Too much fast food and too little exercise can harm the liver, according to a small study published in the clinical journal Gut.   

The findings are based on 18 slim, healthy people (12 men and six women) who took a fast-food challenge for four weeks, and a comparison group, matched for age and sex, who ate a normal diet.   

The fast-food group restricted physical activity to no more than 5,000 daily steps and ate at least two fast-food meals (preferably in well-known outlets) every day. The aim was to double caloric intake and increase total body weight by 10%–15% to gauge impact on liver health.   

Blood samples were taken before the challenge began and at regular intervals throughout the study period so liver enzyme and fat levels could be checked. Liver damage is often identified by symptomless increases in enzymes. One enzyme, alanine aminotransferase (ALT), usually registers higher in people who regularly drink large amounts of alcohol or who have been infected with the hepatitis C virus. In a significant proportion of people, however, there is no obvious explanation.   

Too much fat in the liver indicates damage and is known as “fatty liver.” At the end of the four weeks, those in the fast-food group had put on an average of 14.3 pounds. Five increased their weight by 15%, and one person put on an extra 26 pounds in just two weeks.   

Sharp increases in ALT occurred after just one week on the fast-food diet. ALT levels more than quadrupled over the entire period. In 11 participants, ALT rose to levels indicative of liver damage, with increases linked to weight gain and higher intakes of sugar and carbohydrates.   

Only one participant developed fatty liver. Others’ test results showed a steep rise in liver cells’ fat content, which is associated with insulin resistance (a precursor to diabetes and cardiovascular disease). No such changes were seen among those who continued to eat their normal diet.   

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Illustration courtesy of the National Institute of Diabetes and Digestive and Kidney Diseases