How much keto (low carb) diet when combined with protein intake is likely to hamper muscle growth?

Question

I am 37 years old male, recently went on the keto diet for 3 months and lost ~13 kg (but still have a way to go), to sustain the gains I started working out to gain more muscles and increase my passive metabolic rate.

This paper states that keto is good to maintain existing muscles, however, for maximum muscle gain, one needs cortisol and insulin, which are both being depleted by lack of carbs.

If I would use protein (whey powder isolate + daily protein intake from diet) of 2g per kg of lean mass and stick to intermittent fasting (14hr, 10pm - 2pm) until an hour before exercise, and then continue on keto (~50g carb per day), how big of an effect on reduced muscle growth I could expect? Are we talking half the growth or something like 10% muscle gain reduction? I understand that it's not a perfect science, still, a ballpark figure should be possible.

Answer 1

Can you build muscle doing what you’ve outlined? Sure, but there are a few issues to keep in mind. To what degree? Is it possible to give an accurate percentage example? No. You might be able to find studies illustrating average outcomes of these different issues, but that won’t answer your individual response nor the combined effect.

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Issue 1 - Low Carb

• Carbs are important for fueling high effort activity. A low carb diet will likely hinder high effort activity performance. What this means is that your capacity to provide a high quality stimulus for muscle growth. Additionally, having less carbs means less muscle glycogen stores which are a short term means of increasing muscle size.

Issue 2 - Energy Balance

• Keto is usually pursued for losing weight. This is known as a negative energy balance. Gaining weight (positive energy balance) is optimal for muscle growth, maintaining weight (neutral energy balance) is suboptimal, and losing weight (negative energy balance) is least optimal for muscle growth. Can you maintain or gain weight on keto? Sure, would that resolve this specific issue? Yes. But keto was specifically designed to make it difficult to not be in a negative energy balance.

Issue 3 - Intermittent Fasting

• Fasting is not eating. Not eating means missing out on chances to keep muscle protein synthesis high. Muscle protein synthesis is how your body builds muscle. Is fasting going to make the difference of building muscle or not? Probably not, but it will hold back progress somewhat. Additionally, if you are trying to maintain or gain weight, fasting will make this harder to do.