What Kevin Hall’s Research Teaches Us About Eating Behavior and Addiction

What happens when people eat nothing but ultra-processed food for two weeks? And what does that say about how this kind of food affects our appetite and weight? In 2019, American researcher Kevin Hall, based at the National Institutes of Health (NIH), designed a unique experiment to answer these questions. He later expanded on this work in 2021 with an analysis of the addictive potential of food. His findings are among the most important in nutritional science in the past decade.

The Experiment: Two Identical Diets, One Key Difference

In his now-famous 2019 study, Hall invited 20 participants to stay for four weeks in a tightly controlled clinical research facility. Each participant followed two diets for two weeks each, in randomized order. Crucially, the diets were matched for calories, macronutrients (protein, fat, and carbs), fiber, salt, and sugar. The only difference was the level of processing:

• The ultra-processed diet included foods such as white bread, breakfast cereals, ready meals, and processed meats.
• The minimally processed diet consisted of freshly prepared chicken, whole grain pasta, vegetables, fruits, and nuts.

Participants were allowed to eat as much as they wanted (ad libitum). All food intake was weighed and carefully tracked.

The Outcome: A 500 Calorie Difference Per Day

The results were striking. During the ultra-processed phase, participants ate an average of 508 more kilocalories per daythan during the unprocessed diet. They gained weight and body fat. During the minimally processed diet, they lost weight—even though they had the same freedom to eat.

The overeating happened unconsciously. There was no evidence of a deliberate choice to eat more; it was a passive result of the food environment.

Why the Difference?

Hall and his team concluded that the effect wasn’t just about calories or nutrient composition, but how the food was made. Possible explanations include:

• Eating rate: Participants ate faster when on the ultra-processed diet, giving the body less time to register fullness.
• Texture and mouthfeel: Soft, low-fiber foods require less chewing, which may reduce satiety.
• Food structure: Ultra-processed foods are often engineered to stimulate the brain’s reward system more intensely.

This study provided the first causal evidence that ultra-processed foods promote overeating—even when nutrient labels look identical.

Is Ultra-Processed Food Addictive?

Two years later, in 2021, Hall published a paper exploring whether ultra-processed food is addictive in the same sense as substances like nicotine or cocaine. He reviewed the scientific criteria for addiction: tolerance, withdrawal, loss of control, craving, harmful use, and more.

His conclusion: while ultra-processed food encourages overconsumption, it does not meet the criteria for addiction in the same way as hard drugs. There is no clear withdrawal syndrome, and the neurological responses differ significantly. Some individuals may show addictive-like eating behavior, but this often overlaps with factors like stress, sleep deprivation, or mental health.

What Does This Mean for Nutrition Policy?

Hall’s research has important implications. Many dietary guidelines still focus on calories or individual nutrients. But his work shows that the degree of processing is at least as important. Even if two meals are identical on paper, their structure and presentation can fundamentally change how much we eat.

References

• Hall, K.D., et al. (2019). Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake. Cell Metabolism, 30(1), 67–77.e3. https://doi.org/10.1016/j.cmet.2019.05.008
• Hall, K.D., & Kahan, S. (2021). Is Ultra-Processed Food Addictive? Obesity (Silver Spring), 29(9), 1211–1213. https://doi.org/10.1002/oby.23271