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Is goo for you? We test popular energy gels

— By Kristin Kalning, senior technology editor for msnbc.com

What they are: Gu Energy Gel (approximately $8 for a 6-pack), Clif Shot Energy Gel (about $30 for a 24-pack) and Luna Sport Moons energy chews ($14.99 for 12 packets).

What they claim to do: The companies behind the three products (Luna is the women-targeted brand of Clif) say that the products provide easy-to-digest carbohydrates for endurance athletes. The main difference between the products is where these carbohydrates come from: Clif Shots come from brown rice syrup, Luna chews also come from brown rice syrup with some cane sugar and Gu is comprised of maltodextrin and fructose.

All three products contain approximately 100 calories, 25 grams of carbohydrates and little or no fiber, fat or protein. According to the companies who make the products, this streamlined makeup helps speed the carbohydrates to the bloodstream. Gu is the one product that I tried that has caffeine – 20 milligrams for the flavor I tried. That's less than a Diet Pepsi, but more than a single Hershey's Chocolate Kiss. Clif's Shots have flavors with caffeine, but not in the one I tried.

Image: Chocolate Outrage Gu packet

The instructions for all three products recommend eating one serving before start your activity, and about every 45 minutes thereafter. Usually, they suggest having another serving when you finish, to aid in recovery.

Our experience:  I'm hardly a hard-core athlete. I started running about 20 years ago to take off weight, and I've kept it up because it's convenient and inexpensive. But exercising to be thin gets boring after awhile, so I started participating in races — 5Ks and 10Ks to start, but now I'm doing half-marathons and short triathlons.

In my first half-marathon, I heard people at the starting line talking about their "goo," and I didn't have any idea what they were talking about. Midway through the race, I noticed participants with tiny foil packets sticking out of their mouths, running as they sucked at whatever magic elixir was inside. I wanted in on the secret, so I bought a couple of packets and then promptly forgot about them, until a race last year.

At the starting line of that race, I too had my Gu – Chocolate Outrage flavor – which I sucked down 15 minutes prior to starting with a little water, as per the instructions. I didn't notice anything other than adrenaline and some butterflies, but I ate more gel about an hour into the race. I wouldn't seek it out if I needed a chocolate fix, but it tasted OK: very sweet, with a pudding-like consistency.

Trying something new like an energy gel or even a sports drink during a race can be a recipe for disaster – something I didn't know at the time. If your stomach doesn't appreciate the new substance, you're in trouble – with just a Port-a-Let for comfort. It's best to experiment when you're training, and you can call your husband to come fetch you if your tummy's revolting.

My tummy didn't revolt – in fact, I totally forgot about eating the gel until about 15 minutes later, when I felt like I was getting my second wind. My legs felt lighter and faster. That's when I remembered that little packet of sweet I'd slurped a few minutes before. It was the Gu! It had to be the Gu.

After that, I was a convert. I used Gu during training runs and the phenomenon I'd experienced during my race wasn't an isolated one: If I had a packet before my run and then during, I would feel a little lift about 10 to 15 minutes after eating it. But I didn't love the flavor, so I thought I'd try something else. Enter Clif Shots, in Chocolate (I've got no use for vanilla as a general rule).

Clif Shots have a thinner consistency than Gu, and a lighter flavor. I followed the instructions faithfully, eating a packet prior to running and then again about an hour into my run.  And though I felt a slight bump to my performance after consuming, it wasn't as noticeable as the one I'd felt with Gu.

Next up, the Luna Moon energy chews, which I bought in Blueberry. Each packet contains two servings, and a serving equals about six pieces. The instructions on these were a bit different, suggesting that athletes eat half the packet 30 minutes prior to activity, and every 30 minutes thereafter. So I did.

From an operational standpoint, the chews were more work. With a gel, you just rip open the packet, jam the opening in your mouth, and make like a Hoover. With chews, you've got to extract them from the packaging. A small thing, certainly, but here's another: The chews have the consistency of Dots candy. I adore Dots candy, but they're thick and they stick to my teeth. Awesome if you're watching a movie. Not so awesome if you're in the middle of a 10-mile run.

Still, the chews gave me a mild pick-me-up, about the same intensity as the Clif Shot. And I really did prefer the taste of the chews over the energy gels. The gels are definitely more convenient if you're running and especially if you're on a bike, but the sweetness can be too much for some.

What the experts say: 
Studies have shown that athletes can experience performance benefits from replenishing glucose during hard workouts, says Celia Framson, Registered Dietitian. But she cautions that some formulas can cause stomach upset by drawing water out of the body into the gastrointestinal tract and delaying the movement of fluid out of the gastrointestinal tract into the body.

"How much carbohydrate replenishment an athlete needs is highly individual. The trick is to maximize energy repletion while minimizing GI discomfort," she says. "An optimal intake in studies is 60 grams per hour (which gives you 240 calories), but your best bet is to start low and experiment."

I didn't experience any stomach upset while using these products, but I did wonder why I felt slightly more juiced after taking Gu than the other products. "Scientifically, it is plausible that Gu works better in the body because it gets its carbohydrates from a combination of glucose and fructose in a proportion that could be better absorbed by the body than the proportion in the Luna chews," she says. But, "there isn't a ton of science on the subject."

Framson also points out that both the Luna chews and the Clif Shots contain brown rice syrup, which is a complex carbohydrate that takes longer for the body to digest and absorb. "That might not be the best choice for quick energy for your muscles and body tissues, but it may help you if you are running a marathon."

Both Gu and the Clif Shots contain a small amount of electrolytes, which help to regulate hydration in the body. But Framson says the amounts are so small that "they wouldn't make a dent in the amount you lose during endurance exercise."

Bottom line: Gels and energy chews aren't for everyone, and as Framson pointed out, "each individual has to experiment to find the optimal amount and product." Many of my running buddies don't use gels at all – "Can't stand the taste," shuddered my friend Karen. But others – like me – swear they improve performance. The first day I tried Gu I shaved nine minutes – nine minutes! – from my personal best half-marathon time. Was it the energy gel, the flat course or my dogged determination? I don't know, and for $1.25 per pack, I don't much care.  I plan to have my energy helpers with me during every training run, and every race for as long as I can put one Saucony in front of the other. In the middle of a lonely 12-miler, I need all the help I can get.