Photos: Celebrity transformations Filmmaker Kevin Smith in 2008, left, and in 2015. The director of "Clerks" and other movies tweeted in June 2015 that he lost 85 pounds. His secret? Walking 5 miles daily and giving up sugary drinks. In August 2018 he shared on social media that a plant based diet helped him shed 51 more pounds after he suffered a heart attack earlier in the year. Hide Caption 1 of 24

Photos: Celebrity transformations Kelly Clarkson has always been clear she doesn't care what people have to say about her weight. In 2018 the singer/ "The Voice" coach showed off a more slender figure and joked that she had "literally hired Harry Potter and SPANX, it's all like a sausage." Hide Caption 2 of 24

Photos: Celebrity transformations Actor Craig Robinson said in October 2017 that he lost 50 lbs. by giving up alcohol, working out and going vegan. Hide Caption 3 of 24

Photos: Celebrity transformations Macaulay Culkin's new look is winning rave reviews from fans. The actor was spotted in July 2017 having gained weight and cut his hair. The child star, seen on the left in 2014, looked so gaunt in 2012 there was speculation he was on drugs - which he denied. Hide Caption 4 of 24

Photos: Celebrity transformations Jonah Hill fans immediately noticed the actor's new summer bod for 2017 and applauded his fit overhaul as inspiring. Hill credited his recent slim down to advice from "21 Jump Street" co-star Channing Tatum. Hide Caption 5 of 24

Photos: Celebrity transformations "Girls" star Lena Dunham showed off a slimmer physique in March 2017 on the red carpet for the opening of celebrity fitness trainer Tracy Anderson's new studio in New York City. Hide Caption 6 of 24

Photos: Celebrity transformations Talk show host Wendy Williams lost 50 lbs over three years and said in February 2017 that she is keeping the weight off. Hide Caption 7 of 24

Photos: Celebrity transformations Lil' Kim, left, at a 2003 BET event, has stirred controversy over her changing appearance in the past, but these days she's unrecognizable -- and people are talking. Hide Caption 8 of 24

Photos: Celebrity transformations If you know Chris Hemsworth as the mighty Thor -- at left in 2011 -- his diminished size and scraggly face for the upcoming film "In the Heart of the Sea" might come as a shock. Hemsworth tweeted the photo on the right in November 2015. "Just tried a new diet/training program called 'Lost At Sea.' Wouldn't recommend it," he wrote. Hide Caption 9 of 24

Photos: Celebrity transformations "Roseanne" star John Goodman is known for his burly frame in addition to his acting chops. Goodman has slimmed down noticeably, as was evident at a screening of "Trumbo" in October 2015. The photo on the left was snapped just a year earlier at a showing of "The Gambler." Hide Caption 10 of 24

Photos: Celebrity transformations Shonda Rhimes, creator and producer of such hit shows as "Grey's Anatomy," "Scandal" and "How to Get Away with Murder," has undergone quite a transformation. She credits diet and exercise for her loss of more than 100 pounds. Hide Caption 11 of 24

Photos: Celebrity transformations Melissa McCarthy has been shedding weight, and she told Gayle King on "CBS This Morning" that she simply stopped stressing over it. "I feel amazing ... and I finally said, 'Oh, for God's sake, stop worrying about it,' and it may be the best thing I've ever done," she said. Hide Caption 12 of 24

Photos: Celebrity transformations Matthew Lewis, who starred as the shy and unassuming Neville Longbottom in the "Harry Potter" movies, has grown into a strapping young man. He buffed up for roles as a soldier in "Bluestone 42" and an athlete in "Me Before You." Hide Caption 13 of 24

Photos: Celebrity transformations Penn Jillette told People magazine there was no magic involved in his weight loss from 330 to 225 pounds. The performer, who is half of the illusionist act Penn & Teller, just changed his eating habits to shed 105 pounds and get his high blood pressure under control. Hide Caption 14 of 24

Photos: Celebrity transformations Singer Sam Smith is looking svelte these days (at left, arriving at an event in March 2015) and has credited nutritionist and author Amelia Freer with helping him change his diet and look. Hide Caption 16 of 24

Photos: Celebrity transformations Actress Uma Thurman had some doing double-takes at the premiere for "The Slap" in February 2015. The actress, 44, plays a TV writer in the U.S. update of the Australian show. Hide Caption 17 of 24

Photos: Celebrity transformations The transformation of Zach Galifianakis continued at the Screen Actors Guild Awards in January 2015, where the actor was barely recognizable from years past. What a difference some pounds and some facial hair can make. He first started slimming down in 2013, when he decided to stop drinking. Hide Caption 18 of 24

Photos: Celebrity transformations Kirstie Alley set out to lose 30 pounds in 2014, but she went above and beyond and lost 50. Alley, a paid spokeswoman for Jenny Craig, used the weight loss program to slim down over the past year. Although her size has fluctuated in the past, Alley assured "Today's" Matt Lauer then that "This time, it's different." Hide Caption 19 of 24

Photos: Celebrity transformations When Renee Zellweger arrived at the 2014 Elle Women in Hollywood awards, some people said they couldn't recognize her. Perhaps that's because the "Bridget Jones" star has spent less time in the public eye recently; her last film credit was in 2010. That should change soon: Zellweger's next film, "The Whole Truth," is due in 2015. Hide Caption 20 of 24

Photos: Celebrity transformations Hip-hop artist Missy Elliott proved she can still work it as she flaunted a slimmed-down figure in October 2014 at the launch of designer Alexander Wang's line for H&M. Apparently, an autoimmune disorder is not slowing down the "supa dupa fly" producer, who tweeted a picture of herself in a workout cap that reads "You Can't Workout With Us" after performing at the runway show. Hide Caption 21 of 24

Photos: Celebrity transformations Tom Arnold has lost about 90 pounds since his first child was born in 2013, and he was looking quite thin at the 2014 South by Southwest festival. He'd actually lost the same amount of weight before but regained it when he didn't maintain healthier habits. After his son was born, he knew he needed to make a lasting change. "I saw that little baby, and I thought, 'I gotta stay alive for as long as possible,' " Arnold said. "That's a lifelong commitment." Hide Caption 22 of 24

Photos: Celebrity transformations Jessica Simpson showed off a supersvelte look in a Weight Watchers ad released in February 2014. "I was so insecure -- I couldn't even believe what I weighed," Simpson told "Good Morning America." The singer/fashion mogul says she is feeling better than ever since giving birth to her second child. Hide Caption 23 of 24