There's considerable amount of evidence that willpower is severely diminished if blood glucose get down, and this effect is not limited to humans. And a small sugary drink at the right time is enough to restore it.
We're talking really small numbers. Total blood glucose of a healthy adult is about 5g and it varies within fairly limited range. Then there's maybe 45g in total body waters. Then there's about 100g of glycogen in liver, plus yet larger amount in muscles and other organs, but which doesn't seem to take part in sugar level regulation. For comparison a small can of coke contains 33g - a really small amounts at appropriate times can make a big difference.
This leads to two issues. First, is blood glucose a good explanation for willpower deficiency and therefore akrasia? I'd say there's significant amount of evidence that some effect exists, but is it really the most important factor? Humans are complicated, science knows very little about how we work, and probably half of what it "knows" is false or at best only half-true. Caution is definitely warranted.
And the second issue - if this theory was true - and by manipulating blood glucose levels you could achieve far greater willpower whenever you wanted, what would you do? It seems that exploiting it isn't that easy, and I'd love to hear if any of you tried it before.
Maintaining insulin sensitivity is a whole lot easier, practical and definitely healthier solution than fiddling with sugar intake. After all, in a well-rounded diet your high GI carb intake should be limited to a sane amount of sugar from fruits. Glucose and insulin spikes from consumption of simple carbs would promote insulin resistance on a longer term and render higher blood glucose levels useless from a willpower perspective. To that add hepatic damage and glycation damage from fructose intake.
In healthy individuals blood glucose is regulated extremely strictly; your brain shouldn't crash even several hours after a meal. The reason why so many people experience blood sugar crashes is that they're greatly overeating bad carbs and carbs in general. Currently I'm on a roughly 70:15:15 fat:carb:protein calorie ratio diet and my assessment of mental energy is in favor of it compared to my old ~50% carb diet.
In principle blood glucose can maintained in normal ranges even if there is literally zero carb consumption but sufficient protein consumption, as we're able to turn protein into glucose. This is a reason why carbohydrates are not considered essential nutritients by some people.
Do you have any evidence that willpower problems don't happen on lower carb diet?
My highly detailed food log says I've been eating about 33%:33%:33% calorie-weighed protein/carb/fat 1800kcal/day over the last 17 days, and I have about as many willpower crashes as ever.