They are best for certain types of exercise, says our expert. Here are tips on when to take them and what to look for when choosing a gel.

Those convenient carb-loaded packets work quite well to boost energy “and are best for exercise that’s strenuous enough to raise your heart rate significantly and lasts longer than 60 minutes,” says Julie Brown, RD, manager of Life Time’s nutrition-coaching program. “If your energy level is up for that duration, there’s a good chance that you’re going to need some refueling.”

The gel form “allows the body to access the glucose or fructose in them more quickly than if you were having to digest a piece of fruit or another source. You’ll tend to feel more energy within a few minutes.”

They generally contain 100 calories, Brown says, “enough for about 45 minutes of activity.” Then, at that point you can consume another dose. But the time factor will vary depending on your weight and your exertion.

Avoid the temptation to wolf down a packet right after your workout, unless you feel that your blood sugar is really tanking. “It’s always best to rehydrate using an electrolyte-enhanced sports beverage after exercising,” she says. “And then go for a balanced meal that includes protein and complex carbs.”

When choosing a gel, study the label, she advises. “Some have additional ingredients like caffeine or electrolytes, for example, and you need to be aware of that, and whether those ingredients are right for you.”

But the most important caveat, she notes, “is training with the products you’re going to consume on race day, so you can experiment with amounts and timing to figure out just what you need.”