A more sedentary lifestyle in your 30s may cause your muscles and connective tissues to contract, but it also tends to make your waistline expand. Even if you're active, staying slim can be a mighty challenge because the body uses 12 fewer calories a day for each year past 30. This is also the decade in which extra body fat creeps up. In fact, if you're not vigilant, your body can quickly exceed the recommended level of 18 to 22 percent lard content [source: Brant].
The solution is simple: Eat less.
This might not sound like very much fun, but by relying on portion-control guidelines, you could turn the whole thing into something of a game -- and because you're likely already eating things in bigger portions than what's recommended, you might even lose weight. When you plan your next meal, keep in mind that one portion of pasta or rice is about the size of a hockey puck. A portion of cubed cheddar or cottage cheese is about the size of four dice, and a proper portion of protein (like a chicken breast or burger) is about the size of a deck of cards [source: Mayo Clinic]. If these sizes sound too small, fear not -- you can have multiple portions. Just try to keep it all within the boundaries of a plan like the Mayo Clinic Healthy Weight Pyramid.