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Comment by Aurornis | Hacker News Reader

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.

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I’m not going to pretend to be an expert here, but I remember a study that found gut bacteria composition predicted whether or not an individual was chocolate-craving or not in individuals eating identical diets: https://pubmed.ncbi.nlm.nih.gov/17929959

It makes sense for chocolate given that cocoa flavanols are prebiotic fiber for GABA-secreting bacteria which of course affects the parasympathetic nervous system.

The paper didn’t say that the microbiome was driving food preferences.

It measured some bio markers and some dietary preferences and claims some correlation.

The correlation is that what you eat fuels the microbiome. So your diet influences the microbiome by fueling or starving different bacteria.

Complex theories about causality going the other way through complex chains of flavonoids to bacteria to neurotransmitters to the parasympathetic nervous system sound impressive with all of the big words, but it’s such a complex theory that would need other testing to even begin to understand if there was something there.

Testing the other direction is easy and obvious. You can grow many bacteria in a Petri dish and see that some grow better or worse with different nutrients.

>but it’s such a complex theory

What's the theory now? I didn't propose any specific theory -- just noted a mechanism of influence.

You have to be careful with microbiome research because it’s a buzzword that gets crammed into a lot of research papers to imply something bigger. This is a single paper from Nestle Research Center (yes that Nestle) from 2007 that doesn’t even cite a number of people sampled in the abstract.

They didn’t run any experiments trying to change the diet or microbiome. They just correlated dietary preferences with some markers that might be correlated with the microbiome.

The paper does not say anything about how changing the microbiome might change preferences. The simplest and most well tested explanation is that dietary preference are driving the microbiome.

There’s a lot of woo-woo microbiome discussion out there that misses the really obvious basics of how the microbiome comes to exist and thrive: What you eat is what the microbiome eats, so changing what you eat will change the composition of bacteria that thrive. People who prefer chocolate are correlated with people who prefer sweet diets. High sugar intake is proven to alter the microbiome.

putting aside what others have commented about the truastability of studies and similar

- what all studies show is some vague "craving" for something generic, e.g. the link between iron deficiency and craving for ice

- but what you see in autists is often a far stronger effect and not just for eating something, but also against eating most other food. A craving for chocolate does not remove appetite and willingness to eat other food. It just makes you really want to eat chocolate.

- more important the way autistic people get fixated on a monotonous diet is far more specific then any effects we have observed from gut bacteria or other similar sources AFIK. Like lets say your gut bacteria might make you crave fish. You autism on the other hand might make you crave dino formed fish sticks with a specific texture. And there is just no way gut bacteria care about your fish sticks being dino formed or the specific texture of them... But a autistic person often does care, quite a bit even.

Yes, restrictive behaviors and powerful adherence to familiar routines are a general feature of autism. It’s not specific to food. The comments trying to draw complex links between food cravings and these behaviors are missing the fact that this type of behavior is not limited to foods. Food is only one area where it might manifest.

Mine is just an anecdotal experience but I have Asperger's and the reason I eat the same food is not about disliking other food. I actually like most food. I eat the same stuff because:

1) Eating the same food frequently means I don't have to spend any of my cognitive cycles on deciding what to eat.

2) I know that this food will sit alright with my digestive system. Everyone I know personally on the spectrum has stress-related digestive issues.

For snacks and small meals I will eat citrus or microwave a small sweet potato and this helps with Vitamin C.

Autistic adults are not children and using dino-shaped fish sticks probably does not represent most adults on the spectrum.

But is it driven by a desire for the diet, or a desire against things that provoke an undesirable reaction?

I am forced into an extremely limited diet to avoid provoking my body any more than I have to. And, notably, one of the first reactions used to be something not tasting as good as it used to (or in one case tasting worse than it had.) It doesn't always happen but when it does it's a near 100% accurate test--the only time it ever fooled me the actual culprit turned out to be something my wife put on her face.

I saw what happened to my mother (very similar path, but started much later in life), I already knew how to isolate what was giving me trouble before it ever happened. Most people don't, though, especially when dealing with things where it isn't high on the ingredients list (or, sometimes, not at all--they are strong about requiring manufacturers to list what they put in, but there is no such requirement about noting what they fail to take out from a natural source. Not to mention being allowed to specify that most evil of ingredients "artificial flavors". The second most evil being "natural flavors.")

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