Start Off Slow
Even if you're a jump-straight-in-the-pool kind of person, it's better to take a one-step-at-a-time approach when intermittent fasting. If you go from eating three meals and three snacks a day to eating only 500 calories a day (5:2 plan), you're more likely to have terrible side effects, including headaches, dizziness, and even nausea.
There's no rush! Take several weeks to ease into the plan you've chosen. If you're especially hungry on a certain day or not feeling well or expecting your period, listen to your body and eat if you need to. You may also need to modify your workout routine in the beginning — hunger from intense workouts will make it much harder to stick to your plan.