Why aren't there diet exercise drinks?

Not one, but two threads out of 7-11 tonight. Go me.
I would sorely love a big glug of Gatorade after my 45 minutes on that damn stationary bike, but the stuff is loaded with 'non-fructose corn syrup" and it’s attendant calories, defeating one of the main points of working out.

Have I just discovered a market niche, or is there some intrinsic reason you couldn’t make a sports drink with splenda/saccharine/aspartame?

There are, Propel for one. It’s pretty widely available.

I don’t know about Propel, but burning calories is not necessarily the goal of all sports activity. Other common goals are to build strength, speed, endurance, and have fun. Gatorade replaces the lost carbs in a way that supposedly is very efficient for the body to use (hellifIknow). It also replaces salt to some degree, very important. It’s also a lot lighter in calories than sodas.

Another vote for Propel. I love the stuff, and a 22oz bottle only has 30 calories; it’s not loaded with sugar. I really like the light taste of it as well. I usually drink a bottle a day.

There’s the flaw in your premise, right there.

Gatorade contains glucose, fructose, and sucrose for a couple of reasons.

Admittedly, a big part of it is to mitigate the taste of all that necessary salt and potassium. Having a variety of sugars does this more effectively than sticking to one type of sugar, and certainly better than using an artificial sweetener. If you replaced the sugars in Gatorade with saccharine or aspartame, it would be difficult to swallow. Splenda might be a bit better, but still not as palatable as that trinity of sweet.

More importantly, the sugars are a source of quickly-available energy. That’s a benefit that would be absent in an artificially-sweetened Gatorade. The supposed purpose of Gatorade is to adjust your body chemistry in order to increase endurance. Heavy exertion depletes blood sugar as well as fluid and electrolytes. A blood-sugar boost helps to improve your stamina – and there’s not much point in replenishing your electrolyte balance and fluid levels if you’re going to get fagged out because your blood sugar is dropping.

Coca-Cola’s “Powerade” line now has a 10-calorie version.

If your main concern is replacing salts, I have a recipe for sugar-free “gatorade” that my hub uses. You basically make two quarts of kool-aid or lemonade and use whatever artificial sweetener you prefer, then add ½ teaspoon each of salt and salt substitute (Nu-Salt).

If your goal is to burn calories, stay away from any sport drink. Plain water is just fine for re-hydration.

Even if the drink has just 30 calories, you don’t need it. How many calories did you burn in the 45 minutes of exercise? Let’s say 600. Those 30 calories are 5% of your workout. Normal Gatorade would be about 150 calories (25% of your workout).

Sports drinks are useful if you are training for performance reasons. They help to replace the calories you use and aid in recovery. But they are counter-productive if you are trying to lose weight. They just replace the calories you lost without giving you any significant benefit.

So if you’re training for a marathon, drink Gatorade. If you’re trying to lose weight, drink water.

The same goes for the energy bars. You should only eat them if you are training for performance reasons (unless it’s some form of meal replacement). If you’re trying to lose weight, they just replace the calories you burned during the exercise.

Oh, and I guess to answer the OP, the reason there aren’t many diet exercise drinks is because that would be counter to their main goal. The main purpose of exercise drinks is to provide energy (which are calories). The main goal is not to be a refreshing drink.

A performance athlete needs to have plenty of carbohydrates floating around so they they don’t deplete their available energy stores. They want a drink which provides hydration and energy. A zero-calorie exercise drink would not be useful to them.