The problem with taking in carbs too close to training is that actually depress the central nervous system.
It’s somewhat counter intuitive because we know that carbs are what fuel hard training.
The problem is that those carbs have to be in a « usable » state for that to happen.
We want to have elevated glycogen stores when we train (glycogen is how the body stores carbs in the muscle and the liver so that it can be broken down and converted to ATP for use as energy).
The problem with eating carbs too close to training is that the digestion of carbs raises insulin levels and insulin is a central nervous system depressant (it’s also a diuretic and causes sodium reabsorption at the distal tubule in the kidneys).
So when we eat too many carbs close to a training session, insulin levels are elevated and you end up feeling sluggish.
This is part of the reason why « post workout » carbs are so important…by eating a lot of carbs AFTER the workout, you take them in when the body is most prepared to transport those carbs into the muscle to be stored as glycogen (so they can be used during your next training session for energy).
There is more involved when you consider the benefits of periworkout carb intake and the effects that insulin have on amino acid uptake into the muscle, etc….but the easiest way to think of carbs when you want to use them to FUEL your training is that you don’t want the carbs in your bloodstream when you’re training….that will make you sluggish.
You want to eat your carbs far enough before training so that they have time to be taken up and stored in the muscle for use during your training.
It doesn’t seem quite so obvious when you think about it…but if you want to best use the carbohydrates you eat to fuel your weight training…the best time to eat them isn’t before you go to the gym…it’s AFTER you went to the gym the day before.