Although regular intake of omega-3 fatty acids is believed to have several health benefits, some questions remain unanswered. Do we all need more omega-3 or just some of us? Is eating fish sufficient or do we need to take supplements? Is there a way to tell if the cells in our body are getting enough omega-3 or if we are deficient? The Omega-3 Index may provide answers to some of these questions.
The Omega-3 Index reflects the relative amount of omega-3 fatty acids within red blood cell membranes. The index can be measured by specific analytical methods using a simple blood sample.
Studies show that a low Omega-3 Index is associated with increased risk of cardiovascular disease, and it has been proposed that raising the index may help to reduce risk.
The fats we consume in our diet are mainly triglycerides. Triglycerides are composed of three molecules of fatty acids attached to a glycerol molecule. The type of fatty acids in triglycerides determines the characteristics of fatty foods, such as whether it is solid or liquid at room temperature.
Chemically, fatty acids are chains of carbon atoms with an organic acid (carboxyl) group (-COOH) at one end and a methyl group (-CH3) at the other end.
Fatty acids differ in chain length. Foods contain fatty acids with a chain length of 4 to 24 carbons, and most have an even number of carbons. Shorter fatty acids are more water soluble and more likely to be liquid.
Measurements of fatty acids in red blood cell membranes can provide important information about fatty acid intake. Recently, the relative amount of omega-3 fatty acids in red blood cells has attracted interest as it may provide information about the future risk of heart disease.
Red Blood Cell Fatty Acids and Omega-3 Index
Five main categories of fatty acids can be found in red blood cells; saturated fatty acids, monounsaturated fatty acids, omega-3 polyunsaturated fatty acids (n-3), omega-6 polyunsaturated fatty acids (n-6), and trans fats.
The omega-3 and omega-6 fatty acids can not be produced by the human body. These fatty acids must come from food and are therefore called essential fatty acids.
The omega-6 fatty acids have more than one double bond in their chain, the first one positioned at the sixth carbon atom from the acid end of the carbon chain. Examples are linoleic acid and arachidonic acid. All of the omega-6 fatty acids can be synthesized from linoleic acid, but linoleic acid itself can not be synthesized by the body and has to be provided in our diet.
Linoleic acid is typically found in vegetable oils (corn, safflower, soybean) and small amounts are found in canola, olive and flaxseed oils.
Omega-3 fatty acids are polyunsaturated fatty acids with a first double bond at the third carbon atom from the acid end of the chain. There are three main types; alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). ALA is found in many vegetable oils (rapeseed, flaxseed, and soybean), canola and olive oils, seeds, and nuts. EPA and DHA are typically found in fatty fish and fish oils.
The fatty acid status of red blood cells can be assessed from a blood sample using highly standardized analytical laboratory methodology. The test result reflects the amount of different fatty acids within the red blood cell membranes. Thus, we can tell how much of the fats is trans fat, saturated fat, omega 6, and omega-3 respectively.
The Omega-3 Index reflects the relative amount of EPA + DHA in red blood cells. It is expressed as the percentage of the total amount of fatty acids present. In fact it’s quite simple; if 8% of all the fatty acids present in red cell membranes is EPA+DHA, the Omega-3 Index is 8%.
From a methodological point of view, determining the Omega-3 Index has distinct advantages over determining levels of EPA+DHA in blood or plasma.
The Omega-3 Index and Cardiovascular Risk
It has been hypothesized that the Omega-3 Index may predict the risk of future cardiovascular events such as coronary heart disease and cardiac arrest. If that’s correct, a low Omega-3 Index may be regarded as a risk factor, similar to smoking, high blood pressure and high blood levels of LDL cholesterol.
The average Omega-3 Index in the United States is believed to be between 4-5 %. In Japan, where coronary artery disease is less common and life span longer, the average Omega-3 Index is 9-10% (1). In general, the Omega-3 Index is higher in countries that consume more omega-3 fatty acids.
The Seattle PCA study published in 1995 showed that compared with an Omega-3 Index of 3.3% (the mean of the lowest quartile), an index of 5.0% (the mean of the third quartile) was associated with a 70% reduction in the risk of primary cardiac arrest (2). Similar results were found in the Physicians Health Study published 2002 (3).
In a 2004 publication, Harris and Von Schacky presented data from epidemiological studies and randomized controlled trials showing that the Omega-3 Index was inversely associated with the risk for mortality from coronary heart disease (4). An Omega-3 Index of ≥8% was associated with the greatest protection, whereas an index of ≤4% was associated with the least.
Another study, published 2008 showed that the Omega-3 Index as independently associated with the risk of developing acute coronary syndrome (5). The risk of acute coronary syndrome was 70% lower in individuals with Omega-3 Index > 8 compared with those with an index <4.
Results from The Heart and Soul Study published in Circulation 2010 showed that in patients with stable coronary heart disease, an Omega-3 Index below median was associated with significantly higher mortality than among patients with an index above median (6).
One study found that a higher omega-3 index is associated with increased insulin sensitivity and a more favourable metabolic profile in middle-aged overweight men (7).
All the above data suggest that the Omega-3 Index is inversely associated with the risk of cardiovascular disease. However, these studies don’t prove that there is a causative relationship between the Omega-3 Index and cardiovascular disease nor that improving the index will reduce risk.
What Is the Optimal Omega-3 Index and How Can It Be Improved?
The simplest way to improve the Omega-3 Index is to increase the intake of EPA and DHA by eating marine products rich in omega-3 fatty acids.
Studies show that the Omega-3 Index is influenced by intake of EPA and DHA: every 4 g of EPA and DHA ingested per month increased the Omega-3 Index by 0.24 % (8). The Omega-3 Index is also influenced by age, diabetes, body mass index, gender, physical activity, and some other factors, like social status and alcohol intake (9, 10).
Increased intake of EPA and DHA may have several health benefits (11).
A meta-analysis of 36 randomized trials (12) indicated that fish oils may reduce both systolic and diastolic blood pressure, especially among elderly people with hypertension. Studies also suggest that intake of fish oil lowers heart rate (13).
Several randomized studies have suggested that fish oil improves the function of the endothelium, the innermost layer of our arteries (14).
EPA and DHA are precursors to substances called eicosanoids. Eicosanoids are important regulators of inflammation. Therefore, many experts believe that fish oils may help to reduce chronic low-grade inflammation.
Although some studies have not been able to show that fish oil consumption reduces blood markers of inflammation, other studies (15) have suggested that they may do so when relatively high doses are given (> 2 g/day).
Intake of fish oil lowers blood levels of triglycerides by 25-30 percent (16). Relatively high doses are needed to achieve this effect (up to 3-4 g/day).
What Types of Fish Contains Most Omega-3 Fatty Acids?
Examples of fish that contains most amounts of omega-3 (EPA + DHA) are salmon, herring pickled, tuna Bluefin, mackerel, sardines (canned in oil) and oysters (steamed).
Examples of fish which contains intermediate amounts of omega 3 (EPA + DHA) are swordfish, rainbow trout, sea bass, crab king, walleye, tuna (canned in water) and flatfish.
Examples of fish that contains lower amounts of omega-3 (EPA + DHA) are Halibut, Northern lobster, clams, scallop, haddock, cod, mahi-mahi, shrimp, and catfish.
Recommendations For the Intake of Omega-3 Fatty Acids
For people without cardiovascular disease, most experts recommend eating a variety of fish (preferably oily) at least twice a week to maintain a mean intake of 4-500 mg of EPA+DHA daily.
For those with documented coronary heart disease, a daily dose of EPA+DHA of 1.000 mg per day is recommended, preferably from fish. This can be achieved by eating oily fish 4-5 times a week. Fish oil supplements may also be used.
Many fish oils supplements are available. Important questions have been asked regarding these supplements such as if they are contaminated or if they contain as much EPA+DHA as they say they do?
The International Fish Oil Standards Program may be helpful to check out the quality of different types of fish oils and omega-3 supplements.