For monosaccharides Fick's laws of diffusion - Wikipedia because no digestion is required. There is a 1933 paper which considers an analogous question; how long does it take an individual to absorb a fixed amount of glucose (injected into the stomach). I haven’t read it through but it summarizes a previous result thus, “Pierce, Osgood, and Polansky (3) obtained results which led them to conclude that the percentage of glucose adsorbed appeared to be dependent upon the amount of glucose remaining unabsorbed in the alimentary tract. Cori et al. (4) explain the findings of these investigators on the basis upon the basis that sufficient glucose was not not fed to allow adsorption rate to continue at the initial level.”
The eventual conclusion, emphasis added by me:
The absorption rate of glucose per unit of time from the intestinal
tract of male and female rats of various ages bears a more
constant relation to body surface than body weight. The amount
of glucose absorbed per unit of body surface per hour of time has
been used as the absorption coefficient.
The absorption coefficient is raised by increases in either the
amount or concentration of glucose administered.
Whatever the dose of glucose the rate of absorption as measured
by the method used here decreases with time after it is given.
So they are refuting Cori’s apparent implicit assertion that glucose adsorption saturates within the bounds of the experimental conditions. The experimental subjects were fasting (40 hours) so stomach contents will be minimal. The amount of glucose was around 1g as a 57% (570g/L I assume) solution, about 2mL, fasting stomach size is around 1mL so the glucose was pretty concentrated in the stomach.
The other half of the question is about digestion. I’m not considering rate of adsorption of glucose from things that require complex metabolism; just fast acting carbs that require the chemical degradation that happens in digestion. For sucrose this amount to hydrolysis to get the two monosaccharides out of the compound.
Even in the case of sucrose there are two things going on; acidic breakdown and enzymatic breakdown. If you follow the link in the above page to sucrase you will see that it is introduced in the small intestine; immediately after the stomach.
So adsorption is a function of two processes:
Firstly a simple chemical reaction in solution, the rate of which is expected to be proportional to concentration, the intuitive result. See the wikipedia article for why more complex (multi-step) reactions don’t obey this rule, and;
Secondly the sucrase enzymatic reaction. This presumably depends on the amount of sucrase and, given that the duodenum is not very acidic, is quite likely the only source of glucose+fructose. There are papers on this process, for example, however I suspect many of the dissertations I found were propounding some particular viewpoint or an agglomeration of many modern fads. I did find this 1966 paper which I tend to trust. That paper seems to imply that sucrase hydrolysis will also be primarily dependent on sucrose concentration, regardless of the chemical order of the reaction, because the first step is adsorption which is very likely to be covered by Fick’s law. However, on page 392:
There appeared to be a limiting velocity for both hydrolysis and absorption in all experiments.
Check out the graphs on page 393.
In general I would expect that most scientists would start from the hypothesis that adsorption/digestion/metabolism is proportional to concentration (not amount) and that variations from this are deserving of explanation.
The take-aways are:
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It’s concentration not amount. If you eat a glucose tablet then drink a pint of water it isn’t going to work as fast. If you have an empty stomach adsorption will be much, much faster than if you have just eaten a full meal.
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Glucose is yucky but it works; at least it is better than glucagon.
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Things containing glucose/fructose/etc are likely to be much better than things containing disaccharides or complex carbs; I find glucose tablets are fastest of all but fruit is pretty good. As I’ve commented before, broccoli works.
Also, not related to the above, fast helps avoid the roller coaster. So it’s tempting to eat a tub of ice cream and wait half an hour, I’ve certainly done that, but it is probably better to eat a glucose tablet, wait five minutes (for it to be adsorbed) then eat half a tub of ice cream (there has to be a reward.)
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