Fish oil is hailed for its plentiful health benefits. But new research suggests that the long-term consumption of fish oil or sunflower oil may increase the risk of fatty liver disease later in life.

By studying rats, researchers found that lifelong intake of sunflower oil or fish oil led to changes in the liver that make the organ vulnerable to non-alcoholic steatohepatitis (NASH).

NASH is a more serious form of non-alcoholic fatty liver disease (NAFLD).

Study co-author José Luis Quiles, a professor of physiology working at the University of Granada in Spain, and colleagues recently reported their findings in The Journal of Nutritional Biochemistry.

It is estimated that around 20% of people in the United States who have NAFLD also have NASH, and the condition becomes more common with age.

NASH is characterized by a buildup of fat in the liver—which is not caused by alcohol consumption—inflammation, and liver cell damage. The liver damage that occurs in NASH can increase the risk of liver cirrhosis and liver cancer.

Some of the key risk factors for NASH include overweight and obesity, high blood pressure, high cholesterol, and type 2 diabetes, which are factors heavily influenced by diet.

Studying how dietary fats affect the liver

Building on the link between diet and NASH, the new study from Professor Quiles and his colleagues suggests that the type of oils we consume could play a role in our later-life risk of the condition.

The researchers came to their findings by analyzing the effects of different dietary fats, including sunflower oil, fish oil, and virgin olive oil, on rat livers.

In detail, the team looked at how lifelong intake of each of these oils impacted the structure of the rodents' livers, as well as their effects on gene expression, liver fibrosis (or scarring), oxidative stress, and the length of telomeres.

Oxidative stress is defined as an imbalance between free radicals—which are molecules that can damage cells—and the body's ability to "de-oxidize" these molecules and prevent their harmful effects.

Telomeres are "caps" at the end of each strand of DNA, and they protect our chromosomes. The shorter our telomeres, the greater the damage to our cells.

Research has shown that a reduction in telomere length plays a key role in cellular aging and age-related disease. Telomeres that are too long, however, can increase the risk of cancer.

Additionally, the team looked at how the liver evolves with age as a result of the different dietary fats consumed.

Olive oil best for a healthy liver

The researchers not only found that liver fat accumulates with age, but they also revealed that the type of fat that builds up on the liver is influenced by the type of fats that we eat.

As Professor Quiles points out, "[...] 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 study revealed that fish oil and sunflower oil negatively impacted the liver health of the rats.

Lifelong sunflower oil intake was found to trigger liver fibrosis, and it also altered the structure of the organ, led to changes in gene expression, and increased oxidation in liver cells.

Rats that had a lifelong intake of fish oil demonstrated an increase in age-related cell oxidation in their livers, the team reports, and they also experienced a decrease in electron transport chain activity in the mitochondria—the "powerhouses" of the cell—which impairs cell function.

Fish oil also led to an increase in relative telomere length in the liver, the researchers report. Olive oil, however, was found to cause the least damage to the liver.

Taken together, the team says these findings indicate that virgin olive oil might be the best dietary fat for later-life liver health.

"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. We believe that this study will be very useful in preventing and treating diverse liver diseases."

Professor José Luis Quiles

"[V]irgin olive oil is the healthiest option," he adds, "which has already been proven in relation to diverse aspects of health."

These findings may come as a surprise; fish oil is the most commonly used natural product in the US due to its frequently documented health benefits.

Still, this latest study certainly provides some food for thought when it comes to choosing which fats to include in the diet.