The Confusion Around Omega 3's
In the past year-or-so, I've seen a number of confusing stories circulating news outlets on the subject of Omega 3 supplementation.
Generally, these articles articles tend to focus on the things Omega 3 supplementation does not do.
The mistake that I see the media outlets make over and over is to then - in the interest of producing the most dramatic headline - declare that Omega 3 supplementation is a waste of time and money.
This is a hugely flawed assumption and deeply irresponsible journalism.
To understand why, let's first look at the function on Omega 3's in the body.
Essential Fatty Acids (EFA's): Explained
First, it's useful to understand a bit about these critical fats. EFA's are poly-unsaturated fatty acids that the body requires for optimal health, but cannot synthesize on it's own.
As such, we have to ensure that we are getting these fatty acids in our diet. EFA's are processed by the body for the production of signaling molecules that are used for a huge number of metabolic processes in the body and brain. Without these molecules, these communication pathways in the body sputter to a halt and the body suffers as a result.
As you'll probably recognize, EFA's are classified into two primary categories: Omega 3's and Omega 6's. Both are very much required for complete optimal functioning of the body and brain.
Interestingly though, this doesn't mean that you should just eat Omega 3's and Omega 6's indiscriminately. Omega 3's and Omega 6's compete against each other in the body for the enzymes needed to convert them into the various signaling molecules.
If either Omega 3 or Omega 6 fats are disproportionately present in the body, the signaling molecules derived from these essential fats will be out of balance, creating a number of downstream issues in the body.
Omega 3 vs. Omega 6: The History
Up until the late 70's the fats we ate came primarily from animal fat which made it easy to eat a relatively healthy, balanced Omega 3 to Omega 6 ratio.
The advent of vegetable oils (canola, soybean, corn, etc) has changed this radically. and the average western diet now contains Omega 6's to Omega 3's at a ratio of around 15 to 1 and up to 40 to 1. This is a long way from the ideal ratio most researchers agree on of between 4 to 1 and 1 to 1.This shift to Omega-6-rich oils has had serious consequences for our bodies and brains.
The signaling molecules produced from Omega 6's, while necessary, are also highly inflammatory when allowed to accumulate beyond optimal levels.
While the signaling molecules produced from Omega 3's are also technically pro-inflammatory, they are only minimally so, particularly when compared Omega 6.
What this means is that eating a diet high in Omega 6 oils will lead to systemic inflammation in the body and brain. Unfortunately, because of the proliferation of vegetable oils over the past 40 years, this is a condition the vast majority of people in the western world suffer from.
Inflammation: The Enemy
I've spent a lot of words over the past few years explaining why reducing inflammation in the body needs to be a top priority for anyone seeking dramatic improvements in the way their body and brain looks, feels and performs.
Inflammation in the body quickly leads to inflammation in the brain, which in the short term causes "foggy brain" and in the longer term can lead to more serious conditions like depression, Alzheimer's and other neurodegenerative diseases. 
For more detail on what inflammation is and why it's so damaging for the body and brain, I highly recommend checking out my three part series on inflammation and the best ways to reduce it.
Reduce Omega 6s = Reduce Inflammation
As I mentioned above, Omega 3's and Omega 6's "compete" in the body for the enzymes that convert them into signaling molecules.
Eating a diet that increases your ratio of Omega 3 to Omega 6 will, over time, decrease levels of pro-inflammatory signaling molecules and lead to greatly reduced systemic inflammation. 
Unfortunately, this is not quite as simple as just adding Omega 3 oils to your diet. Being aware of sources of Omega 6's and actively reducing them in your diet is equally as important as adding Omega 3's.
Below is a table of the oils that have the highest Omega 6 content:
Any food containing any of these oils should be on your "use sparingly" or "avoid completely" lists.
Why Supplementing With Omega 3's Is Necessary
It is a reality of living in the western world that high-Omega-6 plant oils are everywhere. Even the most diligent eater will likely have Omega 6 levels that are too high.
You can avoid them entirely at home, but if you go out to eat it's likely that your food will be prepared with vegetable oils that are high in Omega 6's. Reducing Omega 6 consumption is critical and necessary, but it is not enough.
Supplementing with additional Omega 3's is the only way to have your Omega 6 to Omega 3 ratio approach the 4:1 to 1:1 ratio we're looking for.
The Problem With Plant-Based Omega 3 Supplements
There are many sources of Omega 3's, but they are not created equal. Choose the wrong source and your body could not be absorbing even close to the amount you need - or even worse, you could be eating a toxic supplement.
There are a number of plant sources of Omega 3's, but all have their own problems. Hemp oil, walnut oil and several other nut oils are decent sources of Omega 3's, but still have all have far more Omega 6's than 3's. As such, they won't do much to help bring your EFA's into balance.
Flax Oil and Chia are the only two common plant oils that are legitimate sources of Omega 3's. Flax has an Omega 3 to Omega 6 ratio of 40:1, so at first glance it looks like an excellent candidate for supplementation.
Unfortunately, first glances are deceiving. The specific type of Omega 3's found in flax oil and chia (ALA) is not absorbed or used very efficiently by the body. ALA must be converted by the metabolism into DHA or EPA to be useful to the body, and this is an inefficient process in which a lot of ALA goes to waste.
Because of this inefficiency, you would have to eat 40 GRAMS of flax oil to deliver the same amount of useable Omega 3's that 1 GRAM of fish oil would deliver.
Flax oil is also extremely vulnerable to oxidation, a process in which light or heat ruin the oil and make it toxic. It also has organic molecules called ligands that mimic estrogen in the body (yes, this is bad).
So while you could technically eat A TON of flax oil to balance your Omega 3's and 6's, it's certainly not the best route, especially when there are better options available.
If you absolutely need your Omega 3 supplement to be plant based, marine phytoplankton is a far superior (if absurdly expensive) source.
Fish Oil and Krill Oil
At this point, I've settled on high-quality, tested-clean fish oils is the best source for Omega 3 supplementation.
Contamination with heavy metals and other toxins can be an issue with low-grade fish oils, so making sure your fish oil is tested and verified to be free of contaminants is important.
I'm currently using a product from Omega Via that meets all these requirements. It's the highest quality fish oil I've come across and manages to deliver that quality without being unreasonably expensive as some other fish oils are. See Omega Via Pharma-Grade Omega 3 here.
Another option is krill oil, something I've used in the past and would return to again without reservation.
Krill oil is generally free of contaminants as it comes from tiny low-food-chain organisms. Unlike plant sources or fish oil, the Omega 3's in krill oil are in phosphorylated form, allowing for superior absorbability.
The best krill oil I've found for the price is Jarrow Krill Oil.