BMI alternatives: Simpler and better predictors of mortality and disease

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GymThe body-mass index (BMI) is a notoriously inadequate predictor of morality and disease, reports The Atlantic. An emerging body of evidence is finding useful and cheap numbers — such as push-ups, walking-speed and grip-strength — that anyone can track, but aren’t yet being taken seriously.

For those wanting to just generally know whether they are on pace to live an average life or longer, the most common numbers are age and body weight, writes James Hamblin, doctor and author in The Atlantic. And, he says, the US health-care system places tremendous value on the latter, in the form of body-mass index, or BMI, a simple ratio of weight over height. BMI is used to define obesity and “overweight,” and so to stratify risks in insurance and health-care industries. This number has come to be massively consequential in the lives of millions of people, and to influence the movement of billions of dollars.

But, Hamblin writes, despite all this emphasis on body weight, the ability of BMI to predict mortality and disease has been called into question. Its inadequacy is famously evident in examples such as the human muscle-mound Dwayne “The Rock” Johnson qualifying as obese. BMI also ignores the health problems among the “skinny fat” (or “overfat” or “normal-weight obese”).

Health is more strongly correlated with body-fat percentage and distribution than with overall weight, but, he writes, getting an accurate measure of one’s muscle-to-fat ratio is not especially simple – and still draws focus to body image in ways that can introduce its own risks of eating disorders, depression, social isolation, and all manner of things that may be more dangerous than body fat itself.

Except in extreme cases, no single number gives a good idea of whether a person is functionally healthy or not. The common numbers are not directly or easily changeable. However, he writes as these numbers continue to dominate health care, an emerging body of evidence is finding useful and cheap numbers that anyone can track. If these new numbers aren’t being taken seriously, it may be because they seem too obvious.

The speed at which you walk, for example, can be eerily predictive of health status. In a study of nearly 35,000 people aged 65 years or older, those who walked at about 2.6 feet per second over a short distance – which would amount to a mile in about 33 minutes – were likely to hit their average life expectancy. With every speed increase of around 4 inches per second, the chance of dying in the next decade fell by about 12%.

Hamblin writes that walking speed isn’t unique. Studies of simple predictors of longevity like these come out every couple of years, building up a cadre of what could be called alternative vital signs. In 2018, a study of half a million middle-aged people found that lung cancer, heart disease, and all-cause mortality were well predicted by the strength of a person’s grip.

Yes, how hard you can squeeze a grip meter. This was a better predictor of mortality than blood pressure or overall physical activity. A prior study found that grip strength among people in their 80s predicted the likelihood of making it past 100. Even more impressive, grip strength had good predictive ability in a study among 18-year-olds in the Swedish military on cardiovascular death 25 years later.

He writes that another study made headlines earlier this year for declaring that push-up abilities could predict heart disease. Stefanos Kales, a professor at Harvard Medical School, noticed that the leading cause of death of firefighters on duty was not smoke inhalation, burns, or trauma, but sudden cardiac death. This is usually caused by coronary-artery disease. Even in this high-risk profession, people are most likely to die of the same thing as everyone else.

Still, the profession needed effective screening tests to define fitness for duty. Since firefighters are generally physically fit people, Kales’s lab looked at push-ups. He found that they were an even better predictor of cardiovascular disease than a sub-maximal treadmill test. “The results show a strong association between push-up capacity and decreased risk of subsequent cardiovascular disease,” Kales says.

Hamblin writes that usually when studies like these come out, pockets of experts talk about how they should “incorporate it into clinical care” or otherwise take these new metrics seriously to cut down medical costs and to monitor health in ways that are better than body weight. Then the novelty fades, and the system keeps relying on body weight.

But Kales contends that metrics beyond BMI and age have to be taken seriously. This is driven in part by the Americans With Disabilities Act, which mandates that people not be discriminated against in occupational settings based on BMI or age. “Before the ADA, a fire or police department might have a BMI standard where they won’t accept you,” Kales says. “Now they want functional standards.” That is, they want to know whether you can do the job – if you’re fat.

Hamblin writes that the push-up study could reasonably extend beyond firefighters. “Push-ups are another marker in a consistent story about whole-body exercise capacity and mortality,” says Michael Joyner, a researcher at the Mayo Clinic whose work focuses on the limits of human performance. “Any form of whole-body engagement becomes predictive of mortality if the population is large enough.”

That is to say: Health is not simply about push-ups. There’s also nothing magic about grip strength or walking speed. But these abilities tend to tell us a lot. Firefighters with higher push-up capacity were more likely to have low blood pressure, cholesterol, triglycerides, and blood sugar, and not to smoke. People with the lowest grip strengths were more likely to smoke and have higher waist circumference and body-fat percentage, watch more TV, and eat fewer fruits and vegetables.

He writes that essentially, these quick metrics serve as surrogates that correlate with all kinds of factors that determine a person’s overall health – which can otherwise be totally impractical, invasive, and expensive to measure directly. If we had to choose a single, simple, universal number to define health, any of these functional metrics might be a better contender than BMI.

Hamblin writes that a good metric of health should be meaningful, measurable, actionable, and durable. Body weight and BMI aren’t always meaningful or actionable – as many people who’ve tried to change theirs are aware. Other metrics require drawing blood in a doctor’s office or spending money to send saliva or faeces off to a lab to get genetic-sequencing results from companies such as 23andMe or UBiome. You don’t even need to track all your steps every day, unless that’s somehow fun. A single minute of push-ups or grip strength could track progress just as well.

Doing things that produce tangible, short-term results can lead to a domino effect of health behaviours, Hamblin writes. “If someone reads this article and starts doing push-ups, it would be a statement about their general conscientiousness and motivation,” says Joyner, “and that speaks to so many other health behaviours. People who follow guidelines, eat well, get their kids vaccinated – they tend to engage in other healthy behaviours.”

This “conscientious” type of behaviour, Joyner notes, “is about as predictive of mortality as fitness itself.” And unlike BMI, push-ups and the like tend to encourage people to be conscious of what the body can achieve, not body image itself. Conscientiousness, Joyner says, means seeing a connection between how you live and what happens later, and behaving accordingly.

Hamblin writes: “More than pecs or triceps, push-ups build conscientiousness. There, I’ve written my first motto for a high-end gym. But in a real sense, this sort of metric could equip us to cope with a treatment-based health-care system that teaches people that we can do what we like and then be healed with a pill or procedure. The marketing and sale of medical services tears down conscientiousness. Functional metrics of health could help build it back up.”

The Atlantic report

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