A moderator note, because this thread appears in some form every few months and the reassurance above is well placed.
What almost everyone is describing here is telogen effluvium. In plain terms, hair grows in cycles, and at any moment most of your follicles are actively growing while a smaller proportion are resting (the telogen phase) before they release the old hair and start again. A big physiological shock, and both major surgery and rapid weight loss count as exactly that, can push an unusually large batch of follicles into that resting phase all at the same time. They then let go together a few months later, which is why the timing is so consistent: the shedding typically starts around three to four months after the operation, not straight away, which catches people off guard because by then they feel well.
The reassuring part is structural. Telogen effluvium is a shedding of hairs whose follicles are still alive and intact; it is not the follicle dying. Shedding is diffuse, meaning thinner all over rather than bald patches, and for most people it peaks and then settles, with regrowth visible and the density recovering somewhere in the six to twelve month range from when it began. A normal scalp sheds in the region of 100 hairs a day anyway; in this phase that can run several times higher for a while, which is alarming to see but self-limiting.
Where nutrition genuinely comes into it, and where it is worth a conversation with your own team rather than the internet, is that inadequate protein and low iron stores can prolong or worsen the shed. On the bloods, ferritin (your iron stores) is the usual culprit and is often flagged when it drops below roughly 30 to 40 micrograms per litre; zinc and overall protein intake matter too, which is why the standard advice sits around 60 to 80 grams of protein a day once you can manage it. That is the useful version of "eat more protein". The biotin point above is correct: supplementing it does little unless you are truly deficient, which is rare, and high doses can interfere with certain laboratory assays including some thyroid and cardiac tests, so it is worth mentioning to whoever takes your blood. The site's overview of hair loss after the sleeve walks through the same ground, and the piece on vitamins and supplements covers why the iron and protein side is the part that actually earns its place.
Two honest exceptions, and this is where I hand you back to your own clinicians who have your notes and can examine you. If the shedding is still going strong past twelve months, or if it is patchy and defined rather than diffuse, or if it comes with other things such as fatigue, feeling cold, or changes that do not fit, that is no longer a tidy post-surgical story and deserves proper investigation, thyroid function being one obvious thing to check. I cannot tell any individual here which of those they are; that assessment belongs with your GP or bariatric team.