It's tricky, make sure you drink enough water, but not too much (too much water washes out too much salt from your body, dropping your blood pressure too low). A backup plan if this doesn't work is bone broth soup. It's not a lot of calories so keeps you in the fasting state and can pick you up pretty nicely. Salt the soup but not too much. The key seems to be to keep your electrolyte levels in the Goldilocks-zone.