I’ve done a fair bot of research into this area lately, and I am lucky enough to work with an international renowned expert in this field (it’s not mine however). His simple advice to me when I’ve discussed it with him: train low race high with regard to carbs for longer endurance events.
This backs up everything I’ve read, but I also found that the type of carbs and the timing of these on race day is also important.
E.g. you should be taking a low GI carb (such as UCAN, but the research I’ve read also indicates a simple waxy maise starch will work at a fraction of the price) with some MCT powder/liquid about 30 min prior to the swim start for an IM/70.3. Taking malto prior to exercise results in an insulin spike, which among other things actually inhibits utilization of fats as energy. In addition the lower GI carb doesn’t have the big drop that malto has with regard to BG serum levels about 60-90 min into exercise, it is much more stable. Once you’re about an hour in it doesn’t really matter what you take with regard to insulin response, as exercise (particularly at IM/70.3 effort levels) does significantly blunt insulin response to the point that you can continue to maximize the utilization fats as a fuel in conjunction with the carbs that you will burn. My preference is Vitargo, as that leaves me with no GI issues due to its low osmolality.
To calculate your energy needs from carbs as a source it’s fairly simple if you get the right testing done. You can get testing done to calculate you RQ at different levels of intensity. From this you can calculate how many typical calories you will burn from fats vs. carbs at a given effort level. From there it’s a simple exercise of taking in the right amount of carbs to replace what you are burning through. From what I have read, on balance I think the trend would be that people take too many carbs during an IM.
I’d guess that the ‘boost’ you’re referring to is actually the sensation of an insulin spike after downing a high GI food. This is significantly inhibited during exercise, and so the only time I think you’d really feel the ‘boost’ is if you are in a glycogen depleted state…or of course if it’s loaded with caffeine.