Consuming too much fish oil can increase the risk of serious liver disease, a study suggests.
Both fish oil and sunflower oil were linked to harmful long-term changes and ageing effects in the liver.
The end result was non-alcoholic steatohepatitis (Nash) - a dangerous inflammatory condition that can lead to cirrhosis and liver cancer.
Fish oil rich in omega-3 fatty acids and vitamin D is widely considered one of the most beneficial superfoods, warding off heart disease, helping to preserve healthy brain function, and protecting against inflammatory disorders.
The World Health Organisation recommends eating one to two servings of oily fish per week, and millions of people take daily fish oil or omega-3 supplements.
Oily fish include salmon, mackerel, sardines, trout and herring.
Scientists in Spain compared the liver effects of three key types of dietary fat - virgin olive oil, sunflower oil and fish oil.
Only olive oil proved to be relatively harmless as it accumulated in the liver over time.
Lead researcher Professor Jose Quiles Morales, from the University of Granada, said: "The alterations caused by the long-term consumption of sunflower and fish oils make the liver susceptible to non-alcoholic steatohepatitis, a very serious disease that may act as a catalyst for other liver diseases such as cirrhosis and liver cancer.
"Virgin olive oil is the healthiest option, which has already been proven in relation to diverse aspects of health."
Nash becomes more prevalent with age, affecting just 5% of teenagers, 39% of adults aged 40-50, and more than 40% of people older than 70.
A battery of different tests conducted on rat livers found that sunflower oil induced fibrosis, gene suppression and high levels of oxidation.
Fish oil intensified the oxidative damage associated with ageing, impaired mitochondria - energy-generating bodies within cells - and altered relative telomere length.
Telomeres are protective caps on chromosomes that can influence biological ageing and cancer.
Professor Quiles Morales said: "The most striking finding is that the type of fat accumulated differs depending on the oils consumed and this means that, regardless of this accumulation, some livers age in a healthier way than others and with a greater or lesser predisposition to certain diseases."
The research appears in the Journal of Nutritional Biochemistry.