at 20 percent protein, say if you're dieting on 1800 calories that's only 90 grams protein per day. I've been curious but feel like this would reduce muscle mass (can't say for sure because I've never tried it though). Has anyone here tried keto with less than 100 grams of protein per day for several weeks in a row and maintained strength in the gym and muscle size?
Yep. Didn't reduce anything for me. Fats are muscle sparing. Eat enough fats and your muscle will be fine. I ate between 80g - 100g protein about 2 months out from my comp and had no issues.
If you also look at clinical research, it says the same. Keep carbs super low, fats high, and protein moderate, and you'll be fine. You don't need 300g protein to build/keep muscle. You just need enough aminos including leucine to complete muscle protein synthesis, which is a minimum of 50g/day. Of course, you'll want more than 50g, especially in the off-season. But precontest you'll be fine with 100g/day if you're eating enough fats (which should make up the rest of your nutrition with some leafy greens thrown in).