Hi Liliana, the mechanism(s) for why MAC and other NTMs cause weight loss is not yet known. In my case (Bronchiectasis and MAC x 3) I experienced weight loss only the first time, second and third times not. I now have M. Abscessus colonization and insidious weight loss with little to no appetite/full feeling/chronic diarrhea and other GI symptoms. GI testing revealed SIBO and malabsorption of fat, which could be related to the NTM or not. Treatment for SIBO completely resolved the diarrhea and greatly reduced other GI symptoms, but that along with digestive enzyme supplementation for the malabsorption has not, thus far, helped me gain weight. I am working with a nutritionist and eat nutritionally dense food. Protein smoothies are essential when I feel fatigued. So in addition to the excellent advice by the previous commenters, I would add testing for malabsorption and maldigestion to eliminate those as possible factors. Best of luck on your journey with NTM.