I was in a similar situation minus the kids.
My ex gained from ~120lbs (at 5'3") to 270lbs over the course of about 1½ years. That might sound insane (and it was) but it was basically about 8lbs per month. You might be wondering what she ate. Oddly enough it was a lot of Starbucks, of all things. She used to get Starbucks doordashed (we don't have doordash here but it was a similar company) constantly, like sometimes even twice (or even three times) in one day. Each time she'd get one or two of those huge Starbucks drinks with shitloads of syrup, plus several pastries and/or sandwiches. This was during the pandemic so she was stuck at home just doordashing Starbucks constantly. And just a lot of junk food and candy in general.
You might also be wondering if she was a feedee. This might surprise you, but she claimed not to be. However she was very into the whole lifestyle, and thought it was "hot" that I had a fat fetish. She even put together a weight gain compilation of herself for me. No, I don't have it anymore.
She got bloodwork done, came back prediabetic, high cholesterol etc, same as your wife. I don't know about your wife but my ex also had PCOS which partly explained how she gained so quickly and why her health plummeted so badly. Also this is a bit off-topic but Irish people are actually genetically predisposed to high cholesterol, even if they're not fat.
She went to this fancy private weight loss clinic that was admittedly effective but still kind of scammy. I say "scammy" because their whole shtick was upselling her on protein bars. The idea was that she was supposed to eat a protein bar whenever she got hungry (and this would somehow stave off the craving without her gaining weight). She spent a fortune on those protein bars. She was the only actual fat person there, everyone else there was like a rich 30-something soccer mom who had put on *maybe* 20lbs after having a kid. This was all before ozempic was invented. Nowadays that clinic just gives everyone ozempic and calls it a day.
She managed to lose 40lbs in about 8 months. Again, insane. But she achieved this by basically just starving herself and pretty much only living off those protein bars, salads, and black coffee. She also took up smoking. Her losing weight wasn't really the reason why we broke up, but it was definitely a factor. She CONSTANTLY complained about being hungry. She'd whine and even cry. God help me whenever she ran out of those fucking protein bars. It was like €100 for a box of 8, which would last her like two days if I was lucky.
This is difficult to explain, but her weight loss was also fraught with mind games. She would constantly try to goad me into feeding her (simply because she wanted to binge eat, and wanted to be able to blame someone else for it rather than herself). She would berate me for feeding her or berate me for not feeding her. I couldn't win.
In the end I broke up with her because she was a total nutjob. After the third time she ended up in the hospital after overdosing on pills, I called it quits.
If that last detail sounds familiar, I have told this story here before, but with less details. I did not actually meet her until after she had already lost the 40lbs; everything before that point is just lore/backstory that she had told me. The relationship itself was actually very short.
To answer your question about what to do about your wife:
Because you have children, I say you should stay with her. You would have to be mad to throw away your marriage for a fetish. I don't know anything about your wife but I would hope she is a lot more sensible and even-keeled than my ex was (particularly when it comes to losing weight). Helping your wife through her weight loss may be a difficult journey for you (especially because of your fetish) but I would implore you to think about your family. Ultimately her health, your love for each other, and your children's wellbeing are all more important than your fetish.
