For short-term fasts of 1-3 days, your metabolism actually speeds up. See C. Zauner, B. Schneeweiss, A. Kranz, C. Madl, K. Ratheiser, L. Kramer, E. Roth, B. Schneider, and K. Lenz, “Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine,” American Journal of Clinical Nutrition, vol. 71 no. 6 (2000), pp. 1511-1515.