Fish Oil and Cholesterol Levels

Health Benefits of Fish Oil

Fish oil is obtained from the tissues of fatty or oily fish. The major constituents of fish oil are the omega-3 fatty acids, DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid).

Besides these omega-3 fatty acids, fish oils usually contain antioxidants such as iodine and selenium. To further prevent the fish oil from spoilage, vitamin E (also an antioxidant) is usually added to fish oil supplements.

These antioxidants inhibit lipid peroxidation and, therefore, preserve the lipids in the oil from oxidative breakdown. Therefore, these antioxidants also contribute to the health benefits of fish oil in humans.

DHA and EPA are known for their anti-inflammatory properties but also have multiple health benefits.

Both omega-3 fatty acids are found abundantly in cold-water oily fish. Therefore, the preferred dietary sources of DHA and EPA are cod (liver), salmon, mackerel, sardines, herring, and anchovies. Although large, predatory fish may have higher levels of omega-3 fatty acids, they are not recommended because they also accumulate toxic compounds such as mercury.

Oily fish do not synthesize omega-3 fatty acids. Instead, they obtain them from feeding on microalgae, plankton, or smaller fish.

Fish oils also contain omega-6 fatty acids but they usually contain more of the healthful omega-3 fatty acids.

Omega-3 fatty acids continue to be the subject of numerous clinical studies. Such studies have investigated the possible benefits of these fish-oil fatty acids in the treatment of certain cancers, depression, attention-deficit hyperactivity disorder (ADHD), Alzheimer’s disease, Parkinson’s disease, and lupus.  

However, the highlight of omega-3 research has been on their effects on mental and cardiovascular health.

Regarding cardiovascular health, the US National Institutes of Health highly recommends fish oil supplementation for the treatment of high blood pressure, cardiovascular diseases, and hypertriglyceridemia (high triglyceride levels).

One of the important mechanisms by which fish oil omega-3 fatty acids improve cardiovascular health is by improving lipid profile through their influence of cholesterol levels.

Fish Oil, Omega-3 Fatty Acids, and Cholesterol

The results of different studies indicate that fish oil supplementation can lower blood triglyceride levels. The effect of fish oil on cholesterol is, however, not so clear-cut.

Triglycerides are a type of blood lipid responsible for the transportation of fat and glucose from the liver. A triglyceride is synthesized from glycerol and 3 fatty acids. Triglycerides are used to make very-low-density lipoprotein (VLDL).

High blood levels of triglycerides can cause the deposition of fat in the arteries and, therefore, increase the risk of stroke and cardiovascular diseases.

Reducing triglyceride levels is one of the main mechanisms by which fish oil omega-3 fatty acids improve cardiovascular health.

Concerning cholesterol, fish oil omega-3 fatty acids can increase the level of “good” cholesterol while lowering “bad” cholesterol. However, this relationship is not straightforward. Fish oil can raise LDL cholesterol in people with high blood triglyceride levels.

By lowering the levels of all non-HDL cholesterols, fish oil omega-3 fatty acids influence more than LDL cholesterol. For example, by lowering triglyceride levels, it also lowers VLDL levels.

Although fish oil only produces a modest reduction in LDL cholesterol level (and it may increase it), it renders LDL safe in the cardiovascular system. By lowering the level of lipoprotein lipase, fish oil prevents LDL cholesterol from being trapped in artery walls and, therefore, reduces the clogging and hardening of the arteries.

Cholesterols’ Little Known Secret

The classification of HDL as the “good” cholesterol and LDL as “bad” cholesterol is a simple generalization to help us understand the different functions of lipoproteins in the body. The truth is that both lipoproteins are simply carriers of cholesterol. They are best described as packages for cholesterol transportation.

In this regard, high-density lipoprotein or HDL transports cholesterol from the blood while low-density lipoprotein or LDL is responsible for carrying cholesterol through the blood.

Therefore, although a high level of HDL cholesterol improves cardiovascular health, a high level of LDL cholesterol does not necessarily worsen cardiovascular health. It is what happens to LDL cholesterol that makes it bad for the heart.

Specifically, LDL cholesterol only turns “bad” when it becomes oxidized by free radicals.

Furthermore, other non-HDL (low-density carriers of cholesterol) can be turned dangerous by these free radicals.

Therefore, the levels of HDL and LDL cholesterols are not the only factors affecting cardiovascular health. Other non-HDL cholesterols besides LDL are also important.

Omega-3 Fatty Acids and non-HDL Cholesterols

Unlike statins and certain natural supplements used for improving lipid profile, fish oil omega-3 fatty acids do not specifically target LDL cholesterol. Since fish oil does not lower LDL cholesterol, it is easy to conclude that fish oil supplements do not improve cardiovascular health.

However, fish oil does lower the levels of all other non-HDL cholesterols.

This is a rather important benefit. It is arguably more important than simply lowering LDL cholesterol like statins. First, all non-HDL cholesterols can be oxidized by free radicals.

Once oxidized, the lipoproteins form plaques on the walls of arteries and directly cause atherosclerosis.

Because there are more non-high density lipoproteins than low-density lipoproteins, lowering non-HDL cholesterol levels is just as important as lowering LDL cholesterol. Lowering total non-HDL cholesterol is the recommended treatment goal after lowering LDL in patients with high triglyceride levels.

Lastly, studies show that LDL levels aren't an accurate predictor of cardiovascular risks. A more accurate factor is the level of all non-HDL cholesterol.

Studies on Fish Oil and Cholesterol

Summary of Past Results

A 2006 paper published in the journal, Atherosclerosis, is the perfect summary of the body of scientific work done to investigate the effects of fish oil on cholesterols.

The authors of the paper did a systematic review of 21 such studies and found that fish oil consumption significantly reduced triglyceride levels, slightly increased HDL cholesterol, and moderately increased LDL cholesterol.

This review also found that high doses of fish oil supplements produced a greater reduction in triglyceride levels than low doses. Also, patients with very high triglyceride levels saw better results from fish oil supplementation.

While it would seem that fish oil did not improve HDL cholesterol nor reduce LDL cholesterol well enough, the authors of this review insisted that fish oil represents one of the effective natural supplements for reducing the risks of cardiovascular diseases.

How fish oil improves cardiovascular health without significantly lowering LDL cholesterol is discussed in the sections above.

Another Perspective on the Fish Oil-LDL Relationship

A 1990 study published in the journal, Arteriosclerosis, Thrombosis and Vascular Biology, provided useful insights into how fish oil affects different lipoproteins and their capacity to transport cholesterol.

For this study, the researchers recruited 33 men suffering from mild hypercholesterolemia and then divided them into 3 groups.

The men in the first group were given 14 g/day of linoleic acid from safflower oil; the second group received 9 g/day of alpha-linolenic acid from linseed oil, and the third group got 3.8 g/day of omega-3 fatty acids from fish oil.

The results of the study showed that only fish oil reduced triglyceride levels. Also, only fish oil raised the level of LDL cholesterol. The fish oil group also benefitted from reduced VLDL apoprotein B.

While all three oils reduced HDL cholesterol, the biggest reduction was seen in the fish oil group.

Besides measuring the levels of lipoproteins, this study also measured the activity of the enzymes required for the lipoproteins to transport cholesterol. And this part of the study provided a unique perspective on the activities of LDL and HDL.

The results of the study showed that fish oil

• inhibited the transfer of cholesteryl ester between LDL and HDL by 23%
• reduced the activity of the plasma enzyme, cholesterol acetyltransferase by 21%
• reduced the production of thromboxane by 9%

What is the significance of these findings? First, the study shows that although fish oil increases the amount of LDL in the blood, it reduces the cholesterol-carrying capacity of LDL.

This is an important result because LDL is just a “package” or carrier for cholesterol. On its own, it is harmless but the chance of hardening arteries increases when LDL carries cholesterol.

Therefore, although fish oil increases the number of LDL in the blood, it does not increase cholesterol levels. It reduces the amount of cholesterol carried through the blood because it prevents LDL from taking up cholesterol.

Another rarely mentioned benefit of fish oil for cardiovascular health is its inhibition of thromboxane synthesis.

Thromboxane is a lipid involved in the formation of blood clots. By reducing its production, fish oil reduces the risk of blood clotting, cardiovascular infarctions, and stroke.

Plasma Cholesterol vs. Lipoprotein Cholesterol

A 1986 study published in The American Journal of Clinical Nutrition provided support for the benefits of fish oil omega-3 fatty acids even when combined with high cholesterol intake.

The study involved 6 participants tested with 3 diets. The participants were first placed on a diet providing 710 mg/day of cholesterol. Thereafter, they ate a diet with lower cholesterol content (190 mg/day) along with 40 g/day of fish oil. Finally, they took the same fish oil supplement and an egg yolk diet with high cholesterol content (940 mg/day).

The results of the study showed that switching from a high-cholesterol diet to a low-cholesterol diet plus fish oil supplement reduced cholesterol and triglyceride levels.

The dietary change and fish oil supplementation lowered VLDL, LDL, HDL, and triglyceride levels as well as the level of apoproteins A1 and B.

When the study participants were switched back to a high-cholesterol diet plus fish oil supplement, only plasma cholesterol rose. There was no significant increase in lipoprotein cholesterol or apoprotein levels.

These results showed that even though fish oil supplements may not reduce the amount of cholesterol in the plasma, it can reduce the amount of cholesterol carried by lipoproteins (VLDL, LDL, and even HDL). Since lipoprotein cholesterols are the determinant of cardiovascular health, fish oil can improve cardiovascular health even when plasma measurements indicate it does not lower cholesterol levels.

Fish Oil + Vitamin E

In a 1991 study published in the Journal of Nutrition, a group of researchers investigated the benefits of combining vitamin E and fish oil on triglyceride and cholesterol levels.

This double-blind, cross-over study involved 12 healthy volunteers given 30 ml/day of two fish oil preparations (containing 0.3 IU/g and 1.5 IU/g) for 3 weeks.

At the end of the study, the results showed that the fish oil containing the higher dose of vitamin E reduced serum triglycerides (by 48%) and fibrinogen (by 11%). The fish oil preparation with the lower dose only reduced serum triglyceride and only slightly.

The results also showed that neither fish oil preparation reduced total cholesterol levels.

The researchers believed that vitamin E inhibited fatty acid peroxidation while the omega-3 fatty acids in fish oil inhibited the syntheses of triglycerides and fibrinogen.

These results suggest that the combination of fish oil and vitamin E supplements can significantly reduce the risks of cardiovascular health.

Fish Oil Omega-3 Fatty Acids + Gamma-Linolenic Acid

A study published in the American Journal of Clinical Nutrition in 2003 investigated the effects of omega-3 fatty acids and gamma-linolenic acid on plasma lipids.

Thirty-one women were recruited for the study and divided into 4 groups. All of the participants received 4 g of a mixture of EPA and DHA found in fish oil. Also, the participants received 0 g, 1 g, 2 g, and 4 g of gamma-linolenic acid (GLA).

Plasma lipid and fatty acid levels were measured during the 28-day duration of the study.

The results showed that total omega-3 fatty acid levels increased in all 4 groups. Also, LDL cholesterol was reduced only in the group receiving 4 g of EPA + DHA and 2 g GLA. Only this group got the best benefits from this combination treatment.

Therefore, this study established that the cholesterol-lowering properties of fish oil omega-3 fatty acids can be improved by adding gamma-linolenic acid. However, adding a high dose of a GLA supplement may reduce the effectiveness of the combination.