Hormonal

Chronic resistance exercise training increases skeletal muscle LAT1 protein content, but this increase is significantly blunted by concurrent leucine or whey protein supplementation compared to a placebo.

If you are doing resistance training, you will build muscle regardless of whether you take leucine or whey protein supplements. Interestingly, your body's expression of the LAT1 transporter (which moves leucine into cells) increased significantly more when you took a placebo compared to when you took leucine or whey. This suggests that supplementing with these specific compounds might blunt this specific molecular adaptation, even though it doesn't stop muscle growth. You don't need to avoid them, but don't expect them to maximize every single molecular marker.