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I was a bit confused about the relationship between "omega-6" and "omega-3" fatty acids, Linoleic Acid and Alpha-Linolenic acid, whether they are good for you, and what the best sources are. This is what I turned up:

Overview

  • Humans require some fat in their diets, as they cannot create some essential kinds from other sources.
  • Western diets generally contain too much fat, particularly saturated fat.
  • We also eat the wrong ratio of polyunsaturated fats - we need to eat more omega 3 fatty acids. We also need to change the ratio of omega 3 to omega 6 in the favour of omega 3. The recommended omega 6 : omega 3 ratio is 2:1. Typical western diets are nearer 10:1!
  • Particular fish, flax oil and some seeds and nuts are good sources of polyunsaturated fats with a good ratio of omega 3.

  • Humans require some fat in their diets, as they cannot create some essential kinds from other sources.
  • Western diets generally contain too much fat, particularly saturated fat.
  • We also eat the wrong ratio of polyunsaturated fats - we need to eat more omega 3 fatty acids. We also need to change the ratio of omega 3 to omega 6 in the favour of omega 3. The recommended omega 6 : omega 3 ratio is 2:1. Typical western diets are nearer 10:1!
  • Particular fish, flax oil and some seeds and nuts are good sources of polyunsaturated fats with a good ratio of omega 3.

Omega What?

Fats are classified by the length of their carbon chain backbone, the number of C=C carbon double bonds, and the position of the first C=C bond - the "omega number". For example, the omega-6 fatty acid Linoleic Acid has a "18:2w6" structure - 18 carbon atoms, 2 double bonds, the first of which is after position 6 in the chain.

There are two main types of polyunsaturated essential fatty acids, commonly known as "omega-6" (W6) and "omega-3" (W3) fatty acids. They differ in the the position of their first carbon-carbon double.

Essential Fatty Acids

The most commonly discussed W6 fatty acid is Linoleic Acid (LA). The body can make gamma-linolenic acid (GLA), dihomo-gamma-linolenic acid (DGLA), arachidonic acid (AA) from linoleic acid (LA,18:2w6).

There are at least 3 kinds of omega-3 fatty acids: LNA (alpha-linolenic acid, 18:3w3), EPA (eicosapentaenoic acid, 20:5w3) and DHA (docosahexaenoic acid, 22:6w3).

EPA and DHA occur mostly in fish and seafood. LNA occurs mainly in seeds and nuts. EPA and DHA can also be made by the body from alpha-linolenic acid

Sources

For W3 generally, Flax seed is the best source. However, it only contains LNA, not EPA or DHA, so might not have the same health effects as eating fish.

Flax seed over 50% LNA
Hemp seed oil about 20% LNA
Pumpkin seed oil Between 1% and 15% LNA
Canola oil up to 10% LNA
Walnut oil between 3% and 11% LNA

Name Fat Content of seed (%) LNA W3(%) LA W6(%) LA+LNA w3+w6(%) LNA:LA (Ratio)
flax 35 58 14 72 4:1
hemp 35 20 60 80 1:3
pumpkin 46.7 0-15 42-57 57 (0:60) - 1:4
soybean 17.7 7 50 57 1:7
walnut 60 5 51 56 1:10

Walnuts

I was particularly intrested in walnuts, so I did a bit of investigation.

Walnut Oil:
Fat Type Percentage
Saturated, total 10%
Monosaturated, Total 14%
Polyunsaturated, total 76%
- Linoleic 61%
- Linolenic 15%
Total Fatty Acids 100%

Further Reading


The text and table below this line came from the now defunct webpage for the course "The Chemistry of Human Life" at http://www.homepage.montana.edu/~bchem280/omega.html - this link is now broken, so I've mirrored the information here.

BCHM 104N The Chemistry of Human Life

Omega 6 and Omega 3 Fatty Acid Content of Common Foods (g/100 grams)

In the column below headed "omega-6," you will find the total omega-6 fatty acid content of each listing. The next three columns list the content of the individual omega-3 fatty acids: LNA (alpha-linolenic acid, 18:3w3), EPA (eicosapentaenoic acid, 20:5w3) and DHA (docosahexaenoic acid, 22:6w3). The quantities refer to grams per 100 grams of food. (100 grams is approximately one small serving.)

The information comes primarily from The Health Effects of Polyunsaturated Fatty Acids in Seafoods, Edited by Artemis P. Simopoulos, Robert R. Kifer, and Roy E. Martin, and published by Academic Press, Inc. in 1986.

Note: "Tr" signifies trace amounts "--" signifies lack of reliable data for a nutrient known to be present. The best sources of omega-3 fatty acids are highlighted.

SEAFOOD

Type of Food Omega-6 LNA EPA DHA
Anchovies 0.2 -- 0.5 0.9
Bass, fresh water 0.4 Tr 0.1 0.2
Bass, striped -- Tr 0.2 0.6
Bluefish 0.4 -- 0.4 0.8
Burbot 0.1 -- 0.1 0.1
Carp 0.8 0.3 0.2 0.1
Catfish, channel 0.7 Tr 0.1 0.2
Cod, Atlantic Tr Tr 0.1 0.2
Cod, Pacific Tr Tr 0.1 0.1
Eel, European 0.5 0.7 0.1 0.1
Flounder 0.1 Tr 0.1 0.1
Haddock Tr Tr 0.1 0.1
Hake, Atlantic 0.1 Tr Tr Tr
Hake, Pacific 0.2 Tr 0.2 0.2
Halibut, Greenland 0.5 Tr 0.5 0.4
Halibut, Pacific 0.2 0.1 0.1 0.3
Herring, Atlantic 0.4 0.1 0.7 0.9
Herring, Pacific 0.6 0.1 1.0 0.7
Mackerel, Atlantic 1.1 0.1 0.9 1.6
Mackerel, King 1.0 -- 1.0 1.2
Ocean perch 0.3 Tr 0.2 0.1
Pike, northern 0.1 Tr Tr 0.1
Pike, walleye 0.1 Tr 0.1 0.2
Pollock Tr -- 0.1 0.4
Rockfish, brown 0.3 Tr 0.3 0.4
Rockfish, canary 0.1 Tr 0.2 0.3
Sablefish 0.5 0.1 0.7 0.7
Salmon, Atlantic 0.7 0.2 0.3 0.9
Salmon, chinook 0.6 0.1 0.8 0.6
Salmon, chum 0.4 0.1 0.4 0.6
Salmon, coho 0.7 0.2 0.3 0.5
Salmon, pink 0.4 Tr 0.4 0.6
Salmon, sockeye 0.6 0.1 0.5 0.7
Sardines, canned -- 0.5 0.4 0.6
Sea bass, Japanese 0.1 Tr 0.1 0.3
Shark, unspecified -- -- Tr 0.5
Smelt, rainbow 0.1 0.1 0.3 0.4
Snapper, red 0.2 Tr Tr 0.2
Sole 0.1 Tr Tr 0.1
Sprat 0.2 -- 0.5 0.8
Sturgeon, Atlantic 0.6 Tr 1.0 0.5
Sturgeon, common 0.1 0.1 0.2 0.1
Swordfish -- -- 0.1 0.1
Trout, brook 0.3 0.2 0.2 0.2
Trout, lake 1.4 0.4 0.5 1.1
Trout, rainbow 0.6 0.1 0.1 0.4
Tuna, albacore 0.3 0.2 0.3 1.0
Tuna, bluefin -- -- 0.4 1.2
Tuna, skipjack -- -- 0.4 1.2
Whitefish, lake 0.7 0.2 0.3 1.0
Whiting, European -- Tr Tr 0.1
Wolfish, Atlantic 0.2 Tr O.3 0.3

Crustaceans

Type of Food Omega-6 LNA EPA DHA
Crab, Alaska king Tr Tr 0.2 0.1
Crab, blue 0.1 Tr 0.2 0.2
Crab, Dungeness Tr -- 0.2 0.1
Crawfish 0.2 Tr 0.1 Tr
Lobster, European Tr -- 0.1 0.1
Lobster, northern Tr -- 0.1 0.1
Shrimp, Atl brown 0.2 Tr O.2 0.1
Shrimp, Japanese 0.5 Tr O.3 0.2

Mollusks

Type of Food Omega-6 LNA EPA DHA
Abalone -- Tr Tr --
Clam, littleneck -- Tr Tr Tr
Clam, Japanese -- -- 0.1 0.1
Clam, softshell 0.2 Tr 0.2 0.2
Conch 0.1 Tr 0.6 0.4
Mussel, Blue 0.1 Tr 0.2 0.3
Mussel, Mediterranean 0.1 -- 0.1 0.1
Octopus 0.1 -- 0.1 0.1
Oyster, eastern 0.3 Tr 0.2 0.2
Oyster, European 0.1 0.1 0.3 0.2
Oyster, Pacific 0.3 Tr O.4 0.2
Scallops 0.1 Tr 0.1 0.1
Squid, Atlantic 0.1 Tr 0.1 0.3
Squid, short-finned 0.1 Tr O.2 0.4

Fats and Oils

Type of Food Omega-6 LNA EPA DHA
Butter 1.8 1.2 -- --
Canola oil(rape seed oil) 22.2 11.2 -- --
Chicken fat 19.9 1.0 -- --
Duck fat 11.9 1.0    
Flax seed oil (linseed oil) 12.7 53.5 -- --
Margarine, hard, soybean 19.4 1.5 -- --
Rice brain oil 33.4 1.6 -- --
Safflower oil 77 1 -- --
Mayonnaise, soybean 37.1 4.2 -- --
Soybean oil 51.1 6.8 -- --
Walnut oil 52.9 10.4 -- --
Wheat germ oil 54.8 6.9 -- --

Nuts and Seeds

Type of Food Omega-6 LNA EPA DHA
Beechnuts, dried 18.4 1.7 -- --
Butternuts, dried 34.0 8.7 -- --
Chia seeds, dried 3.4 3.9 -- --
Hickory nuts, dried 20.9 1.0 -- --
Soybean kernels, roasted 11.2 1.5 -- --
Walnuts, black 34.2 3.3 -- --
Walnuts, English 32.3 6 -- --


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