Eating a low-calorie diet, whether purposely or not, can significantly affect energy levels. The basal metabolic rate (BMR) is the amount of calories the body expends at a resting state. This is the minimum amount of energy (remember: calories equals energy) your body needs to simply survive and perform its daily functions. Consuming fewer calories than your body’s BMR causes many processes in the body to slow down (think: metabolism, digestion, energy production, oxygen delivery to the brain and muscle tissues, etc.). Consequently, this may cause you to experience fatigue and weakness. It is also challenging to meet nutritional needs on a low-calorie diet. A lack of vitamins and minerals can negatively impact energy production and well-being.