Check some articles on oral rehydration solutions (ORS), a generally accepted recipe for emergency rehydration drinks in situations where IVs are unavailable or impractical.
For a few years I've mostly relied on
DripDrop ORS for hot weather workouts here in Texas. DripDrop is blended to the usual ORS formula, but available in convenient water-resistant single serving Mylar packets, making it ideal for tucking into a small saddle bag, jersey pocket, etc., for use as needed on the road.
I usually carry at least two water bottles in warm/hot weather, one of which is premixed with DripDrop, and I carry spare packets. I've used them twice for folks who were experiencing heat exhaustion and dehydration, and they felt better within 15 minutes. In one case an experienced but out of shape cyclist had brought only plain water but felt unwell after about an hour in warm, humid June weather.
In the other case a tri-athlete in training had taken way too much sodium in the form of salt pills, an outdated technique that should have been abandoned decades ago. She was vomiting up all of the plain water her friends were offering. But she was able to hold down the DripDrop mixed with about 8 ounces of cold water.
Sugar is crucial to an effective ORS. Check the DripDrop website for details, although similar information is available elsewhere online.
Even when I'm not dehydrated I find sugar-free electrolyte/energy drinks cause my stomach to feel upset, burning and burpy. Adding just a little plain sugar helps. Occasionally I will try sugar-free drink mixes but I'll always add a little sugar, sometimes in the form of pre-sweetened creatine powdered drink mixes. Works for me. But I have no blood sugar problems, diabetes, etc. Folks who do should consider other options, or monitor their blood sugar while trying new drink mixes.
Regarding sugar as fuel, rather than as an adjunct to a rehydration mix, it depends on my workout level. Sometimes I'll do fasting workouts. Other times I'll eat normally before a moderate workout. But for high intensity intervals and hard extended workouts, yup, I'm chugging sweetened drinks, eating Clif gels, even gobbling donuts at convenience stores. My stomach will tell me quickly when I've had too much sugar too quickly, but it doesn't really take much, just a little at fairly regular intervals, maybe every 15 minutes, during a longer ride. Pretty much the same as most training plans suggest.