Food and diet

Fast food damages the liver

“Fast food ‘is as bad as booze’,” is the headline in the Daily Express today. People who were “fed fast food twice a day for a month began to show signs of liver damage after only a week”, the newspaper says. The Sun says, "People who gorged junk food and shunned exercise for four weeks had liver enzyme changes that usually indicate alcohol abuse.”
The newspaper reports are based on a Swedish study of 34 people that looked at the effects on the liver of a diet high in saturated animal fat when levels of exercise are kept to a minimum. The researchers discovered that after a month, there was an increase in the levels of liver enzymes used as an indicator of liver damage. However, the importance of this enzyme change will need to be tested further and it is smaller than the changes associated with long-term alcohol consumption. The study does provide a further reason to avoid overeating (especially food high in saturated fat) if one is needed.

Where did the story come from? 

Dr Stergios Kechagias and colleagues from the fast food study group based at Linköping University in Sweden, carried out this research. The study was financially supported by the university and the Medical Research Council of South East Sweden. It was published in the peer-reviewed medical journal: Gut .

What kind of scientific study was this? 

This was a non-randomised controlled trial, which recruited 12 healthy men and six healthy women (average age 27 years) and matched them for age and sex with another 18 people. Both groups were recruited by advertising. All, except one recruit, were students and most were medical students.

Eighteen recruits agreed to eat two fast food meals a day (doubling their calorie intake) with the aim of increasing their body weight by 5–15%. They were instructed not to walk more than 5,000 steps a day and to make no changes to their normal weekly alcohol consumption.

At the start of the study, blood samples were collected and a range of liver function tests was carried out. Various body measurements were also taken, including weight and abdominal circumference. The fat content (triglyceride levels) of the liver cells was estimated with a special MRI scanner using a technique known as proton nuclear magnetic resonance spectroscopy. All of the measurements were repeated at the end of the study four weeks later.

What were the results of the study? 

After four weeks, the volunteers eating the fast food managed to increase their weight from an average of 67.6kg to 74kg, and their waist circumference by 7cm on average.

Over the four-week period, their serum ALT level (one of the liver enzymes measured by the researchers) increased from an average 22.1 to 69.3 units/L. In 11 of the 18 subjects, this enzyme was over the normal level for their sex at the end of the study.

The fat content of the liver cells measured by spectroscopy showed an increase from 1.1% to 2.8%. These results were statistically significant. Other enzyme levels measured by the researchers did not change significantly.

What interpretations did the researchers draw from these results?

The researchers say overeating can induce significant increases in the levels of the enzyme ALT in less than four weeks. They say that their study clearly shows that when doctors are investigating patients elevated ALT levels, they should ask questions about alcohol intake but also “explore whether recent excessive food intake has occurred”.

What does the NHS Knowledge Service make of this study? 

This experimental study has demonstrated important changes in a liver enzyme and in fat content of the liver in association with high-energy food and weight gain. Some limitations to this study mean that it is not possible to say that this effect is causal or that it would apply to people who ate fast food but did not put on weight so rapidly.

  • The study was not randomised, meaning that we are unable to be sure that the volunteers for the study did not differ from the control group by being selected in a way that might affect the results.
  • The researchers did not collect data on alcohol intake and this is an important factor as it is possible that there were important differences in alcohol intake between the two groups, or differences in intake for the group that ate fast food over the four weeks of the study.
  • The upper limit of normal range for the liver enzyme ALT varies from 19 units/L in women to 30 units/L in men. The researchers did not attempt to report the results separately according to the sex of the volunteers. With small numbers of participants in the trial it is possible one or two very large increases in enzyme levels might raise the average level and it would be important to know more details about whether this occurred and its effect on the results. One man, for example, increased his ALT from about 30 units/L to about 450 units/L in three weeks. This is a very large increase, which could have been caused by illness.
  • No details of the content of the fast food diet has been given by the study, other than that it should be rich in protein and saturated animal fat; ideally, hamburger-based. Therefore, no interpretations should be made on any particular foods or fast food chains involved. In fact, as the study has been conducted in Sweden, fast food brands or retail outlets may not be the same or directly comparable to those in the UK or in other countries.
  • The researchers state that the participants in the study “were not representative of the general Swedish citizen”, as their intake of fibre was higher than the norm and all the volunteers were lean.

Enzyme and fatty liver changes have been observed alongside intentional forced feeding and rapid weight gain in this study. The study by its design cannot eliminate the possibility that this has happened by chance or that it is associated with other factors that the researchers did not measure. The study population used here is very small and much larger studies will be required to investigate this effect further.

NHS Attribution