This seems like an appropriate question to answer on Veteran's Day. Thank you to those serve or have served.
Question: Do you know if I can join the military after having weight loss surgery? Joining the Army has always been a dream of mine.
Answer: Unfortunately, it is my understanding that you can not join the military after having weight loss surgery. According to Health Affairs Policy 07-006: “Bariatric surgery represents a major and permanent change in your digestive system that requires strict adherence to a specific dietary regimen that may interfere with operational deployment. A history of bariatric surgery is already a bar to military service and receiving bariatric surgery while on active duty may be grounds for separation.”
For some health conditions you can ask for a medical waiver but again in my reading it does not seem likely it would be approved and it is quite the process. To be certain ask your recruiter.
Some personal thoughts... those that serve in the military are often in need of many thousands of calories a day due to their energy output. Your adapted anatomy post weight loss surgery simply can not hold that quantity any longer. Consider training or serving in triple digit desert heat with only the contents of your pack with you, you would likely be unable to meet your post-operative needs in those conditions and not meeting them would be detrimental to your health making you a liability to yourself and your platoon.
If your dream is military service (it's a noble one for sure) may I suggest an adapted dream of serving in some capacity as a civilian on a military base in the U.S. or abroad. For more information about those opportunities:
U.S. Army: https://armycivilianservice.usajobs.gov/
U.S. Air Force: https://afciviliancareers.com/index.php
U.S. Marines: http://www.usmc-mccs.org/careers/#.UoZz9xBQHGE
U.S. Navy: http://www.secnav.navy.mil/donhr/Pages/Default.aspx
U.S. Coast Guard: http://www.uscg.mil/civilian/default.asp
P.S. Do not consider not sharing your complete health history. There isn't a part of you that isn't checked if you get my drift and not being forthright is not an option.
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Health Affairs Policy 07-006 - Policy on Bariatric Surgical Procedures for Active Duty Service Members
DODI (Department of Defense Instruction for Medical Standards for Appointment, Enlistment, or Induction in the Military Services), (PDF). This is the official document used by Military Entrance Processing Station (MEPS) doctors to determine medical eligibility for military applicants.