Can not eating enough calories cause fluid retention and constipation?

True starvation: can have a variety of problems including vitamin deficiency. You might be too young to remember the big bellied starving kids in Africa TV commercial campaign, but that's fluid retention from starvation (thiamine deficiency). Heck, half my patients have constipation from lack of fiber.....

Starvation?: There is no long-term substitution for a healthy, well-balanced diet. Very simplistically, weight gain is achieved if the individual takes in more calories than he/she burns; weight loss happens when calories burned exceed calorie intake. Diet modification often results in GI & non-GI manifestations, but perhaps you need some guidance. Ask your doctor at this time for a check-up & his/her advice.