On days that you are heading to the gym make sure that you plan to eat something before and after you workout. A little bit before your workout your body needs some quick energy in the form of a light carbohydrate. This provides energy while exercising that will help you perform at a steady rate.
Post workout it is also important to eat a little something. Many people are not hungry after working out but it is important to eat some form of protein afterwards. Eating protein up to 30 minutes after a workout aids in muscle recovery and gets amino acids to the muscles. Make sure that the protein you choose is lower in calorie or considered a "lighter food". Your body doesn't need a lot of protein to help with muscle recovery. One of the most recommended things to consume post workout is chocolate milk. Chocolate milk is known to be the perfect carbohydrate to protein ratio. This will also aid in recovery and is highly recommended for athletes post workout.
If you are at an impasse and are still not sure what to eat pre and post workout here are some healthy options:
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http://www.webmd.com/diet/what-eat-before-during-after-exercise?page=2