What is underfueling?

weight loss woman in weighing scale underfueling


  • Underfueling refers to not consuming enough calories or macronutrients to adequately fuel your body and support your activity level or goals. This can lead to issues like fatigue, loss of muscle mass, hormone imbalances, and poor performance.


  • Underfueling often happens unintentionally when people restrict calories too much while trying to lose weight or don’t properly calculate their needs. Athletes who don’t consume enough carbohydrates to fuel their workouts are also at risk.
  • Signs of underfueling include chronic fatigue, inability to gain muscle or strength, constant hunger, irritability, loss of menstrual cycle, and frequent illnesses.


  • A bodybuilder in a deficit who isn’t eating enough protein or calories to maintain muscle mass.
  • An endurance athlete training for a marathon but not consuming enough carbohydrates daily.
  • Someone on a very low calorie diet trying to lose weight rapidly.

Related Terms

  • Calorie deficit
  • Macronutrients (protein, carbs, fat)
  • Micronutrients (vitamins, minerals)
  • Basal metabolic rate (BMR)
  • Activity level

Common Questions

  • How do I calculate my calorie and macronutrient needs?
  • What are signs I may be underfueling?
  • What should I do if I think I’m underfueling?

Do Not Confuse With

  • Overeating – Consuming excess calories and gaining weight
  • Bad nutrition – Eating poor quality foods with low nutrient density
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