Do you need carbs with your post-workout protein?

Short answer

No. Adding 50 g of carbs to protein raised insulin 5-fold, yet muscle building stayed the same, and adding carbs daily for 8 weeks made no difference to muscle or strength. Carbs are useful for refilling fuel quickly when you train again within 8 hours.

Do you need carbs with your post-workout protein?
'Carbs → more insulin → more muscle building' — does this formula hold?
A common belief is that your post-workout protein shake needs a banana or rice cake on the side for muscle to grow. The logic: carbs raise insulin, and insulin helps build muscle. We checked this formula step by step.
Do added carbs raise insulin?
They do, 5-fold
After a leg workout, 9 people had either 25 g of protein alone or 25 g of protein plus 50 g of carbs. With carbs added, insulin rose 5 times more and blood sugar 17.5 times more. The first link in the formula holds.
Did muscle building rise too?
It did not
In the same 9 people, muscle building and muscle breakdown were the same in both conditions. When 10 people had protein with no carbs, a little, or a lot, muscle building ran at 0.10%, 0.10% and 0.11% per hour, the same. A trial comparing 30 g and 90 g of carbs in 13 people also found no difference.
What about adding carbs every day for 8 weeks?
No difference in muscle or strength
20 people took 30 g of protein daily for 8 weeks, and 30 g of protein plus 50 g of carbs daily for another 8 weeks. They ate 485 kcal more per day in the carb period, but lean mass (muscle and everything except fat), muscle thickness and best squat did not differ.
So what are carbs for?
Refilling muscle fuel, and protein did not help with that
An analysis of 29 trials (246 people) covering the first 8 hours after exercise found that carbs refilled muscle fuel (glycogen) faster than water. For a 70 kg (154 lb) person, that was about 70 g per hour. But adding protein to the carbs did not refill fuel any faster.
Does low fuel block muscle growth?
With less fuel, muscle building was the same
16 people drained fuel from one leg by cycling, then did leg presses with both legs; the low-fuel leg built muscle at the same rate. A review of 49 studies also found that, when fed, workouts of up to 10 sets per muscle did not improve with extra carbs. Benefits showed up in cases like training twice a day.
Bottom line: protein is enough for muscle
Carbs matter when your next workout is close
Added carbs raised insulin but not muscle building, and adding them daily for 8 weeks did not change muscle. After training, 25-30 g of protein is enough, and carbs can come from regular meals. Only when you train again within 8 hours is it worth having carbs right away.