Diabetes mellitus is a chronic metabolic disorder that affects more than 500 million people worldwide, and its prevalence continues to rise. While strict glycemic control remains the cornerstone of diabetes management, even well‑controlled patients face an elevated risk of long‑term complications including neuropathy, nephropathy, retinopathy, and cardiovascular disease. In recent years, interest has shifted toward nutritional supplements that might offer adjunctive protection against these outcomes. Cod liver oil, a traditional supplement rich in omega‑3 fatty acids, vitamin A, and vitamin D, has gained particular attention. But does the current body of evidence support its use specifically for preventing diabetic complications? This article provides a comprehensive review of the existing research, the underlying mechanisms, and the practical considerations for incorporating cod liver oil into a diabetes management plan.

What Is Cod Liver Oil?

Cod liver oil is a dietary supplement extracted from the liver of Atlantic cod (Gadus morhua). Unlike fish oil derived from the body of oily fish, cod liver oil naturally contains high concentrations of fat‑soluble vitamins—particularly vitamins A and D—in addition to the long‑chain omega‑3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Historically, cod liver oil was used to prevent and treat rickets because of its vitamin D content. Today it is promoted for a range of health benefits, including anti‑inflammatory effects, cardiovascular support, and immune modulation. For individuals with diabetes, these properties are theoretically appealing: chronic low‑grade inflammation and oxidative stress are central drivers of diabetic complications, and both omega‑3s and vitamin D have been shown to influence inflammatory pathways.

One tablespoon (approximately 14 grams) of cod liver oil provides roughly 4,000–5,000 IU of vitamin A, 400–600 IU of vitamin D, and about 2.5 grams of omega‑3 fatty acids (a mix of EPA and DHA). These nutrient densities make cod liver oil a potent supplement, but also one that requires careful dosing to avoid excess intake of fat‑soluble vitamins.

Potential Mechanisms for Preventing Diabetic Complications

To understand why cod liver oil has been studied in the context of diabetes, it is useful to examine the biological pathways that link its components to diabetic pathology.

Anti‑Inflammatory Effects of Omega‑3 Fatty Acids

Diabetic complications are driven, in part, by a state of chronic inflammation. Hyperglycemia triggers the production of advanced glycation end‑products (AGEs) and activates the nuclear factor‑kappa B (NF‑κB) pathway, leading to increased expression of pro‑inflammatory cytokines such as tumor necrosis factor‑alpha (TNF‑α) and interleukin‑6 (IL‑6). EPA and DHA can inhibit these pathways. They serve as precursors for specialized pro‑resolving mediators (SPMs) like resolvins and protectins, which actively resolve inflammation rather than simply suppressing it. By reducing systemic inflammation, omega‑3s may help slow the progression of atherosclerosis, nephropathy, and other vascular complications.

Cardiovascular Protection

Cardiovascular disease remains the leading cause of death in people with diabetes. Omega‑3 fatty acids are well‑known for their ability to lower triglycerides, modestly increase HDL cholesterol, and improve endothelial function. A meta‑analysis of randomized controlled trials found that EPA and DHA supplementation reduced the risk of major cardiovascular events, particularly in populations with elevated triglycerides or existing cardiovascular disease. Since diabetes confers a two‑ to fourfold increased risk of heart disease, even modest improvements in lipid profiles and vascular health could be clinically meaningful.

Role of Vitamin D in Glucose Metabolism and Immune Function

Vitamin D receptors are present on pancreatic beta cells, and vitamin D deficiency has been linked to impaired insulin secretion and insulin resistance. Observational studies consistently show that individuals with low vitamin D levels have a higher risk of developing type 2 diabetes, and some clinical trials suggest that vitamin D supplementation may improve glycemic control in those with prediabetes. Furthermore, vitamin D modulates immune responses and may reduce the risk of infections—a common concern in diabetic patients, who are more susceptible to foot ulcers and urinary tract infections. Cod liver oil provides vitamin D in a bioavailable form, which could offer dual benefits for both glucose metabolism and infection prevention.

Antioxidant Actions of Vitamin A

Vitamin A (retinol) is essential for maintaining epithelial integrity and immune function. In the context of diabetes, oxidative stress depletes endogenous antioxidants, and vitamin A can help scavenge reactive oxygen species. Some evidence indicates that retinoids may also influence adipocyte function and insulin sensitivity. However, the high vitamin A content in cod liver oil means that supplementation should be monitored to avoid toxicity, which can paradoxically harm the liver and bone health.

Current Scientific Evidence: What the Studies Show

Despite the plausible mechanisms, direct evidence linking cod liver oil specifically to the prevention of diabetic complications is limited. Most research has examined omega‑3 supplements in general, or fish oil, rather than cod liver oil specifically. Moreover, many studies have focused on surrogate endpoints—such as lipid levels or inflammatory markers—rather than hard outcomes like amputation rates, end‑stage renal disease, or mortality.

Omega‑3 Supplementation and Diabetic Complications

A 2019 systematic review and meta‑analysis of 40 randomized trials involving over 3,600 participants with type 2 diabetes found that omega‑3 supplementation significantly reduced triglycerides by about 0.34 mmol/L and modestly lowered C‑reactive protein (CRP), a marker of inflammation. However, there was no significant effect on fasting glucose, HbA1c, or insulin resistance. The authors concluded that omega‑3s could improve cardiovascular risk factors but noted that the evidence for preventing microvascular complications was insufficient.

Another 2020 Cochrane review examined omega‑3 fatty acids for the prevention of cardiovascular disease in people with diabetes. It found moderate‑certainty evidence that omega‑3s reduce triglycerides and may slightly reduce the risk of coronary heart disease events (relative risk 0.88, 95% CI 0.78–0.99). Yet, there was little evidence of benefit for stroke, heart failure, or all‑cause mortality.

Cod Liver Oil‑Specific Studies

Few studies have used cod liver oil as the intervention. A small pilot trial from 2015 gave 10 g of cod liver oil daily to 20 patients with type 2 diabetes for 8 weeks. The results showed a significant decrease in triglycerides and an increase in HDL cholesterol, but no change in HbA1c or fasting glucose. A larger observational study from Norway, where cod liver oil consumption is common, reported that regular use was associated with a lower incidence of type 2 diabetes—but this was a correlational finding that could be confounded by healthier lifestyle habits.

Regarding diabetic neuropathy, some animal studies suggest that omega‑3s can reduce oxidative stress in nerve tissue and improve nerve conduction velocity. However, human trials are lacking. One small randomized trial in patients with diabetic peripheral neuropathy found that omega‑3 supplementation for 12 weeks improved pain scores and nerve function metrics, but again, the supplement used was fish oil, not cod liver oil.

Key Studies and Their Limitations

It is important to critically evaluate the available evidence. The following studies represent the most relevant data for the question at hand:

  • ORIGIN Trial (2012): This large randomized trial tested omega‑3 supplementation (1 g daily) in over 12,000 people with impaired glucose tolerance or diabetes. After 6 years, there was no significant reduction in cardiovascular death or major vascular events. However, the dose was relatively low, and the participants were already well‑treated with statins and other therapies.
  • REDUCE‑IT Trial (2019): This landmark study tested a high‑dose EPA ethyl ester (4 g/day) in patients with elevated triglycerides and cardiovascular risk, including many with diabetes. It found a 25% relative risk reduction in major cardiovascular events. This suggests that high doses of omega‑3s, particularly EPA, may be beneficial—but the dose is far higher than what cod liver oil provides, and the formulation was different.
  • VITAL Trial (2019): A large randomized trial of vitamin D (2000 IU/day) and omega‑3s (1 g/day) for primary prevention of cancer and cardiovascular disease. The omega‑3 component did not reduce cardiovascular events overall, though a prespecified subgroup of people who did not take statins did appear to benefit. The vitamin D component had no effect on cardiovascular events or diabetes incidence. This trial used fish oil, not cod liver oil, and the vitamin D was given separately.

These studies highlight the complexity of translating supplement research into clinical practice. Dose, formulation, baseline nutritional status, and concomitant medications all influence outcomes. For cod liver oil, the combination of EPA, DHA, vitamin A, and vitamin D may have synergistic effects—or it may introduce risks that outweigh benefits.

Potential Benefits for Specific Complications

Diabetic Neuropathy

Neuropathy, affecting up to 50% of diabetic patients, results from nerve damage due to hyperglycemia, oxidative stress, and inflammation. Omega‑3 fatty acids may protect nerves by reducing pro‑inflammatory cytokines and improving blood flow to the vasa nervorum. A 2021 systematic review of omega‑3 supplementation for diabetic neuropathy included 5 randomized trials with a total of 336 patients. The pooled results showed significant improvements in pain scores and nerve conduction velocity in the treatment groups. However, trials used varying doses and durations, and none specifically tested cod liver oil. Vitamin D deficiency is also associated with neuropathic pain, so the vitamin D in cod liver oil could theoretically contribute additional benefit.

Diabetic Nephropathy

Kidney disease affects approximately 20–40% of people with diabetes. Inflammation and fibrosis are key drivers of renal decline. Some animal models have demonstrated that omega‑3s can reduce proteinuria and glomerular damage. Human trials are mixed: a 2018 meta‑analysis found that omega‑3 supplementation reduced urinary albumin excretion in diabetic patients, but the effect was small and not consistent across studies. A more recent 2022 trial using high‑dose EPA (4 g/day) showed a trend toward slowing estimated glomerular filtration rate (eGFR) decline in patients with chronic kidney disease, but again, the dose is not achievable with cod liver oil.

Diabetic Retinopathy

Retinopathy is a leading cause of blindness in working‑age adults. Omega‑3s are integral to retinal structure, and DHA is a major component of photoreceptor membranes. Laboratory studies suggest that DHA can inhibit the growth of abnormal blood vessels in the retina. A large cohort study in the US (the AREDS2 trial) found that omega‑3 supplementation (1 g/day) did not reduce the risk of progression to advanced age‑related macular degeneration, but no equivalent trial exists for diabetic retinopathy. Some observational data suggest that fish consumption is associated with a lower risk of retinopathy, but confounders such as overall diet quality limit interpretation.

Practical Considerations: Dosage, Safety, and Drug Interactions

For diabetic patients considering cod liver oil, several practical factors must be addressed.

Typical doses range from 1 to 2 teaspoons (5–10 mL) per day, providing 400–800 IU of vitamin D, 800–1,600 IU of vitamin A, and 1–2 grams of omega‑3s. This is generally considered safe for most adults. However, for individuals who are pregnant, have liver disease, or take medications that affect vitamin A metabolism (e.g., retinoids), a lower dose or alternative supplement may be necessary.

Risk of Vitamin A Toxicity

Chronic intake of preformed vitamin A (retinol) above 10,000 IU/day can lead to hypervitaminosis A, causing symptoms such as nausea, dizziness, blurred vision, and eventually liver damage and bone loss. Since cod liver oil is concentrated in retinol, it is possible to exceed safe levels with regular consumption of more than 1 tablespoon per day, especially if diet already provides significant vitamin A from animal sources. The tolerable upper intake level for adults is 3,000 mcg RAE (10,000 IU) per day. One tablespoon of cod liver oil typically contains about 4,000 IU of vitamin A, leaving little margin for additional supplementation.

Blood‑Thinning Effects

Omega‑3 fatty acids have mild antiplatelet effects. For patients taking anticoagulant or antiplatelet medications (e.g., warfarin, apixaban, aspirin), high doses of omega‑3s may increase bleeding risk. The risk is low at typical cod liver oil doses, but it is prudent to monitor INR if taking warfarin and to discuss supplementation with the healthcare provider.

Quality and Purity

As with any fish oil product, contamination with heavy metals, PCBs, and dioxins is a concern. Cod liver oil, because it is derived from the liver, can accumulate fat‑soluble toxins. Consumers should choose products that are molecularly distilled and certified for purity by a third‑party organization (e.g., USP, NSF International, or IFOS). The omega‑3 content also varies between brands; label claims should be verified by independent testing.

Comparison with Other Omega‑3 Sources

Cod liver oil is not the only option for omega‑3 supplementation. Fish oil capsules (from the body of oily fish like salmon, mackerel, or sardines) provide similar amounts of EPA and DHA but contain negligible levels of vitamins A and D. Krill oil contains omega‑3s in phospholipid form and includes astaxanthin, an antioxidant. Algal oil is a plant‑based source of DHA (and sometimes EPA) suitable for vegans.

For diabetic patients specifically, the added vitamin D in cod liver oil may be advantageous if vitamin D status is low—which is common in people with diabetes, as many are deficient due to decreased sun exposure, obesity, or kidney impairment that impairs vitamin D activation. However, patients with adequate vitamin D levels may not need the extra vitamin A, and the risk of toxicity must be considered.

If the primary goal is cardiovascular risk reduction with omega‑3s, a high‑dose EPA product (like icosapent ethyl) may be more effective, but such formulations require a prescription. For general support, cod liver oil can be a practical choice, provided the vitamin A intake is kept within safe limits.

Incorporating Cod Liver Oil into a Diabetes Management Plan

Supplements should never replace a healthy diet, physical activity, and prescribed medications. However, for diabetic patients who wish to try cod liver oil, the following steps can help maximize benefits and minimize risks:

  • Consult with a healthcare provider before starting, especially if you have kidney disease, liver disease, or take blood thinners.
  • Check your baseline vitamin D and vitamin A levels. Routine testing can help avoid toxicity and guide dosing. For vitamin D, many experts recommend targeting a serum 25‑hydroxyvitamin D level of 30–50 ng/mL.
  • Start with a low dose (e.g., 1 teaspoon per day) and monitor for any gastrointestinal discomfort or allergic reactions.
  • Choose a high‑quality, purified product from a reputable manufacturer. Look for “molecularly distilled” or “ultra‑refined” on the label.
  • Do not exceed the recommended dose. More is not better, and the risk of vitamin A toxicity increases with higher intakes.
  • Track your blood glucose and lipid levels to see if any changes occur. While many people may not see a dramatic drop in HbA1c, improvements in triglycerides or inflammatory markers may be a positive sign.
  • Reassess after 3–6 months. If no clear benefit is observed, it may be reasonable to discontinue or switch to another supplement.

Conclusion

Current evidence suggests that cod liver oil may offer some health benefits for individuals with diabetes, particularly through its anti‑inflammatory and cardiovascular effects mediated by omega‑3 fatty acids and vitamin D. The anti‑inflammatory actions of EPA and DHA, combined with the immune‑supporting roles of vitamins A and D, provide a plausible mechanism for slowing the progression of diabetic complications. However, direct evidence from randomized controlled trials specifically testing cod liver oil against hard endpoints—such as amputation, dialysis, or blindness—remains scant. Most of the supporting data come from studies of omega‑3s or fish oil in general, often at doses higher than those obtainable from cod liver oil.

For diabetic patients who are vitamin D deficient or whose diets lack omega‑3s, cod liver oil can be a convenient and effective supplement when used responsibly. Yet, the potential for vitamin A toxicity and drug interactions means that supplementation should not be initiated without professional guidance. A personalized approach that considers baseline nutritional status, comorbidities, and concurrent medications is essential.

Until larger, longer‑term trials confirm a direct cause‑and‑effect relationship, cod liver oil should be viewed as a complementary strategy—not a substitute—for standard diabetes care. A balanced diet rich in fruits, vegetables, whole grains, and lean proteins, alongside regular physical activity and optimal glycemic control, remains the foundation for preventing diabetic complications. For those who choose to add cod liver oil to their regimen, ongoing monitoring and periodic reevaluation of its efficacy and safety are warranted.

Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any new supplement, especially if you have a chronic health condition.