The weird thing that makes Sags gain weight...
Your ruling planet Jupiter tends to expand everything it touches. So for Sagittarius natives, it's crucial to adopt a healthy diet and fitness plan, so your waistline isn't the thing expanding throughout your life.
You may find it tough to count calories, since freedom is your thing. But you can get behind anything that's going to make you happier and healthier.
Eating lots of organic fruits and vegetables puts you more in touch with the earth, which you love, and adding hikes in nature burns calories. You won't even notice you're exercising!