My son started beast at approx 170, and dropped to around 150 during Beast. He's now at 155, 5'7" and around 15% BMI.
He was a Varsity athlete in soccer and lacrosse during high school, and quite fit. He has turned into quite a runner at WP, and was in the fastest group during beast. He was ready for Beast, but got only better during the summer. He said running up the hills was tough, but really imporved his running.
Something to be aware of with the weight fluctuations. Your weight at the start of Beast will be noted, and may influence what company sports activites you will participate in. My son got assigned wrestling in the 170lb weight class during the first semester. He had never wrestled before, and at the start of the semester was at approx 150. It was tough for him being a wrestling novice, and wrestling the other 170 pounders. He dislocated his shoulder twice while wrestling, and he had never had that type of injury before. This put a damper on his participation in physical activities in the latter part of the semester, and affected his IOCT grade. He also had to defer some of the phys-ed classes to the next semester.
His advice, get fit before Beast, and be prepared for the toughest workout you'll likely ever do. It would be great if you were at the optimum weight at the start of Beast. He has focused on his running and working out in the weight room to keep his muscle mass at the right ratio.
He lamented that he slacked off during winter break, and it took a few days to get back into the swing of things. He won't be repeating that again.