1. This can be heavily mitigated by going to the gym, or doing some kind of resistance exercise. Yes, human beings are required to eat the right amount of food and also exercise in order to maintain health. There is no magic bullet.

2. You would only really be lethargic for a small amount of time as your body adjusts to blood sugar levels. It's important to note that people who are losing weight are not low on energy because they are lacking calories. They have a ton of calories, they just aren't very good at using the fat to maintain blood sugar (yet). This should be roughly a 1-4 week transition, and again exercise will help a lot with it.