There are 2 parts to this question:
- Which type of omega-3?
- How much is "enough?"
ALA Alpha-Linolenic acid 18:3, n-3
EPA Eicosapentaenoic acid 20:5, n-3
DHA Docosahexaenoic acid 22:6, n-3
I'll call ALA "short-chain" (18 carbons in the chain) and EPA and DHA "long chain" (20 and 22 carbons in the chains).
Many plants are good sources of the short-chain omega-3 (ALA), e.g. flax, walnuts, and greens like spinach, romaine, and kale if you eat a lot. (Omega-3s are polyunsaturated fatty acids and so can be found in higher concentrations in fatty plant foods.) Just 7 walnut halves provide about 1.3 grams of ALA.
The Institute of Medicine (IOM) has set an Adequate Intake (AI) for undifferentiated (short- and long-chain n-3s lumped together) n-3 of 1.6g/day/men and 1.1g/day/women.
For differentiated (ALA only): "An AMDR for α-linolenic acid is estimated to be 0.6 to 1.2 percent of energy. The lower boundary of the range meets the AI for α-linolenic acid."
So, 0.6% of a 2000 calorie diet is 12 calories or 1.3 grams, 1.2% of a 2000 calorie diet is 2.7 grams. Those 7 walnut halves (or a couple teaspoons of ground flax) should do it for most people for short-chain n-3. You'll get even more with the rest of the day's food.
Since animal foods, particularly seafood, are a better source of the longer-chain omega-3s (EPA and DHA) than plant foods, vegans may have difficulty getting enough. But what is enough? The IOM (in 2002) did not set an AI for EPA and DHA. Our body can convert some of the short-chain ALA to the more potent EPA and DHA but it's variable and inefficient. Many things affect the conversion, including age, gender (women convert more than men), health, and other things eaten including possibly the amount of omega-6 fatty acid. (I'll have more to say about this omega-6:omega-3 ratio in an upcoming post.)
This workshop in 1999:
Workshop on the Essentiality of and Recommended Dietary Intakes for Omega-6 and Omega-3 Fatty Acids, Journal of the American College of Nutrition, 1999
Said 0.65 grams of EPA+DHA a day were an adequate intake.
Is Docosahexaenoic Acid (DHA) Essential? Lessons from DHA Status Regulation, Our Ancient Diet, Epidemiology and Randomized Controlled Trials, Journal Of Nutrition, 2004
Says that DHA is probably essential, meaning we don't convert enough short-chain ALA for health (brain and retina) and we need to eat it directly. (We're better at making EPA from ALA.)
Towards Establishing Dietary Reference Intakes for Eicosapentaenoic and Docosahexaenoic Acids, Journal of Nutrition, 2009
Recommended that the IOM set a DRI for EPA+DHA of about 0.50 grams.
The only plant sources I know of for the longer chain n-3s are:
- Purslane which contains EPA but not DHA (0.01mg/g which is not very much).
- Mustard greens/red leaf lettuce/buttercrunch lettuce which contain DHA but not EPA (but at very low concentrations, 0.001/0.002/0.001 mg/g respectively).
- Seaweeds which can contain very small amounts of EPA and DHA depending on the type.