Supplements With Evidence In Mild Cognitive Impairment
Quick Read Mild cognitive impairment is a grey zone between normal age-related memory changes and dementia, and it may be
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Cod liver oil contains omega-3 fatty acids (EPA and DHA) plus vitamins A and D. These omega-3s are incorporated into your cell membranes and help regulate inflammation and support brain and heart function. Research dating back to the 1970s, when scientists studied the Greenlandic Inuit, consistently shows that people with higher omega-3 levels have better health outcomes across multiple organ systems.
The research appears conflicting because study results depend heavily on dose and baseline omega-3 status. Most Western adults have low omega-3 levels, so they benefit from supplementation. However, a typical 1,000 mg fish oil capsule provides only 180-300 mg of actual EPA and DHA, which is below the 840 mg daily dose where benefits have been demonstrated. For meaningful effects on heart and brain health, you need at least 1.5 grams of combined EPA and DHA per day.
Cod liver oil specifically is worth choosing over standard fish oil if you live in northern climates, as it also provides vitamin D, which is commonly deficient in the UK during winter months. Quality matters: buy from reputable manufacturers, check labels carefully for actual EPA and DHA content, and store products in the fridge to prevent rancidity.
Verdict: Cod liver oil has strong evidence supporting its use at adequate doses of 840 mg or more of combined EPA and DHA daily, though most commercial supplements fall short of this threshold.
Long before omega-3 became a buzzword on supplement shelves, Norwegian fishermen were doing something that looked, to outsiders, almost medieval. They were spooning thick, pungent oil skimmed from the livers of Atlantic cod, and giving it to their children. Not because they had read a study. Because generation after generation, they had noticed that children who took it were healthier, stronger, less prone to the bone-softening disease that ravaged communities further inland. They didn’t know about vitamins A and D. They had never heard of eicosapentaenoic acid. They just knew it worked.
By the mid-1800s, cod liver oil had made its way into European medicine as a treatment for rickets and tuberculosis. By the 1970s, scientists studying the Greenlandic Inuit, who ate extraordinary quantities of marine fat and had astonishingly low rates of heart disease, began to suspect those fishermen might have been onto something far bigger [10]. What followed was one of the most productive, complex and occasionally confusing research journeys in nutritional science.
Here’s what decades of that research has actually taught us, and what it means for someone trying to make a sensible decision today.
To understand the research, you first need to understand what you’re actually swallowing. Cod liver oil is not a single nutrient. It’s a combination of several biologically active components, most notably the long-chain omega-3 fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), along with fat-soluble vitamins A and D.
EPA and DHA are not the kind of fats your body can easily manufacture on its own. You depend almost entirely on what you eat, or supplement, to maintain adequate levels [11]. Once in the body, they are incorporated into cell membranes throughout every organ system, influencing how cells communicate, how inflammation is managed, and how the brain and heart function.
Here’s a useful way to think about it: most of the fat in your brain is DHA. The retina of your eye is built substantially from DHA. EPA, meanwhile, acts more like a regulator, dampening inflammatory signals, supporting blood vessel health and moderating the kind of chronic, low-grade inflammation that underlies many diseases of ageing.
The vitamins A and D in cod liver oil add another dimension. Vitamin D plays well-established roles in immune function, bone health and mood, and deficiency is extremely common in Northern European populations, particularly in winter. Vitamin A supports immune defence, vision and cell growth.
Understanding that cod liver oil is doing several things at once, through several different mechanisms, helps explain why the research story is complicated, and why that complication is actually interesting rather than deflating.
The scientific era of omega-3 research began in earnest in the 1970s, when researchers noticed that the Greenlandic Inuit, despite eating a diet extraordinarily high in fat, had remarkably low rates of cardiovascular disease. The leading hypothesis was their enormous intake of marine-derived polyunsaturated fatty acids, EPA and DHA, consumed through fish, seal and whale [10].
This observation sparked decades of epidemiological research. The conclusion from that population data has remained remarkably consistent: high intake of fish and high measured levels of omega-3 fatty acids in tissues are inversely associated with cardiovascular events and total mortality [11]. In plain English, people who get more omega-3, whether through fish or supplements, tend to have better cardiovascular outcomes.
This is the bedrock finding, and no one has seriously overturned it.
Evidence grade: Conflicted, but here’s why, and what the weight of evidence actually says
If you’ve followed the omega-3 story over the past decade, you may have noticed something puzzling. One major study reports a dramatic benefit. Another reports none. Headlines swing between “fish oil protects your heart” and “fish oil supplements useless.” How can the same substance produce such different results?
The answer, it turns out, has more to do with how the trials were designed than with whether omega-3 actually works.
A landmark 2019 review analysed three large randomised controlled trials published in short succession [9]. The most striking finding came from a trial in 8,179 patients, mostly with existing coronary heart disease, who were given 4 grams per day of a highly purified EPA-only product. This group saw a 25% reduction in major adverse cardiovascular events (p<0.001). Two other large trials, using approximately 840 mg/day of combined EPA and DHA, showed significantly reduced risk of coronary heart disease and cardiovascular death, particularly in people who consumed little or no fish [9].
So far, so impressive. But other trials, often using lower doses, or a combination of EPA and DHA rather than EPA alone, produced neutral results. A 2021 analysis argued that combination EPA+DHA capsules should not be used to reduce cardiovascular risk, while noting that EPA as standalone therapy may be more promising [7].
Why the difference? A 2023 review in the *Internal Medicine Journal* offers the clearest explanation [11]. The critical insight is this: omega-3 supplementation is fundamentally different from drug therapy. A statin reduces LDL cholesterol in everyone, regardless of their starting level. Omega-3 supplementation works by correcting a deficiency, so if your baseline omega-3 levels are already adequate, you will see little or no benefit. If they’re depleted, the benefit can be substantial.
This is where the concept of the “omega-3 index” becomes important. The omega-3 index measures EPA+DHA as a percentage of all fatty acids in red blood cells, a far more accurate measure of true status than asking someone “do you take fish oil?” [12]. An optimal index is between 8% and 11%. In most Western countries, the average is well below this threshold [12]. Studies that didn’t measure or correct for baseline omega-3 status essentially enrolled a mixed group, some already replete, some severely deficient, and averaged out the results to get… not much.
A 2019 paper in the German medical journal *Der Internist* put it bluntly: many large intervention trials were “not positive” due to methodological issues, specifically failing to measure baseline omega-3 status or adequately dose to target [12]. Trials that got the methodology right showed significant reductions in sudden cardiac death, fatal and non-fatal myocardial infarction, and other cardiovascular outcomes.
The UK Biobank, one of the largest population health databases in the world, with hundreds of thousands of participants, tells a broadly consistent story. A 2025 review of all published studies linking fish oil supplement use to disease outcomes in this dataset found 44 significant, favourable associations for fish oil use, and just one unfavourable association (a small increased risk of osteoarthritis) [4]. Meta-analyses using blood biomarkers of omega-3 status reported 11 statistically significant favourable relationships with major disease outcomes, and zero unfavourable ones [4].
The headline-grabbing story about fish oil increasing atrial fibrillation risk, also drawn from UK Biobank data, has since been questioned on methodological grounds [4].
Evidence grade: Strong for higher doses; promising-to-conflicted for supplement-range doses
One of the clearest messages from the accumulated research is that dose matters enormously, and most off-the-shelf fish oil capsules don’t deliver enough to produce measurable clinical effects.
UK dietary guidelines recommend more than 450 mg of combined EPA+DHA per day, a figure last revised in 2004 and based mainly on epidemiological data [1]. But most randomised controlled trials use doses of 840 mg/day or more, and therapeutic effects on inflammation, triglyceride levels and blood pressure appear to require more than 1.5 grams of EPA+DHA daily [1].
Cognitive benefits are also likely to require these higher intakes [1].
Now look at what’s on the shelves. A 2023 cross-sectional study of 2,819 unique fish oil supplements in the US found that the median daily dose of combined EPA+DHA was just 600 mg [15]. Only 24 out of 255 supplements from leading brands, less than 10%, provided 2 grams or more per day [15]. Meanwhile, 74% of products made at least one health claim, and 62% specifically claimed cardiovascular benefit, often at doses the evidence doesn’t support [15].
A similar New Zealand analysis of 10 popular fish oil products found that only three would be likely to confer benefits across heart, brain and joint health even at the maximum recommended daily dose [13]. The good news: 90% of products accurately stated their EPA and DHA content on the label, and mercury was not detected in any sample [13].
The practical implication is clear: if you’re taking one standard 1,000 mg fish oil capsule per day, which typically provides only 180-300 mg of combined EPA+DHA, you are very likely below the threshold where meaningful effects have been demonstrated.
Evidence grade: Promising, stronger for deficiency states and specific populations
The brain is approximately 60% fat by dry weight, and DHA is one of its most abundant structural components. It is involved in neuronal membrane fluidity, synaptic signalling, and the resolution of neuroinflammation, the kind of chronic, low-level brain inflammation increasingly implicated in cognitive decline.
Suboptimal omega-3 index levels correlate with cognitive impairment and major depression in population data [12]. And given that typical Western intakes of EPA+DHA are below 200 mg per day [1], well under what the research suggests is optimal, a substantial proportion of the population is running their brain on insufficient supplies of its own preferred structural material.
For cognitive benefits specifically, current evidence suggests intakes above 1.5 grams per day are likely required [1]. This is a higher bar than most people are meeting, whether through diet or supplementation.
There is also a compelling visual health angle. A 2025 systematic review of 87 randomised controlled trials examining omega-3 supplementation and eye health found strong evidence that DHA supports visual development in infants and benefits children, particularly those with ADHD, in terms of visual processing [2]. In adults with dry eye symptoms, the evidence was mostly positive. For age-related macular degeneration in older adults, the picture was more mixed, with the review noting that optimal dosing and duration still need to be established [2].
Here is where cod liver oil differentiates itself from standard fish oil. A standard fish oil capsule provides EPA and DHA, the omega-3 fatty acids, but not vitamins A and D. Cod liver oil provides all three.
In the UK, vitamin D deficiency is widespread, particularly among older adults and during the winter months when sunlight is insufficient to trigger skin synthesis. The National Diet and Nutrition Survey consistently shows that large proportions of the UK population fall below optimal vitamin D levels, and the consequences for bone health, immune function and mood are well established.
Vitamin A deficiency is less common in the UK but genuine in those with restricted diets or poor fat absorption. It plays a key role in immune defence, vision and mucous membrane integrity.
The dual vitamin payload in cod liver oil is one reason it remained a staple of Northern European health practice long before the omega-3 story was fully understood. Those Norwegian fishermen were, inadvertently, addressing multiple nutrient gaps in a single spoonful.
Evidence grade: Observational data, practically important
Not all cod liver oil or fish oil products are equal, and the research raises some useful flags.
A 2019 authentication study noted that cod liver oil’s high market price makes it a target for adulteration with cheaper vegetable oils, a problem that sophisticated analytical methods are now being developed to detect [14]. A 2025 quality analysis of commercial fish oils in China found that only around 65% of encapsulated products met oxidation standards, and many showed significant variation in actual EPA and DHA content relative to label claims [3].
The oxidation issue matters. Rancid fish oil, oil that has degraded through exposure to heat, light or oxygen, not only smells and tastes unpleasant but may actually deliver less of the EPA and DHA on the label, and potentially produce compounds that counteract some of the intended benefits. Good manufacturing practice, cold storage and purchase from reputable manufacturers matters more than brand recognition alone.
Science is a journey, not a destination, and the honest picture of cod liver oil and omega-3 research still has significant gaps.
The optimal dose is genuinely unclear. We know the UK recommendation of 450 mg/day is probably too low, that RCTs use 840 mg/day or more, and that effects on inflammation and triglycerides seem to require above 1.5 g/day [1]. But we don’t yet have a definitive dose-response map for cognitive outcomes, and we don’t know whether different people need different doses based on genetics, diet or existing health status.
EPA vs DHA, does it matter? Several cardiovascular trials suggest that EPA-only supplementation may be more beneficial than combined EPA+DHA, particularly at high doses [7, 9]. This is biologically plausible, EPA and DHA have different metabolic roles, but it remains an active area of debate, and it’s not yet clear whether this applies to cognitive or visual health outcomes.
The phospholipid question is unresolved. Most fish oil supplements deliver EPA and DHA bound to triglycerides or ethyl esters. But marine phospholipids, the form found in krill oil and some cod liver oils, may be absorbed and utilised differently by the body. A 2020 review concluded that marine phospholipid-bound omega-3s show promising differential bioavailability and biological effects, but called for much more intensive research before firm conclusions can be drawn [10].
Cognition in older adults remains an area of uncertainty. While population data links low omega-3 index to cognitive impairment, the randomised trial evidence for supplementation improving cognition in healthy older adults is not yet strong enough to make definitive claims [1, 2].
Testing is not routine. Ideally, you’d know your omega-3 index before deciding on dose. In practice, most people can’t or won’t do this regularly. A point worth understanding: the research suggests the vast majority of Western adults are running at suboptimal levels anyway [12].
So what would a sensible, well-informed person actually do with all this?
Here is the honest summary: cod liver oil and fish oil have some of the richest evidence bases in nutritional supplement science. The research isn’t perfect, and where it’s conflicted, the conflict usually comes down to dose, baseline status and study design, not to omega-3 itself being ineffective. The weight of evidence, from the Greenlandic Inuit studies of the 1970s through to the UK Biobank analyses of 2025, consistently points in the same direction: higher omega-3 status is associated with better health outcomes across multiple organ systems.
On dose: The most important practical insight from this research is that you almost certainly need more than a standard 1,000 mg fish oil capsule provides. Look for products that deliver at least 840 mg of combined EPA+DHA per day, and ideally 1.5 grams or more if your primary concern is cardiovascular or cognitive health [1, 9, 11]. Read the label carefully: “1,000 mg fish oil” and “1,000 mg EPA+DHA” are very different things.
On cod liver oil specifically: If you’re in the UK, north of latitude 50, and eating a reasonably typical diet, you are probably low in vitamin D for at least five months of the year. Cod liver oil addresses omega-3 and vitamin D simultaneously, making it a genuinely practical, multi-benefit supplement. The fat-soluble vitamins here are worth noting: vitamin D at typical cod liver oil doses is safe and sensible; vitamin A is fat-soluble and does accumulate, so don’t take additional high-dose vitamin A supplements on top. Stick to standard cod liver oil doses.
On testing: An omega-3 index test would tell you exactly where you stand, but it’s not widely available, not cheap, and not practical to repeat regularly. Given that the vast majority of Western adults have suboptimal omega-3 status [12], supplementing daily at a meaningful dose is the rational default, not something to wait for a test to confirm.
On quality: Buy from reputable manufacturers. Look for products that state their EPA+DHA content clearly (not just total fish oil content). Store in the fridge. If it smells rancid, replace it [3, 14].
On safety: At normal supplementation doses, fish oil and cod liver oil have excellent safety profiles. The atrial fibrillation scare from early UK Biobank analyses has since been questioned [4]. Mercury was not detected in any of the tested products reviewed [13]. The risk calculus here is straightforward: the risk of running chronically low on EPA, DHA and vitamin D is well-documented and real. The risk of supplementing at normal doses is very small.
Those Norwegian fishermen didn’t have 1.77 million research papers, which is exactly how many the Vitacuity team analysed to bring you this breakdown. But they had centuries of observation and a healthy instinct for what their bodies needed. Three hundred years later, the science is finally catching up with the spoon.
[1] Sustainable and available sources of omega-3 fatty acids for health: are the current dietary recommendations, food sources and legislation fit for purpose? (2025). *Proceedings of the Nutrition Society*. DOI: 10.1017/S0954422425100127 | https://pubmed.ncbi.nlm.nih.gov/40458055/
[2] Impact of n-3 polyunsaturated fatty acids supplementation on visual health throughout the life cycle: A systematic review (2025). *Prostaglandins, Leukotrienes and Essential Fatty Acids*. DOI: 10.1016/j.plefa.2025.102686 | https://pubmed.ncbi.nlm.nih.gov/40499235/
[3] Systematically Investigating the Qualities of Commercial Encapsulated and Industrial-Grade Bulk Fish Oils in the Chinese Market (2025). https://pubmed.ncbi.nlm.nih.gov/40361705/
[4] A review of the relationships between reported fish oil supplement use and health outcomes in the UK Biobank (2025). *Prostaglandins, Leukotrienes and Essential Fatty Acids*. DOI: 10.1016/j.plefa.2025.102711 | https://pubmed.ncbi.nlm.nih.gov/41072116/
[7] Fish Oil Supplements for Prevention of Cardiovascular Disease: The Jury Is Still Out: CON: Fish Oil is Useful to Prevent or Treat Cardiovascular Disease (2021). https://pubmed.ncbi.nlm.nih.gov/34149081/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8211000/
[9] Sea Change for Marine Omega-3s: Randomized Trials Show Fish Oil Reduces Cardiovascular Events (2019). *Mayo Clinic Proceedings*. DOI: 10.1016/j.mayocp.2019.04.027 | https://pubmed.ncbi.nlm.nih.gov/31627938/
[10] Inflammation and cardiovascular disease: are marine phospholipids the answer? (2020). *Food & Function*. DOI: 10.1039/c9fo01742a | https://pubmed.ncbi.nlm.nih.gov/32270798/
[11] Omega-3 fatty acids and cardiovascular prevention: is the jury still out? (2023). *Internal Medicine Journal*. DOI: 10.1111/imj.16283 | https://pubmed.ncbi.nlm.nih.gov/38105550/
[12] Confusion about the effects of omega-3 fatty acids: Contemplation of study data taking the omega-3 index into consideration (2019). *Der Internist*. DOI: 10.1007/s00108-019-00687-x | https://pubmed.ncbi.nlm.nih.gov/31591673/
[13] Are over-the-counter fish oil supplements safe, effective and accurate with labelling? Analysis of 10 New Zealand fish oil supplements (2020). https://pubmed.ncbi.nlm.nih.gov/32994616/
[14] Chemometric tools for the authentication of cod liver oil based on nuclear magnetic resonance and infrared spectroscopy data (2019). DOI: 10.1007/s00216-019-02063-y | https://pubmed.ncbi.nlm.nih.gov/31401671/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6834736/
[15] Health Claims and Doses of Fish Oil Supplements in the US (2023). *JAMA Cardiology*. DOI: 10.1001/jamacardio.2023.2424 | https://pubmed.ncbi.nlm.nih.gov/37610733/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10448371/
This article is for informational purposes only and does not constitute medical advice. Food supplements should not be used as a substitute for a varied and balanced diet and healthy lifestyle. If you are pregnant, breastfeeding, taking medication or have a medical condition, consult your doctor before taking any supplement. These statements have not been evaluated by the Food and Drug Administration (FDA) or the Medicines and Healthcare products Regulatory Agency (MHRA). This product is not intended to diagnose, treat, cure, or prevent any disease.
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