Cast your mind back several hundred thousand years. Picture people, just like you or me, but not quite. They look like you or me, but in modern parlance we’d say they’re “not the full shilling.”
But they’re a curious, resourceful bunch, and they begin to find ways to catch, cook and eat a particular food source: seafood.
This marks a turning point for these early humans. The cerebral cortex, the area of the brain that makes us human through development of executive function and emotional regulation, rapidly expands with introduction of marine foods to the diet (1).
And, to quote Carl Sagan, “civilization is a product of the cerebral cortex.”
Fast forward to the mid 20th Century, and consumption of these nutrients start to plummet. Yet these polyunsaturated fatty acids eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (AA) remain vital to human brain health.
Dietary intake, and the balance of these fatty acids, influences neurodegenerative disease and cognitive function throughout the life cycle (2; 3).
There is an important caveat, particularly to those of you following plant-based diets.
You’ll often hear about “omega-3’s”, but there are differences within this group of fatty acids. Without getting into too much much chemistry, omega-3’s from plant sources like flax, hemp or chia seeds are comprised of alpha linolenic acid (ALA), which is 18 carbons in length.
EPA and DHA are 20 and 22 carbons in length, respectively.
The body has process of converting ALA into EPA and DHA, but the conversion is minimal. While there is some increase in EPA from increased ALA intake, ALA will not increase DHA levels (1).
This is because we evolved eating a direct source of DHA, in the form of seafood.
The relevance of this is that our requirement for direct sources of DHA is absolute (more on this in the advice at the end).
Concentrations of DHA and AA are highest in brain tissue, and particularly vital during the growth and development of the infant brain (2).
Due to the DHA content of breastmilk, breastfed children show greater cognitive function than children fed infant formula lacking DHA (2).
DHA content in breastmilk reflects DHA in the maternal diet (4), and the more maternal intake of DHA increases, the greater the DHA content of breastmilk (5).
In population studies (which don’t show causation), fish intake of just once to twice per week has been consistently correlated with lower risk of neurodegenerative diseases, including Alzheimer’s Disease (AD) and dementia (3; 6; 7).
DHA is the predominant fatty acid in nervous system cell membrane phospholipids, while EPA may be considered primarily a precursor molecule and is particularly low in brain tissue (8).
They have distinct roles in brain health.
With neurodegenerative disease like Alzheimer’s Disease (AD), the reduced risk noted with fish intake relates to DHA only, not EPA (7).
Conversely, in relation to depression, prevention of depression is seen with EPA only, not DHA, notwithstanding that EPA is largely absent from brain tissue (9).
Inflammation in brain tissue is linked to all neurodegenerative disease, and both EPA is a vital precursor to anti-inflammatory mediators, known as resolvins for the ability to resolve inflammation, in the brain (10).
In this respect, the with the pro-inflammatory AA is important. The role of AA, like EPA, is primarily in cell signalling pathways (2), and both fatty acids compete directly for incorporation into membranes (11).
Supplementation with EPA has been shown to reduce the AA/EPA ratio (11).
As EPA is the precursor to anti-inflammatory resolvins, out competing pro-infammatory AA may reduce neuroinflammation (Serhan 2004).
High brain frequency waves in the brain relate to emotions like anger and aggression, while low frequency waves like alpha waves, promote cognition, calm and relaxed awareness (11). Increasing EPA, and modulating the AA/EPA ratio, increases low brain frequencies and reduces negative mood associated with higher brain frequencies (11).
With particular regard to depression, it appears that EPA is the fatty acid primarily associated with improvement of symptoms (Sublette 2011).
A potential mechanism of action of EPA in relation to depression is through preventing atrophy of the amygdala, through higher cerebral plasma EPA levels (9). This may be plausible as EPA is low within brain tissue itself, but currently the significance of EPA would appear to be in its role as precursor molecule to anti-inflammatory mediates (8; 10).
DHA is particularly high in brain phospholipids (8).
In AD, amyloid precursor protein (APP) is broken down through one of two pathways; the non-amyloidogenic pathway or the amyloidogenic pathway. β-amyloid, the protein that builds up in plaque formation in AD, is a result of APP breakdown through the myloidogenic pathway.
Phospholipid membranes enriched with DHA may divert APP through the non-amyloidogenic pathway, reducing β-amyloid levels (12).
DHA may also directly suppress the amyloidogenic pathway (12).
Amyloidogenic activity is increased by inflammation under the influence of AA, which is attenuated by DHA; thus reducing neuroinflammation may also suppress the amyloidogenic pathway (13).
The balance of DHA and AA is important. Metabolism of AA generates oxidative byproducts that are toxic to the brain, but the products of DHA metabolism are neuroprotective (14).
DHA in this way acts as an antioxidant for the brain, but may decrease brain DHA content(Hashimoto 2002), and expose the brain to oxidative damage from neurotoxic AA metabolites (14).
The balance of DHA and AA concentrations in phospholipids may thus be central to the development of Alzheimer’s and dementia.
There would appear to be individual and distinct roles for EPA and DHA in brain health.
The benefits of EPA are primarily through modulation of inflammation and depression (10; 11).
DHA deficiency appears to underlie neurodegenerative disorders (12; 14).
Advice for consumption depends on whether someone eats fish, eats animal produce but hates fish, eats a plant-based diet, or perhaps is suffering from depression.
There are specific recommendations for pregnancy, but that’s for the next post looking specifically at the importance of the essential fatty acids for infant brain development.
There is also the question of balance of nutrients, as the Western diet is characterised by excessive omega-6 intake, of which AA is one, and we have seen how balance of EPA and DHA with AA is important to brain health.
The next article will deal specifically with pregnancy and DHA requirements, through term and breastfeeding.
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