I had the privilege of being in a 7-year University of Washington study after my RYGB in 2007. The vampires! They took so much blood, I thought I was going to die!
And of course, there were dozens of other tests, as well. A nutritionist was part of it, too. The study is online somewhere. But the current head of surgery is here:
Long-term Weight-Loss Seen with Gastric Bypass - Department of Surgery
My advice to anyone having the surgery is, Make sure you cover all your bases and be 100% compliant with your post-op instructions.
We are here to support you, not to give medical advice. I've read many posts from people who had nightmarish experiences afterward.
Most questions asked here should be asked of your doctor and bariatric team. And you should have an annual exam specifically to check out your revised digestive process, because you can be headed to malnutrition while showing no real signs.
If your doctor doesn't give you a comprehensive plan, or tells you you dont need to augment with supplements, it may be time to find a new doctor.
Think of supplements like chicken soup for a cold: it couldn't hurt!