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.

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

• "Fats for Life" article by Life Extension Foundation - http://www.lef.org/magazine/mag2001/july2001_report_fat_02.html
• http://www.curezone.com/gallstones/fatspage.html
• THE A-Zs of OMEGA-3's - Nice overview, with a section on mental health effects - http://www.bipolarchild.com/newsletters/0501.html
• Omega-3 Fatty Acids and Psychiatric Disorders (preprint) - http://www.groupadpsych.org/pdf%20files/omega3.pdf - A scientific review of a very large number of papers and studies, which seems to support the idea that Omega-3 fatty acids are very important for mental health.

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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	--	--