Per
Calories in Dextrose, there are 106 calories per ounce of Dextrose or 4 calories per gram.
If a tab is 4 grams, that's about 16 calories and it takes roughly 100 calories to walk a mile -- that's about 6 or 7 tablets.
I'm sure these are excellent as a quick fix when you go low-sugar and need to stay conscious. They are not designed to keep you going on the trail.
Just my $.02