GU gives the most concentrated form of immediate energy with no stomach bulk or distress, is quickly and efficiently absorbed, and will maintain your blood sugar at the optimal level for a great workout start to finish. During a workout or race, no matter what type, you need to quickly absorb carbohydrate-based calories to replace the glucose you are burning at 400 - 600 per hour or you will begin to lose concentration and energy. The fat, fiber and protein found in bars are very slow to absorb and create unwanted bulk in the stomach.
GU is created for the body during exercise, and because of its form and ingredients is absorbed much faster than a bar so you will feel the energy almost immediately.

Ingredients

MALTODEXTRIN (GLUCOSE POLYMERS), FILTERED WATER, FRUCTOSE, GU AMINO ACID BLEND (LEUCINE, VALINE, ISOLEUCINE, HISTIDINE), POTASSIUM AND SODIUM CITRATE, GU ANTIOXIDANT BLEND (NATURAL VITAMIN E AND VITAMIN C), SEA SALT, NATURAL VANILLA, CITRIC ACID, CALCIUM CARBONATE, SODIUM BENZOATE, POTASSIUM SORBATE, GU HERBAL BLEND [CHAMOMILE, COLA NUT (HAS CAFFEINE), GINGER], FUMARIC ACID, PECTIN.

GU energy gel comes in both caffeinated and non-caffeinated flavors.
Strawberry Banana and Lemon Lime do not have any caffeine in them.
Mandarine Orange, Vanilla, Chocolate, Triberry and Plain all have 20mg of caffeine per packet.
Espresso Love & Jet Blackberry flavors have 40mg of caffeine.
Caffeine consumption during exercise improves performance in several ways: it sharpens focus and perks you up; but more importantly, it helps the body produce more power, reduce the pain of hard efforts, and may even tap fat for fuel during exercise. All of this prolongs your ability to exercise at a high intensity. We use just enough caffeine to jump start this benefit and no more."