The main reason to consume carbs post-workout is to replenish the muscle glycogen that you burned during the workout. During the workout, your main fuel source is glycogen from the muscles. Glycogen is the storage form of glucose. It is composed of glucose molecules strung together in long strings with numerous branches. The glucose molecules are broken off of the glycogen chain as they are needed to be broken down to generate ATP. Research confirms that the best way to replenish muscle glycogen stores after workouts is to consume high-glycemic carbs as soon as possible after workouts.
Some "experts" argue that during a typical weight-lifting workout, you don't burn up enough glycogen to be concerned about replenishing it. To that I say, "What?!" If you deplete ANY amount of muscle glycogen, you should be concerned with replacing it : especially if muscle growth is your main goal. If a trainer claims that people don't need to worry about replenishing glycogen, then either their workouts are very low volume and very low intensity or they haven't done any research.
The research on glycogen levels during weight training shows that workouts consisting of anywhere from 6 to 20 sets and lasting about 15-30 minutes in length deplete muscle glycogen levels by about 30-40%. Since my workouts last at least 60 minutes and sometimes a good 90 minutes, and you're either moving rapidly between exercises and/or using high-intensity training techniques, cardioacceleraton, Tabatas, etc., then most of my workouts will deplete muscle glycogen levels by a good 60-75%. So, take your carb intake seriously and replenish your muscle glycogen stores Not doing so is cheating your muscles and limiting their recovery.
Research confirms that the best way to replenish muscle glycogen stores after workouts is to consume high-glycemic carbs immediately following the workout. Research shows that when carbs are consumed immediately postworkout, a supercompensation of the glycogen stores is possible. In fact, delaying carb consumption by just 2 hours has been shown to reduce the rate of glycogen replenishment by 50%.
Yet some experts argue that none of this matters for most people in the gym. This is due to the fact that some minimal evidence suggests that whether or not you consume carbs immediately after training or 2 hours later, glycogen levels reach the same levels by 24 hours later. But again, there is minimal and weak evidence to fully prove this. What we are sure of is that high-glycemic (fast-digesting) carbs consumed immediately after the workout replenish muscle glycogen levels the quickest.
Replenishing your muscle glycogen levels as quickly and as fully as possible after workouts is important. Stored glycogen in muscle cells pulls water into those cells. This increases the muscle cell volume and therefore muscle fiber fullness. Given that you gain a muscle pump during the workout, which also increases the amount of water in the muscle cells and therefore the volume of the muscle cells, quickly replenishing muscle glycogen levels can help to maintain a higher muscle cell volume for a longer period. This can be important because evidence suggests that the greater muscle cell volume instigates changes in the muscle that lead to long-term muscle growth.
Consuming carbs, especially high-glycemic carbs (such as dextrose), immediately after an intense workout is also great for making you feel rejuvenated. It can take you from feeling wiped out and lethargic after a brutal workout to energetic and well-fueled. Plus, eating high-glycemic carbs after a workout typically satisfies a massive craving for sugary or starchy carbs without a negative impact on your diet progress. I often recommend gummie bears for postworkout carbs, which most people find very satisfiying and helps them bear through strict dieting the rest of the day.