Study reveals effect of fast food on liver

If there was ever a reason to resist the urge to order from the McDonald’s late-night menu, a new study from Keck Medicine of USC and the Keck School of Medicine, revealing the detrimental effect of fast food consumption on livers, may provide just that.

The study, published earlier this month in Clinical Gastroenterology and Hepatology, found that eating fast food — defined in the study as meals obtained from either a drive-through restaurant or one without wait staff — is associated to an increased risk of nonalcoholic fatty liver disease, a condition in which excessive fat builds up in the liver and can contribute to significantly increased mortality.

The study also found that people suffering from obesity or diabetes who consume 20% or more of their daily calories from fast food have severely elevated fat levels in their livers, compared to those who consume little to no fast food. The general population who obtain more than a fifth of their energy intake from fast food has also seen a moderate increase in liver fat versus those who do not consume as much.

While prior research in the field established a link between fast food consumption and diabetes or obesity, this study is one of the first to explore fast food’s negative effect on liver health, said Ani Kardashian, hepatologist with Keck Medicine of USC, assistant professor of clinical medicine at the Keck School of Medicine and the lead author of the study.

“We wanted to see if we could show and link fast food consumption to a higher risk of having fat build up in the liver,” Kardashian said. “What was surprising to me is how many people are eating fast food and how many people are eating over … a fifth of their calories from fast food.”

Kardashian and her colleagues analyzed the most recent data provided in the 2017-18 National Health and Nutrition Examination Survey, the largest annual survey in the country, to explore the impact of fast food on liver health.

Specifically, Kardashian and her colleagues analyzed the fatty liver measurement from approximately 4,000 adults in the survey and compared that to their fast food consumption histories.

Among those surveyed, 52% of them had consumed fast food within 48 hours before taking the survey, and 29% of those who had fast food experienced a significant rise in their liver fat levels, as they consumed one-fifth or more daily calories from fast food.

Moreover, researchers surmounted the challenge of readjusting the data with multifarious factors and discovered that the higher risk of NAFLD brought by 20% of daily caloric intake is applicable to both the general population and those with obesity and diabetes.

“We use data that’s already available in the dataset on age, gender, race, ethnicity, physical activity and income. Then what we did was we created a model where we looked at the relationship between fast food, or more than one fifth of the calories from fast food with the outcome of fatty liver,” Kardashian said. “And in that model, we controlled for all these other variables. So we accounted for the influence of all these other variables in the model.”

It was particularly challenging yet fulfilling for researchers to figure out which factors or variables are appropriate to be included in the study, said Jennifer Dodge, co-author of the study and assistant professor of research medicine and population and public health sciences at the Keck School of Medicine.

“When we got to the phase of the study where we were looking at fast food and its association with liver fat after adjusting for things that may be confounders, like consumption of sugar-sweetened beverages, alcohol intake or physical activity,” Dodge said. “That’s where it’s really interesting so that you can see the whole picture.”

The detrimental effect of fast foods on liver health is extremely alarming to researchers, as fast food consumption has grown exponentially in the past 50 years, especially during the coronavirus pandemic.  

“Fast food is cheap, and it’s easy to access. It’s at every street corner,” Kardashian said. “We are also seeing that … it is more expensive to buy healthier foods. And so as much as we can do as doctors to counsel our patients of the negative health effects of fast foods, we also need larger policy changes that make it easier for people of all incomes to be able to afford healthy foods.”

Whole foods, which are less heavily processed, could be a litmus test for finding healthier food options and safeguarding liver health, said Carin Kreutzer, instructional professor of clinical gerontology and pediatrics at the Leonard Davis School of Gerontology.

“The elephant in the room is ultra-processed foods. And fast foods are ultra-processed. The study that looked at the consumption of ultra-processed foods, which includes fast foods … finds that as much as 10% of all premature deaths in individuals, ages 30 to 69, were due to overconsumption of fast foods,” said Kreutzer, referring to the study that studied 57,000 individuals in Brazil. “In terms of food, it is looking for healthier choices when you are eating out and we are seeing more and more of that where you can go to a place where they do have whole foods, where it’s not heavily processed.”

Kardashian hoped that the study could encourage healthcare providers to offer patients more nutrition education, especially those with obesity or diabetes, who are at a higher risk of developing fatty liver from fast food consumption.

“Ultimately, the message here is that the healthier you eat, and the better foods you put into your body, the better your liver’s going to be,” Kardashian said.

Correction: This article was updated Jan. 30 at 2:58 p.m. to reflect that Keck Medicine of USC and the Keck School of Medicine, which are separate institutions, jointly contributed to this study. Kardashian is a hepatologist with Keck Medicine as well as an assistant professor at the Keck School of Medicine. The Daily Trojan regrets this error.