The Sims 2/Fitness

Every sim aged child or older has a level of fitness in The Sims 2 that can be expressed in The Sims 2. This article explains how to gain fitness and how to lose fitness.
 * Fit
 * Neutral
 * Fat

Gaining Fitness
Fitness is gained by gaining body skill. Any time your sim is working out to the T.V. or stereo, swimming in a pool or using the exercise machine, they are gaining fitness. When you've exercised enough, your sims will start to visibly change. You can also check your fitness level in the personality section of the simology panel. You'll notice that it goes up slowly while your sim is exercising.

Losing Fitness
Fitness is lost by eating. Specifically, eating when the hunger bar is completely full. When your sims have eaten to the point of maximum hunger, any more food they eat is instead switched to the fitness panel and fitness is lost. For lazy sims, this is a challenge if you want them to be fit. Some sims can have wants to be fit and fears of being fat. If you want to keep their scores high, it's a good idea to keep them fit and healthy. Being fat won't cause a disease. It'll just make sims unhappy.