Smith claims that he's lost over "one stone" (14 pounds) in only two weeks.
The 22-year-old "Stay With Me" singer posted a black-and-white photo of himself holding a book written by nutritionist Amelia Freer, who he credits for his dietary success.
"Three weeks ago I met a woman who has completely changed my life," Smith captioned the photo. "[Amelia Freer] has completely transformed my relationship with food. Everyone go check out her incredible book, and start to live healthy. It's not even about weight loss it's about feeling happy in yourself. Love you Amelia & thank you for making me feel so happy inside and out."
The book, "Eat. Nourish. Glow.: 10 Easy Steps for Losing Weight, Looking Younger & Feeling Healthier," was released in January and is Freer's first book.