Can you be too thin to health?

In my previous blog post, I addressed the myth of healthy obesity, explaining that (1) these individuals are still at a greater risk of adverse health outcomes compared to their normal-weight counterparts, and that (2) it is a transient grace period before metabolic dysfunction occurs.

Now I want to look at the other side of this same coin: being too thin.


At what point are you too thin?

In 2016, a massive meta-analysis was published in the Lancet by the Global BMI Mortality Collaboration, addressing the association between BMI and dying from any cause.

The study was spurred by previous research claiming that being overweight or obese was not associated with an increased risk of death compared to being normal-weight. These previous studies did not, however, account for smoking or chronic diseases that could affect BMI and therefore result in reverse causality (i.e., having a disease or smoking causes weight loss and kills you, rather than the weight loss killing you per se).

Accordingly, the the Global BMI Mortality Collaboration restricted their analysis to never-smokers who were free of chronic diseases when the study began. They also excluded the first five years of follow-up data as insurance against someone developing a disease soon after the study started.

Ultimately, they looked at 189 studies and nearly four million adults from 32 countries around the world. The lowest risk of death was associated with a BMI of 20–25, with increases as BMI moved above and below this range, forming a J-shaped curve.


A separate meta-analysis involving 53 studies and nearly 10 million adults reported similar findings: Both lower and higher BMI was associated with an increased risk of death from any cause compared to a BMI of 20–23.

On a related note, it’s worth pointing out that although the risk of death is similar between underweight and obese people, obesity is a far greater problem. In the U.S., roughly 40% of adults are obese, while only 1.4% are underweight.

Anyway, it seems that a BMI of about 20 is the point at which people are too skinny, at least if we go risk of death from any cause.


Why do thin people die?

Commonly, thin folk die because of underlying diseases or lifestyle habits that cause weight loss. Sometimes this is relatively straight-forward: smoking tends to reduce appetite and cause weight loss while also killing you. These people didn’t die from being thin, they died from smoking, and the risk of dying is lower in underweight people when smoking and health status are taken into account.


Other times, being thin can directly cause illness. Becoming thin is hard; why else would only 1.4% of the US population be considered underweight? It usually requires veganism an eating disorder and malnourishment. This can weaken the immune system and make one more prone to infection, and dying from it.

But, even among healthy non-smokers in the graph above, there is an increased risk of death associated with being underweight. Why?


Lost muscle is lost life

Correlating BMI to body fat percentage in adults from the US population, a BMI of 20 corresponds to less than 15% body fat in men and less than 25% body fat in women. These are not low numbers, and there are many healthy athletes and active adults walking around with body fat percentages much lower.

So, while a high body fat mass plays an established role in the health effects of obesity, it likely plays no more than a minor role in the health effects of being too thin. Rather, I believe the risks associated with being too thin are owed to a lack of muscle mass, strength, and functionality, which increase one’s susceptibility to injury and death down the road.

For example, an analysis of 38,000 men from the Health’s Professional Follow-up study suggested that having more fat mass and less lean body mass were associated with a greater risk of dying from any cause over 25 years.

The researchers also conducted analyses between categories of BMI and death from any cause after excluding excluding people in the lowest 5th and 10th percentiles for muscle mass. Being underweight (BMI < 18.5) couldn’t be included in these analyses because there were no longer any participants in the categories, and being marginally underweight (BMI of 18.5–20.5) was no longer associated with an increased risk of death.


When the same analyses were conducted by excluding people in the lowest percentiles of fat mass, it didn’t make a huge difference — having a low BMI after excluding the leanest participants is still associated with an increased risk of death. The risk may even be greater, since excluding the lean folk means that the low BMI is owed to lesser lean mass.


In a separate analysis of the American population, having a BMI of 18.5–22.5 is no longer significantly associated with an increased risk of death when taking muscle mass into account. Greater muscle mass was associated with reduced the risks of death in both younger (<60 years) and older (>60 years) adults even after accounting for unintentional weight loss, chronic diseases, and physical activity levels.

Looking to the elderly population specifically, having sarcopenia, or the age-related loss of muscle mass and function, is significantly associated with an increased risk of dying from any cause, even when looking at studies with more than five years of follow-up, which helps minimize the possibility that both sarcopenia and death were caused by some other variable.

Sarcopenia has also been proposed as the cause of physical frailty, which in turn is significantly associated with a higher risk of disabilities in activities of daily living,  fractures, falls, hospitalization, and being put in a nursing home.


Summing up

Being underweight is associated with an increased risk of death, just as being obese is, but the causes are fundamentally different. The risks of obesity are owed largely to having too much fat mass, while those of being underweight are owed primarily to having too little muscle mass.

Regardless, being underweight is nowhere near as large of a health concern considering how much more prevalent obesity is. That does not mean underweight isn’t a concern, just that focusing on 1.4% of the population rather than the 40% with obesity seems silly.

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