Weight Difference
You gain weight unrelated to diet and exercise. After every adventure or major campaign arc, you must succeed on a DC 8 + (IE) Constitution saving throw. On failure, your weight has increased by IE × 2d6 percent. On success, your weight has decreased by IE × 1d4 percent. For every fifty percent increase, you have a −1 penalty on Dexterity (Acrobatics) and Strength (Athletics) checks. For every ten percent decrease, you experience Temperature Sensitivity for 3d20 days as your body acclimates itself and must succeed on a DC 8 + (IE) Constitution saving throw or experience Fatigue until you succeed on the saving throw, which you can attempt daily after a long rest. You have a +(IE) bonus to Strength (Athletics) checks for swimming.
Real-world Examples
Hypothyroidism, Depression, Insomnia, Menopause, Cushing’s Disease, Polycystic Ovarian Syndrome, Congestive Heart Failure, Sleep Apnea, Edema, Diabetes, Down Syndrome
Assistive Options
Because this gain is unrelated to diet and exercise, little can be done besides acceptance.