I've had a quiet complaint about BMI for most of my twenty years in nursing, so it's genuinely nice to see research catching up. A new study highlighted by SBS suggests body mass index, that single number so many doctors' offices still lean on, may actually be misleading for older people.

The core problem is simple once someone explains it. BMI is just weight divided by height squared. It can't tell the difference between a kilo of muscle and a kilo of fat. For a 25-year-old athlete that's an annoyance. For a 75-year-old, it can blur the whole picture, because aging changes our body composition whether we like it or not. We lose muscle and gain fat, sometimes while the number on the scale barely moves.

This is where the so-called "obesity paradox" comes in. For years, studies showed older adults with a higher BMI sometimes lived longer than their thinner peers, and it puzzled researchers. How could extra weight be protective? The new findings suggest the answer may be that BMI was measuring the wrong thing. Some of those "overweight" seniors carried more lean mass, meaning muscle, and muscle matters enormously as we age. It protects against falls, keeps us mobile, supports metabolism. A BMI that looks slightly high might actually be recording a sturdy, well-muscled body.

Meanwhile someone in the "normal" range could be what researchers call sarcopenic, carrying too little muscle and too much fat at the same time. My mother-in-law was like that late in her life. Tiny on the scale, frail in ways the chart never captured. I remember thinking the number told us almost nothing about what she could actually do, whether she could climb the stairs, carry her groceries, get up from a chair without pushing off the arms.

So what should you take from this if you're over 65, or caring for someone who is? Don't panic over the BMI number alone, and don't celebrate it either. It's one data point, not a verdict. If you want a clearer read on your health, ask about function instead. Can you rise from a chair without using your hands? Walk at a steady pace? Carry a load of laundry up the stairs? Those everyday tasks reflect the muscle mass that BMI can't see.

And if you have the chance, ask your doctor about waist circumference or body composition measurements. They're not perfect either, but they distinguish between fat and lean mass, which is exactly what BMI fails to do.

My son Linton teases me that I've never met a health metric I didn't want to interrogate. Fair. But this one has earned the skepticism. A number that can't tell muscle from fat was never going to tell the whole story, especially for the people who need the most accurate picture of all.