People who are 'underweight' (BMI > 30) have a higher risk of all-cause mortality than people who are 'overweight' (BMI < 19)



Many people find a thin body more attractive than a fat one, and some may think that being 'too thin' is healthier than being 'overweight.' However, a study by

the National Cancer Center examining the relationship between body mass index (BMI) and mortality risk in Japanese people found that people who are underweight have a higher risk of all-cause mortality than people who are overweight.

Body mass index and mortality from all causes and major causes in Japanese: results of a pooled analysis of 7 large-scale cohort studies - PubMed
https://pubmed.ncbi.nlm.nih.gov/21908941/

Body Mass Index (BMI) and Mortality Risk | Current Achievements | Research on Scientifically Based Cancer Risk Assessment and Cancer Prevention Guideline Proposals | National Cancer Center, Cancer Control Research Institute
https://epi.ncc.go.jp/can_prev/evaluation/2830.html

In late August 2024, a post on X became a hot topic, stating, 'When I lost too much weight due to stress, my doctor advised me that it would be better to gain some weight because it would help me tolerate illness better.' In response to this, another user posted, 'People with a BMI that is underweight have a higher risk of all-cause mortality than people with an overweight BMI.'



The reply poster cited a page published by the National Cancer Center titled 'Study on evidence-based cancer risk assessment and cancer prevention guideline recommendations.' This page summarizes the contents of a paper titled 'Body mass index and mortality from all causes and major causes in Japanese: results of a pooled analysis of 7 large-scale cohort studies,' which was published in the academic journal Journal of Epidemiology in 2011 by a team from the National Cancer Center.

BMI is an index of obesity calculated from height and weight, and is calculated by dividing weight (kg) by the square of height (m). BMI values are classified as 'underweight (under 18.5)', 'healthy (18.5 to 25)', 'overweight (25 to 30)', 'obese (30 to 35)', and 'extremely obese (35 or more)'. It is said that the level of obesity can be used to estimate the relationship with health risks and mortality rates.

However, according to the World Health Organization (WHO) standard, a BMI of 25 or more is considered overweight, but in Asians, even those with a BMI of 25 or less are at high risk of type 2 diabetes and cardiovascular disease, so it is not necessarily the case that the same standard can be applied to Japanese people. Therefore, the research team quantitatively evaluated data from a total of more than 350,000 people collected in seven cohort studies to estimate the impact of BMI levels on the risk of death in Japanese people.

Below is a graph showing the relative risk of all-cause mortality based on a BMI of 23 to 24.9. The left side is men (sample size: approximately 160,000) and the right side is women (sample size: approximately 190,000). The black graph shows all-cause mortality, the blue graph excludes early deaths within 5 years of follow-up, and the green graph (men only) is limited to non-smokers. In addition, the graph is statistically corrected to prevent bias in the results due to factors that may affect mortality, such as age, region, smoking, drinking, history of diagnosis of hypertension, history of diagnosis of diabetes, and leisure sports and exercise.



The graph of all-cause mortality risk shows an inverted J-shape for both men and women, with underweight people with a BMI of 14-18.9 at highest risk, and overweight people with a BMI of 30-39.9 at lowest risk. The lowest mortality risk groups were men with a BMI of 25-26.9 and women with a BMI of 23-24.9, suggesting that being thin is not necessarily healthier.

In addition, the graph below shows the risk of death from cancer (red), cardiovascular disease (blue), cerebrovascular disease (green), and other death risks (gray). Although the overall shape is an inverted J, there are some differences, such as the fact that the risk of death from cardiovascular disease and cerebrovascular disease in men is higher in overweight people with a BMI of 30 to 39.9 than in underweight people with a BMI of 14 to 18.9, and that the risk of death from cancer in women is almost unchanged by BMI.



The National Cancer Center stated, 'The results of this study show that the BMI range that provides the lowest risk of death for middle-aged and older Japanese people is 21 to 27.'

As the National Cancer Center acknowledges, BMI has the problem of lacking data to evaluate abdominal obesity, such as waist circumference or waist-hip ratio. Therefore, some researchers recommend the introduction of new systems such as the Edmonton Obesity Staging System (EOSS) to replace BMI.

BMI calculated only by height and weight is not suitable as a measure of obesity, so how should obesity be measured? - GIGAZINE



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