If you need to lose weight, do so gradually, without skimping on essential nutrients or starving yourself of the calories you need for energy. Poor nutrition and inadequate calorie intake can cause fatigue. And remember to eat regularly throughout the day, even when you’re dieting. Your brain needs a steady supply of glucose from food. When the brain’s glucose levels are low, some people feel hungry, fatigued, or both, which can then trigger a bout of overeating.