The 32-year-old singer opened up about her exercise and diet routines in an interview for the magazine's November issue.
Underwood looks healthy and slim on the publication's cover, less than eight months after welcoming son Isaiah with husband Mike Fisher.
"After I had Isaiah, my goal was to take control of my body again," she shared.
"I gained only 30 pounds, which is the recommended amount, and I exercised throughout my pregnancy ... When I got the OK from my doctor, I started exercising [after giving birth]. Now we box and hike trails, among other things."
"I'm vegan and I love bread, but recently, I started making dinner a breadless meal," she added. "I'll have a bowl of oatmeal for breakfast, and I'll make a sandwich on Ezekiel bread for lunch, but in the evening I'll have protein and vegetables and no bread. When I wake up, my belly is flat!"
Underwood and Fisher, a 35-year-old NHL player, have been together since 2008, and married in July 2010.
The couple welcomed Isaiah on Feb. 27, and the singer recently delighted fans by posting a photo of her son playing a miniature piano.
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