What the VITAL 2-year follow-up tells us about vitamin D, fish oil, and autoimmune risk

If you started taking vitamin D and fish oil a few years ago for autoimmune protection, you may have done so on the strength of the original VITAL trial. That 2022 paper made headlines for a reason. A five-year randomized trial showed 22% fewer new autoimmune diseases in people who took 2000 IU/day of vitamin D, with or without fish oil. A follow-up paper in 2024 watched those same people for two more years after the pills stopped. The story split.

Photo: Lieana Slapinsh / Unsplash

What the research found

VITAL was a large U.S. trial of 25,871 adults, men 50 and older, women 55 and older. Participants were randomized in a 2-by-2 design to vitamin D3 (2000 IU/day) or placebo, AND to marine omega-3 (1000 mg/day of combined EPA and DHA) or placebo. The first published autoimmune results came out in BMJ in 2022 [2]. After a median 5.3 years on the supplements, vitamin D cut new autoimmune disease cases by about 22% (HR 0.78, 95% CI 0.61–0.99). Omega-3 by itself trended toward a 15% reduction, but that result did not reach statistical significance (HR 0.85, 95% CI 0.67–1.08).

In 2024, the original VITAL authors published a follow-up in Arthritis & Rheumatology [1]. They observed 21,592 of the original participants (about 83.5% of those who finished the trial) for another two years after the supplements ended. At the 7-year mark, the picture shifted. The vitamin D protective effect had dissipated, with a hazard ratio at 7 years of 0.98 (95% CI 0.83–1.17). Omega-3 held its protective effect (HR 0.83, 95% CI 0.70–0.99 at 7 years), and across the full follow-up, that became the statistically significant result.

The bigger picture

Other recent work helps explain this divergence. The 2024 Endocrine Society guideline on vitamin D and disease prevention reviewed dozens of randomized trials [3]. Their conclusion was specific: empiric daily vitamin D is useful in defined groups, including children 1 to 18, adults over 75, pregnancy, and high-risk prediabetes. The guideline did not endorse routine empiric supplementation for general adults under 75 without an established indication. Routine 25-hydroxyvitamin D testing was not recommended in healthy populations either.

For omega-3, a 2025 analysis of nearly 189,000 UK Biobank participants in Arthritis & Rheumatology [4] found that each one-standard-deviation increase in intake of long-chain omega-3 fats (EPA, DHA, DPA, and SDA) was associated with a 9 to 10 percent lower risk of incident rheumatoid arthritis. The same paper traced part of that effect through immune-regulatory proteins (CD80 and tumor necrosis factor receptor superfamily 4), giving the membrane-incorporation story a mechanism to lean on.

The naturopathic lens

Two different physiological stories are playing out inside the same trial. Vitamin D behaves like a hormone. Its active form, calcitriol, modulates T-regulatory cell function in real time. If you stop taking it, blood levels fall, and the daily immune-signaling input falls with them. Omega-3s install into cell membranes, where they shift which fatty-acid substrates are available for inflammatory mediators. That structural deposit takes time to build, and time to lose. The 2-year carryover seen in VITAL is consistent with that biology.

In clinic, this changes how I talk about supplements with patients who carry a personal or family history of autoimmune conditions. Vitamin D belongs in the "keep in range" category, not the "occasional top-up" category. Dose to a target blood level, then watch the number. Body composition, gut absorption, and daily daylight all change what dose lands the same person at the same serum level.

Omega-3 status reflects what you actually eat, the omega-6 oils competing in the same pathways, and how efficiently your body converts plant-source ALA to EPA and DHA. Plant-only eaters often benefit from a direct EPA/DHA source (algal oil works) because that conversion is poor in most people.

A whole-person frame helps here too. Autoimmune risk is rarely about one nutrient. It tracks with sleep, infection history, gut barrier function, sex hormones, environmental load, and the inflammatory tone of the overall diet. A separate 2025 UK Biobank study in Arthritis & Rheumatology found that greater adherence to a vegetable- and fish-forward eating pattern (the EAT-Lancet diet) was associated with lower RA incidence, with part of the effect mediated through circulating omega-3 metabolites including DHA [5]. Diet pattern and supplementation are not separate strategies. They work on the same pathways from different angles.

How this changes a vitamin D and fish oil routine

If you already take either supplement, or you carry autoimmune risk in your family, start here:

1. If you take vitamin D, take it consistently. Stopping does not bank past benefit.

2. If you are supplementing vitamin D for autoimmune support, knowing your 25-hydroxyvitamin D level lets you dose to a target instead of guessing. The current Endocrine Society guideline does not endorse routine screening in healthy adults under 75, so this is a conversation to have with a clinician who knows your situation.

3. Build omega-3 intake through diet first when you can. Small oily fish a few times a week, ground flax, and walnuts all contribute. Where that is not feasible, a high-quality fish oil or algal EPA/DHA gives the membrane pool something to work with.

4. Pair both with an overall anti-inflammatory eating pattern. Supplements are not a workaround for the rest of the picture.

5. If you have a personal or family history of an autoimmune condition, work out a longer-term nutrient plan with a clinician who tracks your labs over time.

References

1. Costenbader KH, Cook NR, Lee IM, Hahn J, et al. Vitamin D and Marine n-3 Fatty Acids for Autoimmune Disease Prevention: Outcomes Two Years After Completion of a Double-Blind, Placebo-Controlled Trial. Arthritis Rheumatol. 2024;76(6):973-983. PMID: 38272846. DOI: 10.1002/art.42811. [Read more: https://acrjournals.onlinelibrary.wiley.com/doi/10.1002/art.42811]

2. Hahn J, Cook NR, Alexander EK, et al. Vitamin D and marine omega 3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. 2022;376:e066452. PMID: 35082139. DOI: 10.1136/bmj-2021-066452.

3. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2024;109(8):1907-1947. PMID: 38828931. DOI: 10.1210/clinem/dgae290.

4. Chen L, Tan T, Chen Y, et al. Dietary Polyunsaturated Fatty Acid and the Risk of Rheumatoid Arthritis: Insights From Genetic Predisposition and Proteomics. Arthritis Rheumatol. 2025;77(11):1475-1487. PMID: 40356218. DOI: 10.1002/art.43229.

5. Feng J, Pan C, Gou Y, et al. EAT-Lancet Diet Modifies the Risk of Rheumatoid Arthritis Through Metabolomic Signature. Arthritis Rheumatol. 2025;78(1):68-79. PMID: 40546003. DOI: 10.1002/art.43297.

This information is educational and is not medical advice. Decisions about vitamin D, omega-3, or any supplement should be made with a qualified healthcare provider who knows your history and can interpret your lab results.

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