Gene Test May Shape Who Benefits From Omega-3 Fish Oil

A newly identified enzyme appears to determine whether omega‑3 fish oil can help suppress colorectal cancer or potentially backfire, according to research published in Cellular and Molecular Gastroenterology and Hepatology, deepening scientific questions about who actually benefits from widely used omega‑3 supplements.

Researchers reported that a gene called 15‑lipoxygenase‑1, or ALOX15, is critical for the body to convert the omega‑3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) into anti‑tumor compounds in the colon. Without sufficient ALOX15 activity, the study found, EPA and DHA did not provide the expected protective effects against colorectal cancer and in some models could even promote tumor progression.¹

The findings come as clinicians and public health agencies continue to wrestle with conflicting data on omega‑3 supplements, which have been variously linked to modest cardiovascular benefits, neutral or negative trial results, and emerging signals in brain and mental health.

New study: ALOX15 as a gatekeeper in colorectal cancer

In the new colorectal cancer study, investigators focused on how EPA and DHA—the long‑chain omega‑3 fatty acids abundant in fish oil—are metabolized in the gut.

They identified ALOX15 as a “crucial factor” in whether these fatty acids are converted into lipid mediators that can suppress tumor growth in the colon, according to the report. When ALOX15 was present and active, EPA and DHA were metabolized into molecules that reduced inflammation and inhibited colorectal cancer cell proliferation. When the gene was absent or underexpressed, those anti‑tumor pathways were blunted, and omega‑3 exposure in some experimental models coincided with more aggressive tumor behavior.¹

The authors suggested that testing colorectal cancer patients for ALOX15 expression could become important in future efforts to personalize treatment, dietary strategies or adjuvant supplement use. They also cautioned that the gene‑dependent response might help explain why some past trials of omega‑3s in cancer prevention have produced inconsistent or weak signals.

The study was preclinical, but it aligns with broader literature showing substantial heterogeneity in clinical responses to omega‑3 therapy, according to a recent review of study design flaws in omega‑3 trials published in the Medical Research Archives, which concluded that patient biology, baseline diet and dosage may all modulate effects.[^mra]

Evidence on cardiovascular benefits remains modest and contested

The new mechanistic data arrive against a backdrop of decades of conflicting research on omega‑3s and cardiovascular disease.

The American Heart Association has long recommended omega‑3 intake from fish, and in some cases supplements, to help reduce cardiovascular events in people with established heart disease. But the strength and consistency of the evidence remain debated.

A major Harvard T.H. Chan School of Public Health meta‑analysis of randomized clinical trials, released in 2019, pooled data from tens of thousands of participants and found that omega‑3 fish oil supplements were linked with a lower risk of heart attack and some other cardiovascular endpoints.² The analysis suggested that higher doses were associated with greater risk reduction for myocardial infarction and coronary heart disease.

Yet other large trials, particularly those testing standard‑dose fish oil in broad at‑risk populations, have failed to show clear benefit, and some have raised concerns about side effects such as atrial fibrillation. A 2021 commentary from Harvard physicians reviewing newer omega‑3 heart trials concluded that the evidence had become “more complicated,” with prescription‑strength EPA products showing benefit in some high‑risk patients, while lower‑dose mixed EPA/DHA supplements often did not.³

Meanwhile, a 2015 evidence review from the National Center for Complementary and Integrative Health (NCCIH) described the cardiovascular data on omega‑3 supplements as inconsistent, noting that early observational findings in populations with high seafood intake had not been fully replicated in modern randomized trials.⁴

The Mayo Clinic similarly characterizes fish oil supplements as containing EPA and DHA with potential anti‑inflammatory effects but notes that research on their role in heart disease prevention continues to yield mixed results, particularly for individuals without existing cardiovascular disease.⁵

Brain health and cognition: early signals, limited trials

Beyond the heart, omega‑3s have been investigated for their potential roles in brain development, cognitive aging and neuropsychiatric conditions.

A recent systematic review and dose–response meta‑analysis, published in Scientific Reports, found that omega‑3 supplementation was associated with a significant improvement in visuospatial function, with a standardized mean difference of 0.89 (95% CI 0.45–1.33). The authors rated the certainty of this specific cognitive domain finding as high under GRADE criteria, while emphasizing considerable heterogeneity across trials and doses.⁶

Another synthesis of recent omega‑3 research, highlighted in Nutritional Outlook, reported that:

Higher omega‑3 blood levels were linked to a reduced risk of early‑onset dementia, independent of genetic risk factors, in observational datasets.
Elevated omega‑3 status was associated with lower self‑harm risk, suggesting potential protective effects on certain mental health outcomes and calling for targeted randomized trials.⁷

However, clinical evidence remains uneven. For example, a 2024 meta‑analysis of omega‑3 supplementation for anxiety symptoms, incorporating 19 clinical trials with 2,240 participants, found that doses below 2 g per day did not produce significant improvements in anxiety across diverse populations.⁸

The U.S. Departments of Veterans Affairs and Defense, in an information paper on omega‑3 supplements for mild traumatic brain injury (mTBI) published in 2025, noted growing individual use of omega‑3 products among service members and veterans, but reported that current VA/DoD clinical practice guidelines reference nutrition only broadly as a modifier of recovery. The paper concluded that evidence is still insufficient to support specific omega‑3 supplement recommendations in post‑acute mTBI.⁹

National institutes and experts urge caution on supplement claims

The National Institutes of Health Office of Dietary Supplements (ODS), in its health professional fact sheet on omega‑3 fatty acids, underscores that EPA and DHA are important nutrients but emphasizes that evidence for disease‑specific benefits from supplements is mixed. The fact sheet summarizes randomized controlled trial data showing modest or no effects on many chronic disease outcomes, and points to ongoing large studies aimed at clarifying which populations, if any, derive clear clinical benefit.¹⁰

Consumer interest, however, has far outpaced the certainty of the science. Omega‑3 supplements now form a multibillion‑dollar global market, promoted for heart, brain, joint and eye health. That gap between marketing and data has led some cardiologists and researchers to question widespread routine use.

In a feature on omega‑3s from National Geographic, Steven Nissen, chief academic officer of the Heart, Vascular & Thoracic Institute at the Cleveland Clinic, argued that many large randomized trials have not confirmed the dramatic benefits implied by early observational research. “Supplementation with omega‑3 fatty acids have shown no benefits,” he said of several recent well‑controlled studies, while acknowledging that research in narrowly defined patient groups continues.¹¹

A methodological review in Medical Research Archives noted that study design flaws and population differences complicate interpretation of omega‑3 trials. The authors pointed to variations in background diet, baseline omega‑3 status, choice of endpoints, and the use of combination products (omega‑3s alongside other lipid‑modifying drugs) as factors that may obscure true effects or produce apparent futility where benefits might exist in subgroups.[^mra]

Mechanisms and the omega‑3 / omega‑6 balance

Mechanistic reviews continue to explore how omega‑3 and omega‑6 polyunsaturated fatty acids interact in inflammation, cardiovascular function and neurodegeneration.

A 2025 review in Nutrients examined omega‑3 and omega‑6 polyunsaturated fatty acids across cardiovascular, neurodegenerative and metabolic diseases, drawing on studies indexed in PubMed and PubChem between 2010 and 2025. The authors reported that:

Omega‑3 fatty acids tend to generate less inflammatory or pro‑resolving lipid mediators, while many omega‑6 metabolites are more pro‑inflammatory.
The ratio of omega‑6 to omega‑3 intake may influence pathways involved in atherosclerosis, insulin resistance and neuroinflammation.
Clinical data remain incomplete, and the review called for more trials that measure both fatty acid profiles and downstream biomarkers to link mechanistic changes with hard clinical endpoints.¹²

The new ALOX15 findings in colorectal cancer add another layer to this mechanistic picture, suggesting that individual genetic and enzymatic profiles may determine whether EPA and DHA are channeled into protective or neutral pathways in specific tissues.

Ongoing uncertainty, ongoing trials

Government and academic institutions continue to investigate omega‑3s in large cohorts and disease‑specific trials, while clinical guidance remains cautious.

The Office of Dietary Supplements maintains a searchable Clinical Studies Database (CARDS) and supports Botanical Research Centers investigating dietary supplements, including omega‑3s, across health conditions.¹⁰
The NCCIH fact sheet on omega‑3 supplements notes that while serious adverse effects are uncommon at typical doses, the clinical benefits outside certain cardiovascular indications are unproven or modest, and evidence for many advertised uses remains insufficient.⁴

Researchers behind the colorectal cancer ALOX15 study argue that more nuanced, precision nutrition approaches may be needed—where genetic, enzymatic and metabolic markers are incorporated into trial design and clinical decision‑making about omega‑3 use.

Simultaneously, debates over daily fish oil capsules for the general public are likely to continue, as new brain and mental health data emerge and as cardiology trials refine which formulations, doses and patient groups, if any, gain clear advantages from omega‑3 therapy.

References & Links

“Omega-3 fish oil supplements could backfire without this key enzyme,” Cellular and Molecular Gastroenterology and Hepatology via ScienceDaily. ↩ ↩² ↩³
“In major meta‑analysis of clinical trials, omega‑3 fish oil supplements linked with lower cardiovascular disease risk,” Harvard T.H. Chan School of Public Health (2019). ↩ ↩²
“Omega‑3 fatty acids and the heart: New evidence, more questions,” Harvard Health Publishing (2021). ↩ ↩²
“Omega‑3 Supplements,” National Center for Complementary and Integrative Health (NCCIH) fact sheet. ↩ ↩² ↩³
“Fish oil,” Mayo Clinic drug and supplement monograph. ↩ ↩²
“A systematic review and dose response meta‑analysis of Omega 3 …,” Scientific Reports (2025). ↩ ↩²
“What does the latest research reveal about omega‑3s and human health?” Nutritional Outlook. ↩ ↩²
“Efficacy and safety of omega‑3 fatty acids supplementation for …,” meta‑analysis available via PubMed Central. ↩ ↩²
“Omega‑3 Supplements for Mild Traumatic Brain Injury,” Health.mil information paper (2025). ↩ ↩²
“Omega‑3 Fatty Acids – Health Professional Fact Sheet,” NIH Office of Dietary Supplements. ↩ ↩² ↩³
“Omega‑3s are great for your health—but supplements may not be,” National Geographic. ↩ ↩²
“The Role of Omega‑3 and Omega‑6 Polyunsaturated Fatty Acid …,” review article on PubMed Central. ↩ ↩²