Male Speaker: What is the appropriate amount of calories an underweight person should consume to gain weight? Taras Raggio: A person who is underweight should consume 60 to 65% of calories from carbohydrates. The recommended day allowance for a normal individual is 2000 calories, a person who is underweight needs to consume at least 3300 to 4000 calories in order to gain weight, so more than the average recommended daily allowance.