Someone asked me today, "how long do I have after a workout to eat my carbs?"
She had just been at the gym taking a lunch hour fitness class and wanted to know just how much of a free "window" she had left to eat a few carbs. Well, my answer as it almost always seems to be was, "...it depends..."
As anyone with a rudimentary knowledge of post-workout nutrition would tell you, there are several interrelated factors that determine whether the carbs you are eating are going to refuel your muscle glycogen stores, or being stored away as excess body fat (which, since you are reading this I feel safe assuming you do not want to happen).
Factor #1 - How intense was your workout?
Factor #2 - How long has it been since your workout ended?
Factor #3 - What type of carbs are you about to eat?
Factor #4 - What has your carb intake looked like for the past 3-4 days?
There are even more factors than this at play, but I think you get the general concept. I'll try to attack these one at a time.
Factor #1 - Workout intensity has a number of post-workout effects on one's body. Generally speaking, high-intensity exercise (sprinting, a spin class, intense circuit training with weights, etc) will deplete your body's glycogen stores moreso than low-intensity exercise (unless you put in a marathon/triathlon-style workout). So when those glycogen stores are fairly empty, all the more reason your post-workout carbs will be soaked up by the right places (namely your muscles). Not only that, but in a general sense when you worked up a sweat and raised your heart rate during a workout, your body burns (slightly) more calories post-workout while trying to return to homeostasis.
Factor #2 - There is generally considered to be a one-hour window of time post-workout when your carb intake will be shuttled directly to thirsty muscles. However the "lower" you fall on factor #1 (ie: lower intensity training) then the shorter time period you will have for your post-workout carbs to go straight to muscles and skip past the fat stores.
Factor #3 - This one is arguably the MOST important in a lot of ways. So important actually that I'll need to come back to this for another day. But for now, if you absolutely have to eat simple or starchy carbs (white breads, bagels, muffins, etc) then immediately following an intense workout is the absolute best time. The "simplicity" of how easily they are broken down and get into the blood stream is in most cases a bad thing (since it's easily converted to fat) however post-intense workout this actually works to your benefit.
Factor #4 - Think of your body's carb stores (muscle capacity to contain glycogen) as fuel tanks. If you've eaten a carb heavy diet for the past few days, then your tanks are likely either still full, or moderately full even with an intense workout within the past hour. You won't need more post-workout carbs as much as a person who's been going low carb for the past 3-4 days.
Here is another (longer) read on post-workout nutrition:
Key Takeaway: The harder you've worked out...and the lower your carb intake has been before today...the better the opportunity you have to consume some "bad carbs" you've normally been avoiding. Just be sure not to overdo it.