Source for the claim that nicotine use "ruins" gut health?

My understanding is that the relationship between nicotine and gut health (indeed, overall health) is much more complex and nuanced than that. I know that nicotine has a positive effect on ulcerative colitis symptoms for many sufferers.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8895249/#s4

A quote:

Of all the diseases summarized here concerning systemic inflammation, especially in sepsis and endotoxemia, nicotine exerted the most pharmaceutical effect and significantly improved the survival. Next, nicotine is also a potential candidate for treating ulcerative colitis, rheumatoid arthritis, osteoarthritis, multiple sclerosis, and myocarditis; the in vivo data provided a much better foundation. For local inflammation, the nicotine administration route may be more important to avoid its accumulation in other healthy organs—for example, the effect of nicotine on arthritis will be more pronounced when nicotine is directly injected into the focus of infection. Perhaps that is why, in the early years, tobacco was used to treat enteritis as enemas (4). It is evident that nicotine has a significant pro-inflammatory effect on periodontitis. However, the latest research also found that nicotine positively affects periodontitis at a lower dosage. In this regard, we consider that the effect of nicotine on periodontitis is mainly due to the influence of inevitable and original oral microbes. At present, most studies focus on the cellular level, and in vivo studies may be limited due to the difficulty of model construction. Therefore, we recommend that individuals with poor oral hygiene avoid excessive direct exposure to nicotine for oral diseases.