You may think high sugar things will provide all the energy you need to workout. Eating refined sugars causes a a sharp increase in your insulin levels, then drop it fast and causing you to seek more sugar throughout the day. This in turn gets stored as fat in your body.
For extra energy throughout the day or for your workout, try a banana. They are packed with potassium, which aids in maintaining nerve and muscle function
Try making your own smoothies. This way you know exactly what is in it and you also avoid the excess added fat, sugar and preservatives that could have been added
Meals high in fat can take hours for your body to digest, and thus seriously slow down your workout. They will also make you feel heavier and sluggish and diminish your workout intensity and pace. Avoid consuming this even though you are going to the gym. Working out and still eating poorly will not give you the desired results
Energy Drinks and Soda
They contain artificial sweeteners and are carbonated. They are also similar to the refined sugars discussed above. They will also cause a lot of gas and bloating which in turn slow your workouts down
Protein and cereal bars
Watch the nutritional information before buying. Avoid those with added sugar in them or ingredients you cannot pronounce. Also avoid those with tons of calories and so little protein value in them.