A bag of salty sticks disappears in ten minutes in front of a series, and you wonder where it went. A plate of potatoes and chicken does not do that. The difference has a measured cause. There is nothing mystical in the bag.
Short answer: You tend to eat ultra-processed food faster, and in lab trials faster eating went with higher intake. The exact mechanism is still being studied. In Hall's 2019 trial people ate 508 kcal (calories) a day more on an ultra-processed diet, even though both menus offered matched calories and macros. Hall's team also measured eating speed: 48 kcal a minute versus 31.
The trial where both diets offered the same calories and macros
Kevin Hall and his team kept twenty adults on a ward at the US National Institutes of Health for a month (Hall 2019, Cell Metabolism, PMID 31105044). Each person spent two weeks on ultra-processed food and two weeks on unprocessed food, in random order. Both menus matched the calories on offer, energy density, macros, sugar, salt and fiber. The instruction was simple: eat as much as you want.
On the ultra-processed menu people ate 508 kcal a day more. The extra energy came from carbohydrate (+280 kcal) and fat (+230 kcal). Protein intake stayed almost the same. In two weeks on the ultra-processed menu they gained 0.9 kg. In two weeks on the unprocessed menu they lost 0.9 kg. Weight change tracked intake, with a correlation of 0.8.
Notice what did not happen. Thermodynamics worked normally that month. People gained because they ate more, and they ate more because the food made it easy. The calorie math held. The food simply made overeating effortless. The deficit itself is in calorie deficit: how much to eat.
Eating speed, the lever that gets less attention
Hall's trial also measured how fast people ate: 48 kcal a minute on ultra-processed food, 31 on unprocessed (figures as cited by Forde 2020, Current Developments in Nutrition, PMID 32110771; Ciaran Forde was a co-author on Hall's study). More than half again as fast.
Forde and colleagues then pooled five studies and 327 foods. Average intake rate stepped up with the NOVA groups (NOVA sorts foods into four groups by how much industry has processed them; ultra-processed is the fourth): unprocessed 35.5 kcal/min, processed 53.7 kcal/min, ultra-processed 69.4 kcal/min. The authors add an important caveat: the spread inside each group is huge, and NOVA by itself offers no proven mechanism.
Fullness takes a while to catch up with what you ate. If you get the same energy in much less time, you can be well ahead before you feel done. That is a likely explanation, not a settled one.
Texture and density matter alongside the label
Pey Sze Teo and Forde tested this directly (Teo 2022, American Journal of Clinical Nutrition, PMID 35285882). Fifty people ate four lunches in turn: soft and hard in a minimally processed version, soft and hard in an ultra-processed version. The harder versions were eaten more slowly, and people took in 26% less energy from them. The hard minimally processed lunch was the smallest (483 kcal). The soft ultra-processed lunch was the largest (789 kcal). That is about 300 kcal in one meal, and self-reported intake for the whole day moved the same way, by about 15%. Processing still played a part: the lowest intake came from the hard, minimally processed lunch.
A later trial in Wageningen made it even clearer (Lasschuijt 2025, Journal of Nutrition, PMID 40516651). Sixty-nine people got sandwiches that differed only in eating speed and energy density. Slow and energy-dilute: 570 kcal. Fast and dense: 1,143 kcal. A gap of 573 kcal, a doubling. The important detail: every version got 67 to 98% of its energy from ultra-processed ingredients. Same label, double the intake.
Hardness, volume and energy density are levers you can move in a normal shop, without banning anything.
What the big observational data show
A 2024 BMJ review pooled 45 analyses covering almost ten million people (Lane 2024, PMID 38418082). Two of the strongest (class I) findings link a higher intake of ultra-processed food with higher risk of cardiovascular death (RR 1.50) and type 2 diabetes (RR 1.12 for each step up in intake, in a dose-response analysis). A step weaker are all-cause death (RR 1.21) and obesity (OR 1.55).
Before anyone writes a headline, read what the authors say about quality. On the GRADE scale, 22 analyses were low quality, 19 were very low, and only four were moderate. Even the strongest association, 1.50 for cardiovascular death, was rated very low quality on GRADE. These are observational data: a link, not a proven cause. Someone who eats a lot of ultra-processed food often also differs in smoking, movement, sleep and total intake, and all of that lands in the numbers. If you have heart disease, diabetes or another diagnosis, talk to your doctor about your diet.
What I do with clients
Bans work worst of all here. What holds up:
- Put about 80% of calories on whole food. Leave the other 20% for anything that fits the day's intake.
- Keep protein at 1.6 to 2.2 g per kg of body weight a day. It fills you and protects muscle in a deficit. See how much protein.
- Get 25 to 35 g of fiber a day, mostly from vegetables, pulses and fruit.
- Pick the harder, bulkier version of the same food. A whole apple instead of juice, boiled potatoes instead of mash, fruit and yogurt instead of a cereal bar.
- When you eat something soft and energy-dense, put it on a plate and weigh it. A guess by eye is further off than people admit. I see it at almost every first look at a food log.
With my clients, the last point usually saves the most calories. If weighing or logging food makes you anxious, or you have a history of an eating disorder, skip it and talk to your doctor first. A sloppy log is usually a bigger problem than the macro split. The plate itself is in what to eat to lose fat.
Frequently asked questions
Are ultra-processed foods unhealthy by themselves?
Processing does not turn food into poison. Plain yogurt, cheese, canned beans, whey protein and olive oil are processed and they have a place. The catch in NOVA group four is that it is easy to overeat, and it often brings less protein, fiber and micronutrients.
Can I lose fat if I do not cut ultra-processed food out?
Yes, if you hold a calorie deficit. In studies where people ate freely, they ate more of it. When you measure intake, the advantage of whole food is mostly fullness and ease, not a break in thermodynamics.
Do I have to eat slowly?
Slower eating lowered intake in the trials above, so it makes sense to try. A more practical move than watching a stopwatch is to pick food that forces a slower pace by itself: harder and bulkier.
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Apply for coaching Get the free Fat Loss KitSources, so you can check:
1) Hall KD et al., Cell Metab 2019;30(1):67-77. PMID 31105044, DOI 10.1016/j.cmet.2019.05.008
2) Forde CG, Mars M, de Graaf K, Curr Dev Nutr 2020;4(3):nzaa019. PMID 32110771, DOI 10.1093/cdn/nzaa019
3) Teo PS et al., Am J Clin Nutr 2022;116(1):244-254. PMID 35285882, DOI 10.1093/ajcn/nqac068
4) Lasschuijt MP et al., J Nutr 2025;155(8):2602-2610. PMID 40516651, DOI 10.1016/j.tjnut.2025.06.006
5) Lane MM et al., BMJ 2024;384:e077310. PMID 38418082, DOI 10.1136/bmj-2023-077310
Martin Barna
Online nutrition and fitness coach. Since 2013 I have coached 600+ people through fat loss. Plans built on research and on what you can keep up.
Apply for coachingThis article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.