YO, HAVE YOU seen District Attorney Seth Williams lately?

The 47-year-old looks lean and buff. He's dropped 45 pounds - and he did it in just 100 days.

One hundred days.

Insert head shake here, because that's a whole lot of fat to slough off a body in such a short time.

I caught up with Williams last week at the Sporting Club at the Bellevue, where he's been spending considerably more time lately. As we talked about his recent weight-loss results, his voice had the enthusiastic zeal of a recent fitness convert.

He jumped off the bench where we sat not once but twice, midinterview, to pull his T-shirt over his head, revealing a trim waist, bulging biceps and a buff chest.

I couldn't turn away. My journalistic instincts wouldn't let me. He'd undergone a shocking transformation.

In September, I wrote about how Williams was serious about getting in shape. He had a 38-inch waist and, at 5 feet 11 inches tall, he was a portly 243 1/2 pounds. Which meant he was built like a whole lot of other middle-age American men.

Williams, who worked out sporadically, also had high blood pressure.

Back then, America was in the midst of the viral craze of the ALS Ice Bucket Challenge. People were posting videos of themselves dumping frigid water over their heads, part of an effort that raised more than $21.7 million to combat amyotrophic lateral sclerosis, also known as Lou Gehrig's disease.

Williams came up with a plan to continue raising money for ALS research and to help his cousin, Wayne "Fade" Mack, who has it. Williams asked friends and colleagues to pledge $1, or maybe $5, for every pound he lost. I pledged $5 a pound - not thinking he'd lose very much. Yes, I wrote him my check, and rounded it up. He's still collecting on pledges.

"I lost 45 pounds in a hundred days," Williams said. "I couldn't even imagine losing 20 pounds in 100 days, but it became almost like a religious experience for me to help my cousin.

"Everyone had done the Ice Bucket Challenge over the summer," Williams said. "I thought there was something that I could do that was a little bit more significant than just pouring ice on my head."

So he devised his own ALS Challenge and signed up for personal-training sessions with Hannah Purbe, the Sporting Club's fitness director. He also dramatically changed his diet.

Williams stopped adding sugar to his morning coffee and gave up his daily bacon, egg and cheese croissant sandwiches. For lunch, he started to eat rotisserie-chicken drumsticks from the Reading Terminal Market and salads - without croutons.

Dinner was often steak, chicken or fish.

On Thanksgiving, Williams allowed himself to have stuffing and macaroni and cheese but he skipped the key-lime pie.

Williams also started jogging at the Sporting Club once or twice a week and took spin classes. He signed up for weekly boxing classes at Joltin' Jabs on Main Street.

Results were immediate. During the first week of his ALS challenge, he lost 10 pounds.

"Each of these things was a commitment that I tried to fit in. Some weeks I was able to do a lot. Some weeks I couldn't do as much. But I stuck to the no-carbs, no-sugar [diet]," he said. "I think if I had killed myself [with exercise], I probably would have done more. But I'm amazed at how cut up my body is now.

"Anybody who says you can't change your health in your late 40s is wrong. You just have to have a little discipline and you can do it," he added.

I'm inspired that Williams didn't get thrown off his plan by all the traveling he does. Since September, he's been to D.C., Dallas and Thailand.

The sad plight of his cousin, a former barber who was diagnosed with ALS in 2006, must also have been a huge motivating factor. Mack, who lives in a nursing facility, is on a ventilator and has no motor function from the neck down. He operates a computer by looking at the screen.

In the months since Williams started his fitness challenge, Mack and I have exchanged a number of emails, and I've been both amazed at his ability to persevere despite his prognosis. There is no known cure for ALS.

"It's a horrible disease because he still has his mind. He's sharp as a whip but he's literally trapped in his body," said Mack's wife, Angela. "He's faced a lot."

Williams' fundraising efforts moved them both.

"I really don't have the words to express our gratitude" to Williams, she told me. "Just saying thank you doesn't seem enough."

But it will have to suffice.

Blog: ph.ly/HeyJen