There is no scientifically concluded cause of ADHD. Such things take time to pan out.
That said, we do know that it is always related to either genetic predisposition or early brain trauma (like pre-mature birth, or head injury). There’s no scientific work that pins gut bacteria to ADHD, and the probiotic fad is actually making people sick (because unless you have an actual deficiency, taking probiotics spreads bacterial colonies where they shouldn’t go, like the lower intestines). ADHD appears to be a mechanically different brain that lacks some important neural connections between different parts that exist in neurotypical brains. This is confirmed with brain scans.
Alternative treatments are not science based and can be quite dangerous when trust is placed in them over proven treatments. They are based on science-denial and pseudoscience. The scientifically proven first-line treatment for ADHD is stimulant medication. It is proven by generations of study and practice to be the most effective and safest treatment available. And it is very very safe. There are second-line treatments as well, which include medications that aren’t stimulant based, neurofeedback has shown some promise, and cognitive therapy can be useful (but rarely by itself).
Poor diet can cause symptoms that are similar to ADHD, but that doesn’t mean it is ADHD or that poor diet causes ADHD. No ADHD link to diet has ever been found in real science.