Dextrose & fructose aren’t “Slow Carbs”.
This is simplified, but you can think of carbohydrates as molecular chains, with each link in the chain being a glucose molecule. The longer the chain, the longer it takes to break it down in the gut, so the “slower” the release of energy.
“Fast” carbs, i.e. high GI index, means either monosaccharides (one single link, that can be absorbed directly high up in the digestive tract) or short-chain polysaccharides (a few links, broken down quite easily to monosaccharides and absorbed).
Only monosaccharides can pass across the gut wall into the blood, so all carbohydrates have to get broken down into their building blocks of glucose or fructose before we can absorb them. Some common examples:
Mono-saccharides, one link long:
- Dextrose (also known as Glucose or D-glucose)
Di-saccharides, two links long:
- Sucrose (fructose+glucose)
- Maltose (glucose+glucose)
- Lactose (galactose+glucose)
Short chain polysaccharides:
- Maltodextrin, dextrins (3 to ~17 glucose units)
From this point on, your gut bacteria are doing most or all of the work of breaking these down for you:
Medium chain polysaccharides:
- Inulins (~20 to ~3000 fructose units) - a type of soluble fibre, not very digestible.
Long chain polysaccharides:
- Amylose (300-3000+ Glucose units)
- Amylopectin (2000-200,000 Glucose units
- Glycogen (30,000 glucose units)
- Cellulose (10,000+ glucose units)
Amylose and Amylopectin and the building blocks of what we normally call ‘Starch’, things like Brown Rice Carbohydrate, Potato Starch, Psyllium Husk, Tapioca Starch - anything with the words ‘Starch’, ‘Bran’ or ‘Fibre’ in the name, etc… - i.e brown, tough stuff.
Carbohydrates this long and complex are really hard to digest - in some cases the human gut can’t do it at all (cellulose) and in most other cases we rely completely on our gut bacteria to break these down into monosaccharides for us, so we can absorb them.
Also, starches in their crystallized form (i.e. dried & powdered) are even harder to digest than normal, sometimes completely impossible - this is why they’re normally always cooked with water before eating (to gelatinize them), so I would be careful adding powdered starches to Soylent - it might not be very digestible. Probably the worst that will happen is that you’ll get a stomach ache, bad gas and won’t get much energy out of it - it won’t kill you, but it might not be very nice
TLDR; Maltodextrin is probably a fairly good choice, actually. Go much lower GI than that and you’ve switched from a ‘sugar’ to a ‘starch’ and this changes the way they’re digested - which might be a problem if you’re not cooking it.