Nutrient timing is the concept that what you eat should have a purpose based on what you are doing or did. For example, it's not just about how many macros you consume, i.e. the number of carbs or grams of protein, but also about when you eat them!
Before you workout, you might want to make sure you consume macros that will be the fuel source for the upcoming exercise. So eating enough carbs and protein before and after a workout might help the anabolic response of the workout itself. Even eating the right food before bed could arguably help stabilize blood glucose during sleep and eating the right foods before you go into a big business meeting might help you stay focused and not doze off.
Typically, the closer you get to the training/workout/competition window, the more of a focus you would have on carbs/protein and less of fat/fiber. Simple, faster-digesting carbs/meals closer to your workouts with complex carb focused meals at other times of the day is a good rule of thumb, but this will always be specific to the individual athlete.
Taking in post-workout protein and carbohydrates to replenish depleted glycogen stores is also critical for proper recovery. The point is, nutrient timing is a concept that is as simple as eating the right foods at the right time and as complex as knowing how those foods interact with your body to help optimize performance outcomes.
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