What did the NIH study about processed foods and its health impacts?

December 17, 2025 •

2 min read

According to a 2019 landmark study from the National Institutes of Health (NIH), people who eat a diet high in ultra-processed foods consume significantly more calories and gain weight, even when compared to a nutritionally matched, unprocessed diet. This groundbreaking research directly demonstrated a causal link between heavily processed foods and health concerns, providing objective evidence for what was long suspected.

Quick Summary

The NIH's 2019 randomized controlled trial revealed that ultra-processed diets lead to overeating and weight gain. Participants consumed approximately 500 extra calories per day and gained an average of two pounds on the processed diet, even when calories and macronutrients were matched between meal types.

Key Points

Causal Link Confirmed: The 2019 NIH study was the first randomized controlled trial to prove a direct, causal relationship between consuming ultra-processed foods and weight gain.
Excess Calorie Consumption: Participants on an ultra-processed diet consumed an average of 500 more calories per day compared to those on a minimally processed diet, despite being offered nutritionally matched meals.
Significant Weight Change: Individuals gained an average of 2 pounds during the two weeks on the ultra-processed diet and lost the same amount on the unprocessed diet.
Mechanism Hypotheses: Possible reasons for overeating on processed diets include a faster eating pace, hyperpalatability (engineered taste combinations), and lower satiety due to a broken-down food matrix.
Broader Health Implications: Evidence links high ultra-processed food intake to adverse health outcomes beyond obesity, including cardiovascular disease, diabetes, and mental health issues.
Socioeconomic Factors: The study highlighted that convenience and lower cost make ultra-processed foods difficult to avoid for many, pointing to the need for accessible healthy food options.

The Landmark NIH Processed Food Study

In 2019, researchers from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), a division of the NIH, published a pivotal study in the journal Cell Metabolism. This was the first randomized controlled trial to investigate the direct effects of ultra-processed foods on eating behavior and body weight. The study involved 20 healthy adult volunteers who stayed at the NIH Clinical Center for 28 days.

Participants were assigned to eat either an ultra-processed or a minimally processed diet for two weeks each. Both diets were designed to be matched for calories, sugar, fat, and sodium, allowing researchers to focus on the level of processing. Meals were provided ad libitum.

Key Findings: A Direct Link to Overeating and Weight Gain

The study found that on the ultra-processed diet, participants consumed an average of 500 more calories per day than on the unprocessed diet. This resulted in an average weight gain of 2 pounds on the ultra-processed diet and an equivalent loss on the unprocessed diet. The increased calorie intake from the ultra-processed diet came from more carbohydrates and fat.

Why Did Participants Overeat on the Processed Diet?

While the study confirmed a causal link, the exact mechanisms are still being explored. Hypotheses suggest factors like faster eating pace, hyperpalatability, and the food's physical structure may play a role.

Comparing Processed vs. Minimally Processed Diets

A comparison between the ultra-processed and minimally processed diets in the NIH study highlights key differences:


Feature	Ultra-Processed Diet	Minimally Processed Diet
Calorie Intake	Increased by ~500 kcal/day	Stable or decreased
Weight Change	Average gain of 2 pounds	Average loss of 2 pounds
Eating Pace	Faster	Slower

Broader Health Implications and Ongoing Research

Research indicates a link between ultra-processed foods and risks for various health problems, including cardiovascular disease, type 2 diabetes, certain cancers, anxiety, and depression. More research is needed to pinpoint which specific elements of these foods are most detrimental.

Moving Forward: The Challenge of Accessibility

Processed foods are often more affordable and convenient, posing a challenge for those seeking healthier options.

Conclusion

The 2019 NIH study demonstrated a causal link between eating ultra-processed foods and increased calorie intake and weight gain, even when matched for nutrients. Factors like faster eating and hyperpalatability likely contribute. These findings emphasize dietary quality. Understanding the health impacts is crucial, but making healthy choices accessible remains a challenge.

[You can read more about the landmark 2019 study findings directly from the {Link: National Institutes of Health https://www.nih.gov/news-events/news-releases/nih-study-finds-heavily-processed-foods-cause-overeating-weight-gain}].

Future Research and Understanding Processed Foods

Future research continues to explore the exact mechanisms. Understanding these details is crucial for developing interventions to address rising obesity and chronic diseases linked to highly processed diets.

Frequently Asked Questions

Ultra-processed foods are industrial formulations made with ingredients predominantly found in industrial manufacturing.

Yes, it was the first randomized controlled trial to directly test the effects of ultra-processed foods on calorie intake and body weight in a clinical setting.

The study found a direct, causal link; participants on the ultra-processed diet gained weight while those on the unprocessed diet lost weight.

Participants consumed approximately 500 more calories per day on the ultra-processed diet compared to the minimally processed diet.

The purpose was to test if the degree of food processing affects energy intake and body weight, independent of nutritional content.

High consumption has been linked to increased risk of type 2 diabetes, cardiovascular disease, hypertension, certain cancers, and common mental health disorders.

Processed foods are often engineered to be highly palatable with appealing combinations of sugar, fat, and salt.