Am I muscular or just fat? Find out in the back of a truck

Brian Lee’s rig seems just another white box truck zipping along I-5.

Until you see the picture on the side of a half-naked guy.

What’s up with that?

It’s a fat-mobile.

The truck carries a large steel tank used to conduct underwater testing that measures the fat that lurks inside your body. A quick dip gives a percentage of blubber versus muscle.

Like, why would anybody want to know that?

“Your body composition is just another health metric, just like blood pressure or your A1C,” Lee said, referencing the blood test used to diagnose diabetes. “Sometimes it’s a wake-up call.”

Lee has no formal medical training, nor does he give medical advice.

“I’m not a doctor and I don’t play one on TV,” he said.

Lee, an independent contractor for Body Fat Test, takes his truck to gyms and events likely to attract the health conscious. You typically won’t find him at venues that serve corn dogs or fried Twinkies.

He has done thousands of tests in his 12 years on this job where he can set his own hours and wear shorts and flip-flops to work.

The 28-foot Ryder Freightliner rental truck hauled cargo its first 99,000 miles before being retrofitted into a rolling hydrostatic body fat “clinic.”

The gas tank holds 100 gallons of diesel. The dunk tank, on the other hand, holds 300 gallons of chlorinated water.

Lee opens a side door to the cargo hold turned office, puts down steps — and in you go.

Inside are two curtained dressing rooms, a desk, stainless steel tub and, for the heck of it, rubber ducks.

Bring a suit. No skinny-dipping.

Slink into a 2-foot pool of 92-degree water, hold your nose and blow out like a fish. Try not to cry when you get your numbers, good or bad.

Don’t blame the messenger. Lee just punches in the data … and bada-bing bada-boom!

“The computer does the work,” he said. “We have a high-tech proprietary formula.”

In addition to body composition, Lee said the test also measures metabolic rate, caloric burn during activity and how you compare to others of similar age and gender.

At a recent session parked outside 24 Hour Fitness in Lynnwood about a dozen people took a dip, one at a time. This isn’t a communal plunge.

Cost is $49 for newcomers and $35 for follow-ups. Lee doesn’t deal with any medical insurance.

The process begins with a height and dry weight measurement, then a pep talk from Lee.

“Blow fast. Squeeze hard. Because air you don’t get out shows up as fat,” he told Hayley Nuval, 25, as she floated in the tank.

“Lower yourself slowly. You are not racing Michael Phelps in there. I want to see those veins popping out.”

He had Nuval repeat the process several times, allowing long breaks in-between for air. People stay underwater for about seven seconds. There aren’t any buzzers or lights or bops on the head by Lee.

After she toweled off, Lee gave Nuval a printout of her report card.

Nuval took the plunge as a guide for powerlifting and bodybuilding meets.

“It’s good information to know where exactly I am now so I can start setting realistic goals for where I want to be for my next two competitions,” Nuval said.

Powerlifter Dylan Shenefield, 27, took off his “Excuses don’t build muscles” T-shirt and got tanked.

Shenefield described the process this way: “Ever play that game in a pool where you blow all the air out of your stomach and someone uses you as a surfboard? That’s what it seems like.”

There are other ways to get the skinny on fat.

Skin fold tests use calipers, a simple pinching device. Or you can sit inside a computerized, egg-shaped chamber such as the Bod Pod. Or get zapped by a machine with tiny electrical impulses that measure fat mass. Then there’s the proverbial BMI, body mass index, based on height and weight mathematics.

Lee occasionally tests himself in the tank.

“I’m not as lean as I was 13 years ago,” he said. “Let’s just say that I’m a work in progress.”

Lee lives in Mukilteo with his wife, Laura Brent, who owns a land-use consulting firm.

He was a Seattle Times union circulation worker for about 30 years and retired at a relatively young age with a pension.

“I was coaching high school sports and selling real estate. I stumbled onto this,” he said.

While channel surfing, he flipped on a reality show about a woman who had a Body Fat Test test. “I thought, ‘This is the coolest thing ever.’ I did a little Googling,” he said.

He started in 2007 with a former Penske truck with a giant dunk tub. It’s the one that says “Get Tanked” on the side.

He added a second truck a few years ago, this one with a smaller tub that is about 7.5-feet long and 3.5-feet wide, a perfect fit for one of his regulars.

“The dude is huge, one of the biggest humans I’ve ever seen. He’s about 6-foot-8 and pushes 400 pounds,” Lee said. “I had him stand there and got a measure of the width of his shoulders because I’m never going to see a human bigger than him.”

Andrea Brown: abrown@heraldnet.com; 425-339-3443. Twitter @reporterbrown.