Meta analysis here
https://link.springer.com/article/10.1007/s13679-021-00460-y
It concludes In sum, current research suggests that consumption of UPFs may lead to excessive calorie intake, weight gain, and abnormal blood lipids in the short term and progress into long-term health consequences in adulthood. Given that lifelong dietary patterns develop from childhood and continue into adulthood 84, 85], efforts should be taken to reduce children’s exposure and consumption of energy dense and nutritionally poorer ultra-processed foods.
Key mechanisms are:
UPFs induce high glycemic responses, but have low satiety potential 86
Specific features from food processing, such as the inclusion of additives and alteration of the food matrix makes the foods have a softer texture for less chewing and amplifies sensory properties, which delays satiety signaling, and thereby results in an overconsumption of foods 58]. The higher sugar, fat, and salt content in UPFs makes them more hyperpalatable, which in turn could result in a healthy, nutrient-dense diet being displaced with empty calories and a lower-quality diet that results in weight gain 88
UPFs also have been reported to contribute to a gut environment that selects microbes that are associated with inflammatory disease 89]. The modification of the food matrix often changes the fiber and fat content of the foods, which influences the microbiota composition and bacteria–host interactions 88]. Minimally processed or natural foods have intact fibrous cell walls that provide a substrate for fiber-degrading bacteria in the colon and ensure a slow release of nutrients along the digestive tract 90]. However, the nutrients in UPFs are largely acellular, which instead results in an environment that promotes inflammatory gut microbiota that are associated with various cardiometabolic conditions
Another mechanism by which UPF consumption might impact biology or metabolism could involve the endocrine-disrupting chemicals, such as bisphenol A (BPA), often found in the elaborate packaging materials used for UPF products 88]. While the complete mechanisms of BPA remain unknown, there is some evidence that BPA promotes insulin resistance, oxidative stress, inflammation, and adipogenesis, which in turn increases our risk for major CVD conditions, including diabetes, overall and abdominal obesity, and hypertension