Fat burning is not an on/off switch. The body always uses a mix of fuels; the proportions shift as insulin falls after meals. As fasting extends—often around the 12–16 hour range for some—fat oxidation can become more prominent, but timing varies with the last meal and activity. Lower-carb meals and exercise can accelerate the shift. For actual fat loss, the key is sustained energy deficit over days/weeks. A long fast followed by overeating can cancel the benefit. A more predictable plan combines moderate calorie deficit, adequate protein, resistance training, and good sleep. People at hypoglycemia risk should not aggressively extend fasting without guidance.