It is always best to obtain your nutrients from fresh, whole foods. But owing to differences in soil and climate, and variations in vitamin levels found in food, it can be hard if not impossible to determine with certainty that you are getting adequate amounts of all nutrients.
Traditional advice is to take a multi-vitamin to top off nutritional requirements and ensure adequate daily intake of all major vitamins and minerals. And most supplements do contain 50% or more of the USRDA of the major micronutrients (iron, B vitamins, etc).
But what is left out? These days vitamin pills contain a host of pseudonutrients such as extracts and powders from herbs whose usefulness in the diet has not been adequately determined. But what most vitamins do not contain is arguably one of the most important nutrients in food.
Enter Omega-3 fatty acids.
There are three types of Omega-3 fatty acids. For simplicity let's use the abbreviations here. ALA is found in plant sources including leafy green vegetables, flax and chia seeds, and in smaller amounts in other plant foods. DHA and EPA are present in fish and other animal sources. DHA is also found in seaweed. ALA is the parent Omega-3, as your body converts it into DHA/EPA with the aid of the enzyme delta-6 desaturase, but the process is inefficient in diets in which Omega-6 acids predominate. High Omega-6 intakes use up the desaturase enzyme, which is too busy making inflammatory prostaglandins and leukotrienes. This suggests the importance of obtaining DHA and EPA as well as ALA in the diet, especially for lovers of nuts, oils and grains (all high in Omega-6).
Omega-3s are involved in mood, memory, cardiovascular health, and inflammation, among many other things. Dry skin, attention/cognitive dysfunction, and low energy levels may be signs of deficiency. EPA levels are decreased in people with dementia. Depressed patients have lower levels of all Omega-3 fatty acids and experience symptom improvement with EPA supplementation. Studies have shown adding DHA in the diets of people lacking this fatty acid improved memory and reaction time, and Omega-3s also play a central role in the aging process. In fact, the amount of DHA found in the blood is proportional to a cell DNA's telomere length (the lower the level the shorter the length the older the cell/more times it has divided). DHA may prevent age-related brain deterioration, for example as seen in Alzheimer's. Moreover, older vegans with Parkinson's have been witnessed in the clinical setting to have nonexistent levels of DHA in their blood
It may be for these reasons that the Omega-3 index is being touted as the only blood test you'll ever need. But the test itself is expensive relative to the food sources of the acids it detects and must be ordered through your physician. A more expedient and cost-effective option may be to consume adequate amounts of these essential fatty acids in your diet.
But how much Omega-3 is enough? About a gram a day if you're a female, a gram and a half for males, give or take, says the Institute of Medicine. But the Institute does not differentiate among the Omega-3 subtypes. This requirement seems to apply if the source is EPA/DHA. But if your source is the plant-derived ALA, you may need to take upwards of 10 grams (equivalent to about 3 tbsp. of chia seeds), but such a high dose brings with it problems of its own, including an increased risk of prostate cancer and macular degeneration.
It would be more judicious to include a moderate amount of ALA in the diet - say, in the form of 1-2 tablespoons of flax or chia seeds, and then choose the appropriate dietary source of DHA/EPA. If you consume animal products, this may come from fatty fish, free range eggs or grass fed beef. Vegans may wish to include an algae-derived DHA/EPA source, such as the one Joel Fuhrman endorses, which contains around 250 mg of these important fatty acids. Cod liver oil and other fish oils are another option. They are less expensive and contain far more of these fatty acids. Of course, if you shun animal foods and/or are environmentally conscious this may not be the best option, as the oceans are nearly depleted of marine life due to the increased demand for seafood. If you do eat meat, you may wish to supplement with fish oil in lieu of getting levels checked, take for a few months, and note physiological responses - mood, skin, and other indicators of Omega-3 status. Note that it takes about a month for blood levels to equilibrate, and a maximal response is seen with a DHA dosage of 2g/day, though taking this much is probably unnecessary. If after DHA/EPA supplementation an improvement in symptoms is seen, then transition to a plant-based source. It is more expensive, but if you eat fewer nuts, oils, and grains, you may need to take less of it, as you will produce more.
A final note. More important than the amount of EPA/DHA in your diet may be the ratio of these anti-inflammatory fatty acids to Omega-6 fatty acids, which have pro-inflammatory effects. The optimal ratio seems to be approximately 1, meaning equal amounts of Omega-3 and Omega-6. Western diets have intakes of Omega-6 that are 15-16-folder higher than Omega-3. This is due to the preponderance of nuts, grains, meat, and oils, all high in Omega-6. By shunning these foods in favor of fruits, vegetables, beans, and some seeds, you can bring your Omega-6 levels down and decrease your requirement of EPA/DHA as you will have higher levels of the desaturase enzyme responsible for making DHA and EPA from ALA. A good thing. Just make sure to get enough folic acid, an important B-vitamin which enhances blood levels of DHA. Folate is found in beans and leafy greens.
Don't miss including this important nutrient in your diet in sufficient quantities. Your life (quality, longevity) may depend on it.