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Ultra-Processed Food and Weight Gain: What the NIH Study Found

In an NIH trial, people ate about 500 extra calories a day on ultra-processed food. Dr. Sayantan Banerjee explains what UPFs are, why they drive overeating, and easy swaps.

OBESITY REMISSION

Dr. Sayantan Banerjee

10/31/20267 min read

Ultra-processed foods have become one of the most discussed topics in nutrition. In both the US and the UK, they make up more than half of the calories the average adult eats. Books, documentaries and newspaper columns warn about them. Food companies push back. And many people are left wondering what the term actually means, whether these foods really cause weight gain, and what to do about it.

This article explains what ultra-processed foods are, what the strongest evidence shows, why they may make overeating easier, where the science is still uncertain, and how to reduce them without turning your life upside down.

What counts as ultra-processed?

The most widely used definition comes from the NOVA classification, developed by researchers at the University of São Paulo in Brazil. It sorts foods into four groups:

Unprocessed or minimally processed foods: fresh, frozen or dried fruit and vegetables, plain meat, fish and eggs, milk, plain yogurt, rice, oats, beans, lentils, nuts.
Processed culinary ingredients: oils, butter, sugar, salt, flour.
Processed foods: foods made by combining groups 1 and 2, such as fresh bread from a bakery, cheese, tinned vegetables in brine, tinned fish, and simple cured meats.
Ultra-processed foods: industrial products made mostly from substances extracted from foods or made in a laboratory, often with additives such as emulsifiers, flavourings, colourings and sweeteners that you wouldn't use at home.

Common examples of ultra-processed foods include fizzy drinks, many breakfast cereals, packaged snacks, sweets and chocolate bars, mass-produced packaged bread and buns, chicken nuggets, hot dogs and many processed meats, instant noodles, ready meals, many flavoured yogurts, and most fast food.

A simple home test: if the ingredient list includes items you wouldn't find in a kitchen cupboard, such as maltodextrin, hydrolysed protein, modified starch, emulsifiers or flavourings, it is probably ultra-processed.

How much do we eat?

A lot. Studies using national survey data estimate that ultra-processed foods provide more than half of the calories eaten by adults in both the US and the UK, and even more in children and teenagers. These figures have risen over recent decades.

The NIH study that changed the debate

For years, the evidence linking ultra-processed foods to weight was observational. People who ate more of them tended to be heavier, but that could reflect other differences in their lives.

In 2019, Kevin Hall and colleagues at the NIH published the first controlled trial, in Cell Metabolism. They admitted 20 adults to the NIH Clinical Center in Bethesda, Maryland, where every bite of food could be measured. For two weeks, participants ate an ultra-processed diet. For another two weeks, they ate an unprocessed diet. The order was random.

The researchers carefully matched the two diets for the calories, sugar, fat, fibre, salt and protein offered at each meal. Participants could eat as much or as little as they wanted.

The results were striking. On the ultra-processed diet, people ate about 500 more calories a day. They gained about 0.9 kg (2 lb) over two weeks. On the unprocessed diet, they lost about 0.9 kg. They also ate the ultra-processed meals faster.

The unprocessed diet was also associated with higher levels of PYY, a gut hormone that signals fullness, and lower levels of ghrelin, the hunger hormone.

It was a small, short study, but it was the first to show that ultra-processed food itself, not just its nutrient content, can drive people to eat more.

More recent evidence

In 2025, researchers at University College London published a trial in the UK in which adults followed diets based on national healthy eating guidelines for eight weeks, either made from minimally processed foods or from ultra-processed foods. Both diets met the same nutritional guidelines. People lost weight on both, but they lost roughly twice as much, around 2% of body weight compared with 1%, on the minimally processed version.

Large reviews of observational studies, including a 2024 umbrella review in The BMJ, have linked higher intake of ultra-processed foods with a range of health problems, including obesity, type 2 diabetes, heart disease and depression. These studies can't prove cause and effect on their own, but the consistency of the findings, combined with the controlled trials, has increased concern.

Why might ultra-processed foods lead to overeating?

Scientists are still working out the exact reasons, but several explanations are likely.

They are easy to eat quickly. Soft textures and little need for chewing mean we can eat a lot before fullness signals catch up. In Hall's study, people ate ultra-processed meals faster.

They are energy-dense. Many pack a lot of calories into a small volume, so portions look small but provide a lot of energy.

They are designed to be very appealing. Combinations of sugar, fat, salt and flavour that rarely occur in nature can strongly activate the brain's reward system. Researchers sometimes call these hyper-palatable foods.

They lack structure and fibre. Food structure slows digestion and allows nutrients to reach the lower gut, where fullness hormones such as GLP-1 and PYY are released. Many ultra-processed foods are broken down and absorbed quickly.

They are cheap, convenient and everywhere. They are heavily marketed, sold in large portions, and available at every petrol station, shop and vending machine.

Where the science is less certain

It is worth being fair here. Not all ultra-processed foods are equally harmful. Some, such as wholegrain packaged breads, some plain yogurts with added ingredients, and certain plant-based foods, may have a place in a healthy diet. In some large studies, these particular products weren't linked with poor outcomes.

The definition itself is also debated. Some scientists argue that it is better to focus on nutrients, energy density and food structure than on processing alone. And it isn't yet clear which specific features matter most.

What isn't in doubt is that a diet built mainly on minimally processed foods, with plenty of vegetables, fruit, beans, whole grains, nuts, fish and plain dairy, is consistently linked with better health, and is easier to eat in sensible amounts.

Practical ways to eat fewer ultra-processed foods

You don't have to remove them completely. Aim to shift the balance.

Start with drinks. Sugary fizzy drinks and energy drinks are among the easiest swaps. Replace them with water, sparkling water, tea or coffee.

Change breakfast. Swap sugary cereals for porridge (oatmeal) with fruit and nuts, eggs, or plain yogurt with berries.

Upgrade snacks. Swap crisps (chips), biscuits (cookies) and chocolate bars for fruit, nuts, plain popcorn made at home, or vegetables with hummus.

Cook a few simple meals. Learn four or five quick dinners you can make from basic ingredients in under 30 minutes, such as stir-fries, omelettes, traybakes of chicken or fish with vegetables, lentil soups, and pasta with a homemade tomato sauce.

Use helpful shortcuts. Frozen vegetables, tinned beans, tinned tomatoes, tinned fish, microwave rice pouches with simple ingredients, and pre-chopped vegetables are minimally processed and save time.

Read labels. A short ingredient list of recognisable foods is usually a good sign.

Shop the edges. In many supermarkets, fresh foods are around the outside, while more processed items fill the middle aisles.

Plan ahead. Many people turn to ultra-processed food when they are tired and hungry with nothing ready. Batch cooking at weekends or keeping simple ingredients on hand helps.

Simple swaps for US and UK shoppers

Fizzy drink (soda) — Try: Sparkling water with lemon or lime
Sugary breakfast cereal — Try: Porridge (oatmeal) with berries
Packaged white bread — Try: Bakery wholegrain or sourdough bread
Crisps (potato chips) — Try: Unsalted nuts or homemade popcorn
Flavoured yogurt — Try: Plain Greek yogurt with fruit
Chicken nuggets — Try: Oven-baked chicken breast strips
Ready meal — Try: A quick homemade stir-fry or omelette
Biscuits (cookies) — Try: An apple or pear with a piece of cheese

The cost question

Ultra-processed foods are often cheaper per calorie, which is a real barrier for many families. Some minimally processed foods are also very affordable: oats, eggs, tinned beans and lentils, tinned tomatoes, frozen vegetables, bananas, potatoes and rice. Cooking in larger batches and using leftovers can bring costs down further.

Reading labels in the US and UK

Food labels can help you spot heavily processed foods and compare similar products.

In the UK, many packaged foods carry front-of-pack traffic light labels, showing whether a portion is high (red), medium (amber) or low (green) in fat, saturated fat, sugar and salt. Mostly green and amber is a good sign. A product with several reds is best kept for occasional use. The ingredient list on the back is still the best way to judge processing.

In the US, the Nutrition Facts panel shows calories, nutrients and added sugars per serving. Check the serving size first, since many packs contain more than one. The FDA has also been developing a front-of-package nutrition label, so labels may change.

In both countries, ingredients are listed in order of weight, so the first few ingredients make up most of the product. If sugar, or one of its many forms such as glucose syrup, dextrose or fructose, appears near the start, the product is likely high in added sugar.

Labels such as "natural", "wholesome", "high protein" or "low fat" don't tell you whether a food is ultra-processed. Many protein bars, low-fat yogurts and "healthy" snacks are heavily processed.

Children and family eating

Children and teenagers in both the US and the UK get an even larger share of their calories from ultra-processed foods than adults do. Family habits make a big difference. Eating together, cooking simple meals at home, keeping fruit and vegetables visible and easy to reach, and making water the default drink all help. Involving children in shopping and cooking makes them more likely to try new foods. Change is easier when it is gradual and shared, rather than imposed on one family member alone.

How my book can help

Food structure is one of the key themes of The Obesity Exit Strategy. The book explains why the form of food matters for hunger, insulin and GLP-1 signalling, and how to build a way of eating that supports your biology rather than fighting it. See the book page.

Frequently asked questions

Do ultra-processed foods cause weight gain? In a controlled NIH trial, people ate about 500 more calories a day and gained weight on an ultra-processed diet compared with an unprocessed one.

What are examples of ultra-processed foods? Fizzy drinks, packaged snacks, sweets, many cereals, processed meats, ready meals and fast food.

Is all processed food bad? No. Frozen vegetables, tinned beans, plain yogurt and cheese are processed but not ultra-processed, and fit well in a healthy diet.

How can I tell if a food is ultra-processed? Look for ingredients you wouldn't use at home, such as emulsifiers, flavourings, modified starches or sweeteners.

Do I need to cut them out completely? No. Reducing them and making minimally processed foods the base of your diet is a realistic goal.

Are ultra-processed foods addictive? Some researchers think certain hyper-palatable foods can drive compulsive eating, but this is still being studied.

Are plant-based meat alternatives ultra-processed? Many are, as they are made from isolated proteins with additives. Some can still fit into a healthy diet, but beans, lentils, tofu and tempeh are less processed options.

Are artificial sweeteners a problem? Drinks with sweeteners are better than sugary drinks for weight and glucose in the short term. Water is the best long-term choice. Research on long-term effects of sweeteners is ongoing.

Does cooking at home help with weight? People who cook at home more often tend to eat fewer ultra-processed foods and fewer calories. Even a few home-cooked meals a week can help.

What is the single easiest change? For many people, cutting out sugary drinks.