‘Freshman 15’ may be a myth, but beneficial gut microbiome still important

ASU study reveals how microbiome can impact weight gain among college students


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One of the long-held beliefs about college students is the “freshman 15,” the idea that first-year students gain an average of 15 pounds when they first enter college because they don’t know how to manage their food consumption.

There’s just one problem with that belief: It may not be true.

A study by the National Center for Biotechnology Information showed that students gain an average of only 3.2 pounds throughout their first year of college.

But, according to a paper co-written by Corrie Whisner, an associate professor in Arizona State University’s College of Health Solutions, and postdoctoral student Alex Mohr, poor eating habits and weight gain — even if not as substantial as the “freshmen 15” — can lead to health problems later in life.

Their study, published in Gut Microbes, a leading gut microbiome journal, explored the intricate relationship between dietary habits, lifestyle changes and the gut microbiome for college freshmen living in on-campus dormitories, and how the community of microorganisms in digestive tracts plays a pivotal role in overall health.

"To our knowledge, this is the first longitudinal study to observe a statistically significant relationship between weight change and the structure of the gut microbiome in dormitory-dwelling emerging adults during the first year of college," Mohr said.

ASU News talked with Mohr and Whisner, who are also researchers in ASU's Biodesign Center for Health Through Microbiomes, about their study.

Editor's note: The following interview has been edited for length and clarity.

Question: The "freshman 15" is a widely accepted idea. How does your research challenge or support it?

Whisner: I guess I would start by just prefacing what the freshman 15 is now. That’s sort of an outdated term. Now, students are actually only gaining about two to five pounds in their freshman year. The literature is associated with the fact that students are entering college at a greater weight, so there’s maybe less room for adding additional pounds. But that weight gain can continue beyond freshman year. Some citations state that by the end of your second year, you’ve gained about, on average, 10 pounds. So what we’re talking about in our paper is much more subtle shifts in weight, but they’re definitely representative of what’s common in college populations these days. And it doesn’t mean that it’s less impactful.

Q: Can you explain the role of the gut microbiome in weight regulation among college students?

Mohr: That’s pretty complex to answer, but basically, there’s a lot of interactions going on with the foods that we eat, particularly foods that we know are more beneficial for the microbes that are harbored in our gastrointestinal tract, particularly in the lower portion. One thing we noted with the cohort was that they weren’t eating as much fiber as they should. They were well under the amount of fiber needed. Generally, we know that when you’re eating greater fiber, you’re supporting communities of microbes that are more generally associated with health.

Q: Your paper highlighted two specific microbes: Prevotella and Bacteroides. Why are they significant?

Mohr: Bacteroides is generally more associated with a Western-style diet, like a higher-fat diet. Prevotella is associated with a higher-fiber diet. We found a trend where if you have a higher ratio of Prevotella to Bacteroides, that is more associated with someone that is either lower in body weight or has lost body weight — whereas those that stabilized in weight or gained weight had a lower ratio. One of the more important findings in the paper was that we found it was very difficult over the academic year for a person to shift between those two states, basically between a Prevotella-dominated state to a Bacteroides-dominated state.

Q: Isn’t this sort of common knowledge, the idea that if you eat more fiber and less fatty foods you’ll maintain or lose weight?

Mohr: What we’re finding, or what at least this data is suggesting, is that the composition of the microbiome is important in terms of not dictating necessarily, but at least being associated with if you’re going to be predisposed to gaining, losing or maintaining weight.

Q: So, it’s as simple as eating healthier?

Mohr: Unfortunately, it’s not that easy because one of the analyses that we did was looking at how you can shift into a Prevotella-dominated microbiome versus a Bacteroides-dominated microbiome. It’s not as simple as eating more fiber because these microbes are very antagonistic, meaning that they don’t really like to be with each other. The results showed that the baseline composition of the gut microbiome matters, and a reductionist approach of simply eating more fiber is not enough. The broad set of factors examined, including physical activity, sleep, depression and various dietary attributes, show a more personalized approach is likely needed to promote the microbiome states.

Q: Corrie, how would you describe your findings?

Whisner: Our study underscores the importance of the gut microbiome's composition in weight dynamics. The freshman year can significantly impact late-stage adolescents' gut health. While earlier beliefs suggested our microbiome is fixed by age 3, our findings indicate potential for change even during later stages. Harnessing this understanding can profoundly affect health interventions tailored for young adults.

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