Sustainable Seafood 6. The Skinny on Fats: Omega-3 vs Omega-6, etc.
In which the E@L explains why you should eat fatty fish
We hear a lot about fatty acids these days, especially omega-3 fatty acids (commonly noted as Ω-3, or simply n-3), and how good they are for us. You may have also heard that omega-6 fatty acids are not as good. But what are omega-3 and omega-6 fatty acids, and what do they have to do with seafood? (Full disclosure – if you read this entire post, you might qualify for a minor in Lipid Chemistry).
All About Fatty Acids
Fatty acids (FAs) are important molecules in our diet because they have many physiological functions. Generally, they come in two forms: saturated and unsaturated. Saturated FAs consist of a chain of 8 to 22 carbon atoms connected with single bonds. Each carbon can have two hydrogen atoms attached to it, forming ethyl groups (CH2). When each carbon atom has both hydrogen atoms attached, we call it saturated. Saturated fatty acids are uniformly straight chains that can stack on top of each other and lock into place like strands of dried spaghetti, with the consistency of butter. If one hydrogen atom is missing from two adjacent carbon atoms, they can form a carbon-carbon double bond, and we call them unsaturated. The double bond gives the chain a kink, causing it to bend. As a result, unsaturated FAs don’t stack well, like a bad hair day, and have a fluid consistency like olive oil. Fatty acids with a single double bond are called mono-unsaturated, whereas those with many double bonds are called polyunsaturated fatty acids, or PUFAs.
We classify fatty acids by their length, the number of double bonds, and the location of the first double bond 1. Omega-3 FAs have their first double bond located at carbon number 3 (counting from the end of the fatty acid chain), whereas omega-6 FAs have their first double bond located at carbon number 6. One of the most common, and important omega-3 FAs is alpha-linolenic acid (ALA), also known as C18:3n-3 because it has 18 carbons and 3 double bonds, starting at carbon #3. Two other important n-3 FA’s are eicosa-penta-enoic acid, known as EPA (hyphens inserted to assist pronunciation), which simply means it has 20 carbons and 5 double bonds (C20:5n-3), and docosa-hexa-enoic acid (DHA) with 22 carbons and 6 double bonds (C22:6n-3).
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When we think about fats, we usually think about the kind that causes our waistlines to expand with age. Those kinds of fats are storage fats, that usually come as triglycerides (TG). A triglyceride is simply three FA chains attached to a glycerol backbone, like a hand with three fingers. TGs carry fats through the bloodstream to provide energy for our cells and muscles.
Long chain FAs are considered essential for two reasons; for one, they are critical components in cell membranes, function as signaling molecules (for example, in blood clotting), and perform important roles in the immune system. For another, the human body cannot make them, so we must get them in our diet. ALA is abundant in many plant foods, such as flax seeds, walnuts, soybeans, and canola oil. In contrast, EPA and DHA are only made by algae, that are then consumed by plankton, and eventually fish or shellfish. Thus, the only way to get them is to eat seafood (although they are added to a number of products these days including yogurt, juice, and plant-based milks). We can, however, convert a small amount of ALA to EPA and DHA.
Cholesterol – the Good, the Bad, and the Apo
Cholesterol is a wax-like substance that is a component of cells, vitamins, and hormones. Generally, your liver makes all the cholesterol you need. However, we tend to accumulate an excess amount of it from the food we eat (primarily meat and dairy products). Some fats (e.g. palm and coconut oil) stimulate excess cholesterol production. Too much cholesterol can lead to heart disease, heart attack, and stroke. Cholesterol can be characterized as high density lipoprotein (HDL) or low density lipoprotein (LDL). Another important compound is Apolipoprotein (Apo), a protein attached to HDL & LDL. LDL or Apolipoprotein B is often considered “bad” because it carries cholesterol to our cells and causes fatty buildup of plaque (atherosclerosis). HDL or Apo-A is considered “Good” because it carries LDL to the liver for destruction. A combination of high TG and high LDL leads to high risk for heart disease.
There, now you are a lipid chemist!
Omega-3 vs Omega-6
Both n-3 and n-6 FAs are important for the proper functioning of our bodies. Humans evolved eating a diet of green leafy vegetables, nuts, and fruits, all of which had a good balance of both n-3 and n-6 FAs, in a ratio of about 1:1. However, our diet has changed dramatically in recent history. The development of agriculture brought an increased consumption of cereal grains, which are high in n-6 FAs. In the last century, development of cold pressing techniques promoted widespread use of vegetable oils made from corn, safflower, and other grains, which are therefore high in n-6 FAs. In addition, feeding livestock with grain instead of grass increased the levels of n-6 FAs in meat products. As a result, the ratio of n-6 to n-3 FAs in our western diet is now about 15:1 (or, equivalently, a n-3/n-6 ratio of 0.067). This is much different than our body chemistry is adapted to.
Omega-3 FAs are known to promote good cardiovascular health and reduce the risk of coronary events. Consumption of n-3 PUFAs improves fetal development and brain development, promotes longer pregnancies, and leads to fewer premature births. They also suppress autoimmune and inflammatory diseases such as heart disease, reduce macular degeneration, and can help moderate the symptoms of Alzheimer’s disease by reducing cognitive impairment. Increasing the amount of n-3 PUFAs in our diets lowers triglyceride levels and can help prevent certain types of cancer including breast and colorectal cancer, and possibly prostate cancer (but that is still debatable)2. In contrast, high levels of n-6 FAs (relative to n-3s) tend to promote these diseases. Experimental research showed that reducing the n-6/n-3 ratio to 5 had beneficial effects for asthmatics and lowering it to 2.5 suppressed inflammation in patients with rheumatoid arthritis and reduced cell proliferation in colorectal cancer3.
In the accompanying chart, I have plotted the levels of n-3 and n-6 FAs in various foods4. In this graph the data are transformed to their log10 values to fit them on the same plot, and the size of the circle represents the ratio of n-3 to n-6, so larger is better. The straight blue line is the line of equality where the n-3/n-6 ratio is equal to one. From this, we can see some interesting trends. Our typical western diet (yellow dot) is low in omega-3 FAs and high in n-6 FAs (thus above the 1:1 line), with a n-3/n-6 ratio of about 0.67 (small dot; equal to the previously stated n-6/n-3 ratio of 15, but I’m inverting it here to emphasize n-3 FAs). One of the biggest green circles is represented by cod, with n-3 of 280 and n-6 of 20, thus it has one of the highest ratios of n-3/n-6 for any seafood (14.0). Eating more cod would go a long way towards bringing your overall n-3/n-6 ratio back into balance.
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Redfish, New Zealand hake, plaice, Alaska pollock, and wild salmon all have n-3/n-6 ratios similar to cod, ranging from 7 to 11. Farther to the right, herring and mackerel have some of the highest n-3 values, with n-3/n-6 ratios of 6.3 and 7.9, respectively. Notice that farmed salmon has much higher n-3 values than wild salmon (2400 vs 530), which is good, but farmed salmon also has much higher n-6 values, so its n-3/n-6 ratio is lower (1.8 vs 10.6). This is primarily due to the inclusion of plant-based oils in aquaculture feeds to replace the fish oils they would naturally consume. Also note that Tilapia is both low in n-3 and its n-3/n-6 ratio (0.4) due to high levels of n-6. Although anchovies and sardines are both high in n-3 FAs, they both have relatively low n-3/n-6 ratios (0.4 and 0.6, respectively). At the opposite end of the spectrum are beef, chicken, and safflower oil, with extremely low n-3 values and extremely high n-6 values.
Finding a Balance
The problem is not that we consume too much n-6 FAs, but that we don’t consume enough n-3 FAs. Fast food contains a lot of n-6 FAS, because much of it is fried in vegetable oil. Humans should consume at least 500 mg per day of PUFAs. We can get that by eating seafood twice a week, or about 4-6 oz of seafood. Pregnant women should consume twice that amount, about 8-12 oz per week. The best sources of PUFAs are fatty cold-water fish, including cod, pollock, salmon, sardines, and anchovies2. Non-fish sources include walnuts, chia seeds, and flax seeds. In addition to PUFAs, salmon have additional food-specific-compounds, including phosphatidyl-glycerol-phosphate (PGP), that are associated with improved cardiometabolic health indicators, and lower the levels of Apo-B cholesterol, LDL, and total TGs. We can increase the amount of n-3 FAs in our diet, and reduce our n-6/n-3 ratios by taking fish oil, as supplements or pills, but the effect is not as good as eating whole fish. This could be due to the presence of other as-yet-unknown healthful components in fish (such as PGP), or it could be due to the fact that eating more fish replaces less healthy protein sources.
The take home message here is simple. Eat more seafood! Especially fatty fish like salmon and cod. Human populations that include high levels of PUFAs, omega-3 FAs, and fish in their diets tend to live longer and have much lower mortality from cardiovascular and other metabolic diseases. And that’s something that would benefit all of us.
Sources for this Post
1 https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/
2 https://ods.od.nih.gov/factsheets/Omega3FattyAcids-Consumer/
3 Simopoulos, A. P. 2002. The importance of the ratio of omega-6/omega-3 essential fatty acids. Biomedicine & Pharmacotherapy, Volume 56, Issue 8, Pp 365-379. https://doi.org/10.1016/S0753-3322(02)00253-6
4 https://seafood.oregonstate.edu/sites/agscid7/files/snic/omega-3-content-in-fish.pdf
Foods marketed as “healthy” or “heart friendly” often tout their Omega-3 content, but rarely (in fact I’ve never seen it) include nutritional information on their Omega-6/Omega-3 ratio.