Ultra-processed foods (UPFs) are industrial formulations of cheap ingredients and additives with little whole food — think packaged snacks, sodas, instant noodles, and many cereals. In a tightly controlled 2019 NIH study, people ate either an ultra-processed or an unprocessed diet matched for calories, sugar, fat, and fiber. On the ultra-processed diet, they ate about 500 more calories a day on average and gained weight; on the unprocessed diet they lost it.
Researchers think UPFs are easy to overeat because they're energy-dense, soft, quick to eat, and engineered to be moreish. Large population studies also link high UPF intake to higher risks of obesity, type 2 diabetes, and heart disease. You don't need perfection — just shifting more meals toward whole and minimally processed foods makes a measurable difference.