The microbiome might have some modulating effect, but the fidelity of gut-brain axis communication isn’t so strong that our gut microbiome is driving us around with highly specific inputs.

The theories for how gut-brain axis modulation works include altering the balance of nutrients that get absorbed and modulating the vagus nerve, primarily. For someone with autism it might be possible that altering some of these balances could make the condition better or worse, but that’s all theory without much foundation.

What is known, however, is that diet has a massive impact on the microbiome. Even the mechanism for that is obvious: Bacteria thrive on different foods, so if you eat more of one class of nutrients and less of another then the microbiome proportions will adjust based on which ones thrive on that diet.

Yes! My son who has autism would eat anything we put in front of him until age 3, when his weight, appetite and health suddenly and alarmingly crashed. Ever since that episode, he's had a much more restrictive diet and food preferences. Night and day.

They never successfully identified what happened. Just diagnosed it generally as failure to thrive.

You seed the gut with nutrients. having lots of fiber and a varied diet increases the number of species that an adult has which is between a couple hundred to a thousand or so. Our guts are generally dominated by a bunch of beneficial bacteria.

which for many is not the case for a variety of social economic or behavioral reasons. Add in with explosions of bacterial populations due to alcohol or sugar and you can see how we can change our gut biome drastically from week to week.

no and yes

autists have often a much much stronger need for habits and avoidance of change. This includes a change of, or a less repeting/habitual diet. The effect if applified due to autism being commonly comorbid with ADHD and hyper fixation on specific foods being a very common thing (not (mainly) caused by gut bacteria as the effect is too strong and too specific to be "just" a preference caused by gut bacteria)

but this can lead to a imbalance of gut bacteria and that can have an reinforcing effect on wanting a even more monotonous diet, but in the end this is AFIK "just" a secondary reinforcing reason not the root cause

this relies a bit on a healthy brain to make s good correlation. an unhealthy brain might make a bad diet just out of habit/compulsion rather than driven by their biology.

couldn't it just be perpetuated, after the other occurrence too?

just to play devil's advocate that it isn't an opposing possibility