I'm not trying to be a pedant or derail this post, because I 100% agree with it. I do want to give some extra info, because I think it's a little overly general and too many people have absolute butt-wrong ideas about diabetes (fully informed by fatphobia), what it is, and how to handle it. I am nothing if not a consistently broken record on this topic, so here's a bit of it:
We know what causes Type 1 diabetes and have known for a really long time. It's autoimmune, which means at some point, a person's immune system attacks the beta cells in the pancreas, which are the ones that make insulin. Over time–some people are diagnosed earlier than 3 years old; others not until their twenties, and it's not unheard-of to be diagnosed later–the pancreas cannot make enough insulin to support daily function and the person will have to take insulin for the rest of their lives. Some diabetics prefer injections, but plenty of Type 1s use an insulin pump.
Type 2, presumably the one the fatphobes are talking about, is more complicated. But it still isn't true to say it's caused by food, because there is not a single thing linking Type 2 diabetes to food intake or the types of food eaten. The overall cause is insulin resistance, but the more Type 2 is studied (in good studies, not fatphobic ones), the more that insulin resistance seems to be the end result of something else, rather than the specific cause of Type 2. In fact, there are at least six potential causes for Type 2, and that's just what we know now.
PCOS has been singled out as potentially being one cause, hypercortisolemia (Cushing's Disease) has long been associated with the development of Type 2, DNA encoding has been suggested and has merit, and three others I cannot remember off the top of my head. What's interesting is that with the exception of DNA encoding (and this might still be true, I just haven't personally seen any studies talking about it), most of the causes are wrapped up with an overproduction of cortisol–you'll know it as the "stress" hormone but it has so, so many other jobs in the body. One of them is insulin resistance, like when you exercise or haven't eaten for awhile, so that your cells don't take in the usual amount of insulin, which means you're less likely to have a large drop in blood sugar. This is so you don't die when you go for a walk or something.
There's also more types than just 1 and 2, but they are often lumped in with one of those two for simplification depending on if damaged beta cells are involved and/or insulin resistance. Different doctors will classify them different ways. Some people refer to Type 1 as insulin-dependent diabetes, but there are Type 2s who are also dependent on insulin and do very well with that as a therapy. Many Type 2s use injections, but that's just because their doctors are cowards who won't prescribe insulin pumps when we know how well it works for diabetics of any stripe. Or their insurance companies are fatphobic arseholes.
Food management has yet to be shown as an effective therapy in any type of diabetes. Full stop. There is even reliable mounting evidence that it can contribute to insulin resistance, via the same pathway I mentioned earlier, cortisol, because of how cortisol functions in the body.
One of the biggest reasons Type 2 diabetes is on the rise has nothing at all to do with food, and everything to do with moving goalposts. It used to be that anything under 7.0% on a blood test called A1c was considered fine, between 7-8% was pre-diabetes. Then the American Diabetes Association (with the "help" of companies that sell weight loss drugs and non-insulin diabetes drugs) lowered the threshold to 6.5% and said we should start treating that as pre-diabetes. Recently, the threshold was lowered again to 5.7%, which is slightly above normal. The obvious goal seems to be to classify everyone who doesn't have perfect blood sugar as being pre-diabetic, even though there absolutely no good evidence for that whatsoever.
Eventually, the person at the American Diabetes Association who invented the term pre-diabetes admitted that it was a total mistake to have made the label and heavily regrets doing so, with specific pleas to both the Am. Diab. Assoc. and the American Medical Association stop using it at all as any kind of guidance, because it doesn't exist.
Why did he say this? Because out of all of the cases of pre-diabetes, just 2% of patients went on to develop full-blown diabetes (≤8% A1c). 98% of pre-diabetes diagnoses went back to normal without any intervention.
Which shows how much the point still stands: they're just making shit up to give people eating disorders. The association between diabetes and eating disorder incidence is shockingly high. But now you know why it's so fucking bullshit, and there's no excuse for you to keep parroting that nonsense like it's real.
Food does not cause any kind of diabetes and it is entirely insidious, fatphobic and ableist to say it does.