From my point of view as someone who is likely autistic (undiagnosed but do plan to get assessed eventually), I have more autism symptoms now than I used to, and I was malnourished at an early point in my life due to how my birth went.

I used to snack on Arnotts Shapes often when I was younger and beyond that only had small dinners and cereal for breakfast, and I didn't even have a routine, nor was I socially incapable, but I did stim a ton, with me getting seen as fidgety.

Now, I have the same breakfast daily, an omelet consisting of multiple eggs, at least a handful of home grown veg such as silver beet, some garlic and other seasonings, and some wholegrain wraps on the side. I made sure drinking water was also part of the routine to the point I almost drink too much.

However, that doesn't change the fact I stim the same amount, I speak my mind and am very direct, I think, draft and reason about my interests in computing architectures, structural engineering, electrification, and sustainability almost constantly that it stops me from being as social as I could, and I still can't prevent myself from forgetting to go to the bathroom and have showers, especially since those two aren't part of my scheduled routine I've built, they just happen when they need to.

There is some truth in lack of proper nutrition worsening mental and physical issues, but the simple fact is autism is predominantly a difference in how interconnected parts of the brain are and how different it is to a neurotypical. This is also the reason why the people wanting to create an autism vaccine continue to fail.