The study was so small and over such a short period of time, and was so different from normal conditions where people choose what they eat (like, the subjects' meals were being chosen for them, they were only choosing how much to eat) that I think the conclusions are meaningless.
It's a rigorous study in the sense that they collected many different types of data including biological data and subjective questionnaires, but I think it doesn't demonstrate anything at all about "ultra-processed food" because it doesn't define what that is.
In the introduction, there are two different parts that might describe what "ultra-processed food" is, this:
an increasingly industrialized food system (Stuckler et al., 2012) characterized by large-scale production of high-yield, inexpensive, agricultural “inputs” (primarily corn, soy, and wheat) that are refined and processed to generate an abundance of “added value” foods (Blatt, 2008; Roberts, 2008).
and a reference to the NOVA classification of food processing, which you can read about on Wikipedia here. This is a quote from the Wikipedia article (itself a direct quote from a publication) which might help:
Ultra-processed foods are operationally distinguishable from processed foods by the presence of food substances of no culinary use (varieties of sugars such as fructose, high-fructose corn syrup, 'fruit juice concentrates', invert sugar, maltodextrin, dextrose and lactose; modified starches; modified oils such as hydrogenated or interesterified oils; and protein sources such as hydrolysed proteins, soya protein isolate, gluten, casein, whey protein and 'mechanically separated meat') or of additives with cosmetic functions (flavours, flavour enhancers, colours, emulsifiers, emulsifying salts, sweeteners, thickeners and anti-foaming, bulking, carbonating, foaming, gelling and glazing agents) in their list of ingredients
The NOVA system seems to be trying to capture the idea of foods that are created using industrial processes, out of ingredients that were broken down and extracted by industrial processes. It's pretty vague and flawed in some ways: "Flavors and flavor enhancers" could mean anything, including any seasoning at all.
But, NOVA classification at least gives a potential operational definition of "ultra-processed food."
Trouble is, the "ultra-processed" diets in the study don't even follow that definition.
If you download the supplementary materials, you can scroll down to where it shows photographs and descriptions of every meal the participants in the study ate.
Like, the ultra-processed diet includes...whole milk?? How on earth is whole milk "ultra-processed?" It's milk! Canned corn definitely isn't ultra-processed according to the NOVA definition. It's corn...in a can. Ditto with the green beans and peaches.
The Wikipedia for the NOVA classification explicitly says that the following foods fall under category 3, "processed," NOT "ultra-processed:
Examples include cheese, canned vegetables, salted nuts, fruits in syrup, and dried or canned fish. Breads, pastries, cakes, biscuits, snacks, and some meat products fall into this group when they are made predominantly from group 1 foods with the addition of group 2 ingredients.
There is no explanation of whether or how the foods selected for the "ultra-processed" group fit into the NOVA system, but at a glance, it seems like the "ultra-processed" diet includes a lot of things that aren't ultra-processed.
The "unprocessed" diet also includes items that (as per the NOVA classification) are not unprocessed, such as yogurt, spaghetti, and penne pasta. Like I get that it would be hard to make a meal out of only unprocessed foods but spaghetti noodles have been through some industrial processes. They don't grow out of the fucking ground like that.
I don't know, maybe it wasn't the point that every item in the meals fit the definitions, but if they don't all have to fit, it's kind of arbitrary, isn't it?
I appreciate how this paper shows how much more expensive the "unprocessed" diet is, and looking at the pictures you can get a sense of the kind of privilege you would need to eat that way: year-round access to fresh, perishable fruits and vegetables, and lots of time to cook and prepare meals. Whereas the "ultra-processed" meals are mostly easily prepared and shelf-stable.
The creator of the NOVA classification was analyzing "ultra-processed food" in relationship to class, but the authors of this study aren't.
Limiting consumption of ultra-processed foods may be an effective strategy for obesity prevention and treatment.
They're just interested in how making the Wrong individual choices makes people fat.