Health is more strongly correlated with body-fat percentage and distribution than with overall weight, but 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. As these numbers continue to dominate health care, however, 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 in the Journal of the American Medical Association, 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 percent. (Whenever I think about this study, I start walking faster.)
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.
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. Because 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 submaximal treadmill test. “The results show a strong association between push-up capacity and decreased risk of subsequent cardiovascular disease,” Kales says.