Gel packs have become popular in the last few years, especially for use in longer races and training runs. However, few runners are aware of a major problem in using them properly; and in most cases, they are actually a detriment to performance.
Quoting from Owen Anderson's article in "Running Research News," " . . . runners have been slipping them into their digestive systems without paying much attention to how much water is also going into their gullets. As a result, they often end up bellies filled with . . . a gelatinous syrup. This can actually drag intracellular water into the stomach, in effect increasing the risk of dehydration. . . . It seems safer to simply use sports drinks, which are formulated to have the right balance of H2O and carbos."
Here is the problem in simple terms: It is a well-established fact that the maximum transfer of water and carbos from the stomach to the blood stream occurs with a 7% solution. However, the rate of absorption decreases rapidly for rations greater than 7%; there is a significant loss at just 10%. Typical gel packs have about 28 grams of carbos. To produce a 7% solution in your stomach, you would have to drink 13.5 oz of water along with a 28-gram gel pack.
First, drinking 13.5 oz of water is just too much at one pit stop; and second, few runners can accurately judge how much is 13.5 oz. Let's see what the effect is at a typical water stop. You've eaten a gel pack and drank 7 oz of water [from an 8-oz cup, they are rarely full]. The carbo / water ratio is now 13.5%; far exceeding the 7% ideal ratio. Now consider the fact that this gel pack and 7 oz of water combination is repeated several times on the run and it is obvious that you WILL become dehydrated. The rule for computing the proper ratio is: about 1 oz of water for every 2 grams of carbo. Anderson states: 3.4 oz of water for every 7 grams of carbo.
Incidently, the 28 grams, or so, of carbos provide the energy needed to run less than one mile. As a practical matter, they really are not going to measurably affect ones performance.
So then, why do so many gel pack devotes praise the benefits of gel packs? I suspect that because they taste sweet, they simply signal the brain that "sugar is on the way" and the feeling of being hypoglycemic [after or during a long run] will soon pass. From my own experience, I know that a sweet sport drink is a big mental booster at mile 20 in a marathon. I also know it is mainly a mental booster, not a physical one.
My recommendation for using gel packs would be to practice eating about 1/3 [about 9-gms] each time and drinking 7-oz of water. This will yield a ratio of somewhat less than the ideal 7%. However, you'll still get the mental boost without risking dehydration.