He told Access Hollywood: "I gave up sugar. I lost 30 pounds in four months. It's amazing.
"I do Pilates and spin, but I don't do as much yoga as I'd like. When we're shooting and I can't work out, I just have to eat less. So I'm very conscious of that. But sugar was the real killer for me - that was the problem."
Another reason for 53-year-old Alec's new healthy lifestyle could be that he wants more children with his 27-year-old girlfriend, yoga instructor Hilaria Thomas.
The actor - who has 16-year-old daughter Ireland with ex-wife Kim Basinger - recently said: "Have I thought about having more kids? Oh sure, that would be great, that would be heaven, that would be fantastic."
He jokingly added: "As my friend said to me, 'When you have children, typically in a second marriage, when you're older and you get married again to a woman who would have children, you must always remember that you make sure the children attend a college where the commencement ceremonies are held in a facility with a wheelchair accessible ramp.'"