What can I do for shakiness while fasting?

It sounds to me that this is a blood sugar issue because for some reason you're doing it too fast because if you jump into keto and you get on an 18 hour fast and you've never done it before. That might be too high of a step. In this case you want to kind of gradually go into it and let your body adapt at its own pace and then make sure when you're eating, you don't have higher amounts of carbs. Try to keep those carbs really low. Because if you do the intermittent fasting with a higher amount of carbs, then it's going to take longer to adapt and get into keto. And that low blood sugar situation will be harder to resolve.