5. What Korean National Health Insurance Data Tells Us

The most powerful evidence to settle the debate is Korea's National Health Insurance Service population-wide data. An analysis of the entire Korean population over approximately 10 years from 2012 to 2022 (published in the 2024 Korean Society for the Study of Obesity Fact Sheet) yielded very clear results:

• Highest mortality risk (worst): Underweight (thin people)
• Second highest mortality risk: Severe obesity (Stage 2 obesity and above)
• Third highest mortality risk: Normal weight
• Longest-living group: Overweight to Stage 1 Obesity (BMI 23-30)

Koreans genetically have weaker pancreatic function and lower fat storage capacity than Westerners, meaning they tend to have less muscle and fat. This suggests that people at the Korean "normal weight" standard may actually have insufficient energy reserves and be more vulnerable to disease.

In Korea, overweight (BMI 23-25) and Stage 1 Obesity (BMI 25-30) showed the lowest mortality rates. Mortality for the overweight group drops 26% compared to normal-weight individuals. And the Stage 1 Obese -- the heavier group -- showed the lowest rate of all, with mortality dropping 27%.

By the standard of mortality, being thin is the worst. Then the second worst is being extremely obese. The third worst is normal weight. (...) The pattern is the same for both men and women, and for cancer, cardiovascular disease, stroke, and other conditions.

6. Conclusion: What Is Your True Optimal Weight?

In conclusion, there is no need to be overly fixated on the "beauty weight" demanded by society or the "normal weight" standards set in the past. Professor Lee suggests that for Koreans, a BMI of 23-27 is the healthiest longevity weight. For a man who is 172cm tall, this is approximately 68kg-80kg, and for a woman who is 160cm tall, approximately 58kg-69kg.

However, there is one critical prerequisite: you must be metabolically healthy.

Key Guidelines

1. Keep your HbA1c (glycated hemoglobin) below 6.0%. (6.5% and above indicates diabetes)
2. If you are free of diabetes or serious metabolic disease, having a bit of a belly and being slightly plump is actually a "symbol of longevity" and a "healthy energy reserve."
3. As you age, it becomes harder to build muscle and gain weight. Rather than forcing yourself to lose weight to achieve a thin physique, eating well, exercising, and maintaining a moderate weight is far more advantageous for survival in old age.

Use HbA1c 6.0% as your benchmark. If you go above 6.0%, think "I'm starting to have metabolic issues." If you are still in the 5-point-something percent range, think "This plump physique of mine is actually a healthy body type."

Being too thin is obviously a problem, but what we call "normal weight" -- which seemed ideal by every measure -- turns out not to be. The truth is, thin people praising and envying that weight standard was essentially gaslighting.

In Closing

We may have mistaken "thinness" for a measure of health due to media and social pressure all along. But the facts are clear: the strength to fight disease comes from "stored energy." Starting today, instead of stressing over the number on the scale, checking your metabolic markers (such as blood sugar levels) and loving your well-nourished body may be the true beginning of health management.