Scientists only use terms like ultra processed food after defining them in their scientific papers. The problem here is that the media find it difficult to write a short article for the general audience if they have to define things scientifically.
What specifically is bad about UPF foods is still being researched. A few leading ideas are:
- Very little fibre
- Starches are all immediately accessible to digestion and so blood glucose spikes much more than for the non-UPF equivalent
- UPF foods are soft and dry (so weigh less) making it very easy to eat a lot very fast, so you eat too many calories.
- Relatively high in salt and sugar
- Use of emulsifiers. These may change your gut microbiota and also make your gut more leaky causing inflammation
- Use of preservatives and artificial colours
- Frequently have a lot of oil
Low fibre, emulsifiers and preservatives, while lacking variety of phytochemicals found in fresh food is known to change your gut health. People on UPF diets tend to eat more and have higher blood glucose spikes leading to heart disease and diabetes.
Altogether this is a recipe for a shorter, less healthy life
I don't know.
My thoughts are that your total daily intake is more important than considering any single food item. As such, having some UPF in your diet is ok. The problem becomes epidemiologically measurable when, like the UK and US, 60% of calories consumed by some demographics are from UPF food.
And there are almost certainly multiple different things 'wrong' with UPF and so if you fix one problem, you may still be at risk from another. For example in your question, there are a lot of studies showing the importance of fibre in the diet, including those that add bran to whatever the person normally eats. So UPF with lots of fibre, all things equal, is likely less bad than UPF without.
Fat soluble vitamins (A, D, E and K) are interesting in that they don't show benefits above RDA, and in high doses cause a long list of nasty symptoms. In particular, vitamin A in excess is correlated with increased risk of multiple major diseases and even death.