If it is really 2.5% measured with a valid approach (preferably pressure plethysmometry or densitrometry with pulmonary adjustment), you need to find out why. Not difficult to do. Heart rate monitoring over 7 days and heart rate – energy expenditure calibration in a lying, sitting and standing position will provide energy expenditure. A weighed or double plate food intake record will define caloric intake. If you are truly at 2.5%, you may also need to get an estimate of total bone mass and bone density.

Lots of tricks for increasing calories without letting yoiur body or mind think that youi are eating too much. With all of the talk about obesity, low body weight is a problem for ~10% of the population.