Last week I learned something a little distressing: I’m 4 pounds from being obese — at least according to one measure.

In talking to Dr. Steven Heymsfield of the Pennington Biomedical Research Center about body mass index, a formula used to estimate a person’s body fat, he guessed my BMI to be 24 or 25 — the upper limits of healthy or normal. In my street clothes, I look pretty fit.

He was a little surprised when I told him I had calculated my BMI at 29.5, just a few pounds from obese, which is a BMI of 30 or above.

“No way,” he said. “Really? I never would have guessed it.”

It’s common for fit athletes to have BMIs over 30, even though they have extremely low percentages of body fat. That’s one problem with the BMI formula — it can’t differentiate between muscle weight and fat.

But I’m not a super fit athlete. I exercise five days a week, but I’m not skinny. I’m 5 feet 10 inches tall and weigh 205 pounds.

I like to mountain bike, where riders over 200 pounds are called Clydesdales. While they are graceful, beautiful horses, Clydesdales don’t race the Kentucky Derby. They pull beer wagons.

Over the past two years I’ve lost 20 pounds — 10 of those since February — so I’m feeling pretty good.

The doctor’s surprise at my BMI made me curious about my weight and its relation to my health. The Pennington folks offered to put me through a gamut of body composition tests that would detail exactly what I’m made of.

Last Tuesday, I became a lab rat, where I faced a room filled with six machines, each designed to figure body composition in a different way.

Pennington regularly receives contracts to test these machines for accuracy. One device used lasers; another electricity. Some of the machines are on the market; others are prototypes.

I changed into snug shorts, pulled on a skull cap and draped myself in a hospital gown. The most painless tests were first: Two machines made by Tanita that send an electric current through your body. In less than a minute, they printed out results — body fat percentage, muscle mass analysis and dozens of other pieces of data.

Then I had two three-dimensional body scans. For one I stood beneath an arch, arms akimbo, while sensors captured every bit of me. For the second, which was pretty impressive, I stood in a curtained compartment while a laser scanned me up and down.

Shortly a 3-D scan of my body began to appear on the computer monitor. One of the lab assistants described it as a “Pixar-like animation.” Later they emailed me a list of 170 pieces of data gleaned from the scan.

For the final test, I stretched out on a bed while LSU graduate students used an ultrasound machine to examine my biceps, triceps, thighs and calves. On the screen they pointed out my bones and the layers of muscle and fat atop them. Apparently my triceps were more muscular than I thought, my biceps less so.

That’s the way it feels when the test results print out. Some things are much better than you expected. Others are disappointing.

My body fat percentage was between 19 and 23 percent, according to different tests, which means I’m “acceptable.” That’s better than obese, right?

According to the Tanita electrical current machines, my physique was rated “solidly built,” and I had a healthy visceral fat rating, which means I have a decent amount of fat covering my mid-section.

So what did I learn from being poked and scanned by a bunch of strangers? My health is OK, but it could be better.

And from now on there’s no deceiving myself when I step on the scale. It isn’t all muscle.