When your doing high intensity workouts your glycogen stores in your muscles are being depleted. To renew these we needs carbohydrates. You can gain muscle and remain in a state of ketosis (best of both worlds).
A lot of people here are trying to lose weight, I am trying to get lean and trim fat. For somebody just losing weight and to induce ketosis they should be having a minimum of 50g carbs to 100g carbs. If you are going to the gym on your gym days you should be adding 50g carbs ontop of your carb intake. For me this means on my gym days I consume more carbs (roughly 100grams) and on a normal day less (50grams). This keeps me in a constant state of ketosis which I'm comfortable with. If I was feeling brain-fog or constant fatigue I would up my carb intake.
I never eat refined carbs (unhealthy carbs) and always try to get carbs from healthy nutritious source e.g. fruit. I won't mention cardiovascular workouts because the carb requirements are to variable depending on the time duration and intensity. When exercising theres a point where a lack of carbs will induce glycogenisis in the muscles (breaking down the protein enzymes for energy). This would affect my muscle gains!
It's been researched by Spanish scientists that a beer after a workout can replenish you better than just drinking water. This means on Fridays I have no guilt catching up with friends for a couple pints
How Many Carbohydrates Do You Need | BodyRecomposition - The Home of Lyle McDonald
Scientists suggest beer after a workout - Washington Times
-safe is the new risky