Are there any natural sources of this substance?

Any expected side effects, like cancer or something?

It's interesting that this functionalized cellulose promotes body weight loss (in mice) through effects on intestinal microbiota. One of my pet theories is that other functionalized celluloses such as carboxymethylcellulose and hydroxypropylcellulose might be the culprits behind the obesity pandemic, because they are commonly used in ultra-processed foods, which have a close epidemiological association with obesity, and the withdrawal of which seems to reduce obesity in controlled experiments. Moreover, we know there's some kind of intestinal microbiome link in the causality chain, because fecal transplants can induce or reverse obesity in controlled experiments. And we know that in general such functionalized celluloses do alter the intestinal microbiome. However, nobody knows which intestinal commensals promote obesity, so we don't know whether these substances promote obesity or prevent it.

This experiment is a crucial step in that direction, but unfortunately the functionalized cellulose they tested is one that isn't commonly used in food.

Nobody should take my viewpoint on this too seriously, because I don't know much about the field, but while I'm stating it, I'll mention that other industrial polysacccharides like xanthan gum, guar gum, and modified food starch seem like equally strong candidates; that other food additives like titanium dioxide nanoparticles and polysorbate 80 are also candidates; and that the obesity pandemic is almost certainly multifactorial rather than caused by any single cause. Coca-Cola barely contains anything sketchy (just sugar, phosphoric acid, and caramel color) but can reliably induce significant weight gain and loss.