Will keto cause a loss of muscle tone?

No, if you do it incorrectly and if you also do it with the right nutrition. Now, let's talk about fasting. Let's say you're fasting, and you're eating every third day. Can that affect your muscles? It could if you're not taking amino acids over a period of time, but typically if you can maintain exercise that's really the biggest potent stimulator of muscle growth, more than anything. If you have sufficient amino acids and enough exercise and all the other nutrients, you're not going to lose muscle. You'll start losing muscle when you start eating dirty keto, and you're doing fasting incorrectly, and you don't exercise.