As runningman says, maltodextrin is a polysaccharide of glucose. What that means is it is multiple glucose molecules all linked together. It is a somewhatp generic term in that the length of the chain can very greatly. Generally, the longer the chain, the less sweet the product. Maltodextrin will pass through the small intestines as fast (if not faster) than simple glucose or dextrose. Dextrose is simply two glucose molecules bonded together.
The reason maltodextrin is beneficial for your training and race nutrition, more so than simple sugar or dextrose, is how it reacts to water. In the same way your body will regulate your internal hydration to maintain a constant concentration of electrolytes (ie, the more you sweat out salt, the less your body will retain added water unless you also add back the salt since the body wants to maintain a constant concentration of sodium) your gut will suck water from your body into your stomach to maintain a fixed concentration of sugar molecules to water ratio for the digestion process. However, in the case of your gut, the concentrating factor is not the number of glucose molecules, but rather the number of glucose polysaccharides.
If I remember correctly, you can tolerate about a 6% solution. Any more than that, and your gut will flood water to dilute whatever you drank and you may get that bloated feeling. This effectively limits how much sugar you can consume without having stomach issues. On a side note, you can throw in a little fructose as well since your body processes it differently. That will get you a little more sugar/hour.
The great benefit of maltodextrin is if you take a 6% solution of glucose and a 6% solution of maltodextrin (with an average of say 10 glucose molecules per polysaccharide) then you will actually absorb 10 times more sugar without having to suck any more water into your gut. Once the maltodextrin hits the wall of the small intestine, it breaks into single glucose molecules and does one of three things (1) gets burned in the engine right then and there, (2) bonds to other glucose chains in your muscles and liver to replenish glycogen, or (3) gets stored as fat assuming your tank is otherwise full.
I use pure maltodextrin solution on my long training rides and long runs. On the bike I mix up a 1000 calorie solution and put in in a 20 oz. bottle. I then drink plain water and use salt sticks for electrolytes. This way, I can individually control my nutrition, hydration and electrolytes depending on conditions. 1000 calories will get me through a 3 hour ride. For IMCDA, I intend, as of now, to have an extra 20oz bottle at special needs to swap out after the first loop. We will see as June approaches if that will be a viable option.
I really prefer the nothing added maltodextrin. The flavors sometimes can cause as much issues as the bloating if you are spitting up berry flavor and citric acid. If it has to taste sweet to suck it down, you need to HTFU.
There is a great product out there called Vitargo. It is sold at GNC under the brand name Genr8. It is a VERY long chain glucose polysaccharide. It has NO sweetness at all. It mixes cloudy (maltodextrin mixes clear) and tastes like corn starch or some such. It is very easy for me to drink plain and it causes me no stomach issues at all. I prefer it to the plain maltodextrin if only for the lack of sweetness which the maltodextrin has a little of. However, it is WAY TOO expensive for me to train on and recover with. I intend to race on it and will do some long training rides with it in advance of race day, but the bulk of my training will use the much cheaper maltodextrin. I also mix maltodextrin into my protein shake to make my recovery drink. Works great and is easy to hit the carb:protein ratio I am looking for.
If you search for maltodextrin you will find threads which will point you to online sources of the product. I bought a 50 lbs bag of the stuff and split it with a buddy. It was about $1.00 per lbs compared to like $25 per lbs for the Vitargo. However, not all maltodextrin is created equally and the term applies equally to a wide group of glucose polysaccharides with varying molecular weights. You will need to investigate what you are buying and know why you are picking one maltodextrin over another. I definitely don’t recommend going to your local health food store and buying it out of a bin if you cannot identify the properties of that particular product. As a general rule, the longer the polysaccharide chain the better.
YMMV