Do Ultra-processed Foods Cause Weight Gain?


Written By: Heather Gerrish, RDN

Researchers at the National Institutes of Health (NIH) recently conducted a study evaluating the differences between ultra-processed and minimally processed foods. They found some interesting results and has taken the dietetics world by storm as one of the first randomized controlled trials to assess this “assumption” we carry today.

An overview of this study includes the following:

Participants (10 men and 10 women) were enrolled in this study, all of which were brought to the NIH research hospital (for an entire month!) and provided one of two diets. A brief overview was provided in a recent excerpt from NIH:

“For the study, researchers admitted 20 healthy adult volunteers, 10 male and 10 female, to the NIH Clinical Center for one continuous month and, in random order for two weeks on each diet, provided them with meals made up of ultra-processed foods or meals of minimally processed foods. For example, an ultra-processed breakfast might consist of a bagel with cream cheese and turkey bacon, while the unprocessed breakfast was oatmeal with bananas, walnuts, and skim milk...The ultra-processed and unprocessed meals had the same amounts of calories, sugars, fiber, fat, and carbohydrates, and participants could eat as much or as little as they wanted”.

This study provided a controlled environment with strict requirements of researchers and the protocol itself, leading it to provide excellent data and information regarding ultra-processed foods and their impact on our diets.

There has been a wide range of feedback provided for the researchers and individuals who conducted this study, with an overall appreciation for what was done and the rigor in which it was carried out.

What was an interesting result of this study was the higher intake of total calories in the ultra-processed group when both were provided open ability to consume as much as they wanted. Similarly, they were offered the same amount of the above macronutrients in each meal - the ultra-processed group simply ate more.

Why did this occur? Well, that study might be up next but there has been discussion regarding the palatability of both diets and potentially our natural likeness of more savory items (which often are more processed vs your standard vegetable) but the jury is still out in terms of data backed evidence to suggest one reason or the other.

What do you think might be the reason behind participants consuming more on the “ultra-processed” diet?

Another translation of this research is how participants were brought through procedures in this study. In reality, and in all human subjects research projects, it is very difficult to conclude any definitive correlations - not to mention causation! Each participant had to stay at the NIH research hospital for the duration of this study. That in itself is an amazing component of this study - would you be open to staying in a research hospital for an entire month? How would you feel if you were a participant?

With this “control” aspect of the study in place, researchers were able to provide participants the only food (or diets) they were allowed to eat. [Not true for human subjects out in the real world]. This type of control is very unusual for nutrition studies and is unique for longer duration studies.

Further discussing the parameters of this study, participants were provided with each type of diet. After consistent adherence for an entire month [again, WOW], participants were assessed for changes, including weight. It was found that participants who were on the “ultra-processed” diet actually gained more weight than those who were not provided this diet, those who received the more minimally processed diet actually LOST weight!

Although this study is not 100% transferrable to the real world because we live outside of a confined environment and can freely choose what we want to eat each day, this study helps to provide evidence to support the cause for following a less processed diet.

You might be thinking - yeah, I knew this already. BUT now we have evidence to support further studies to find out WHY this occurred in participants and further investigate why participants ate more when provided more processed foods.

From the surface, this research is an important component and progression of nutrition research where such a controlled environment related to diet is not at all the common occurrence, but also provides a reason for further investigation.

The next time you think about the “why” to your rationalization for eating more processed foods, you might think further as to this “why”.


Hall KD, et al. Ultra-processed diets cause excess calorie intake and weight gain: A one-month inpatient randomized controlled trial of ad libitum food intake. Cell Metabolism

(link is external). May 16, 2019.

Heather Gerrish