This study investigated whether vitamin D and marine-derived long-chain omega 3 fatty acids reduce autoimmune disease risk. A nationwide, randomized, double-blind, placebo-controlled trial with a two-by-two factorial design was conducted in the US. In total, 25 871 participants (12 786 men ≥50 years and 13 085 women ≥55 years at enrollment) took part.
Participants were given
- vitamin D (2000 IU/day),
- matched placebo,
- omega 3 fatty acids (1000 mg/day),
- matched placebo.
Participants self-reported all incident autoimmune diseases from baseline to a median of 5.3 years of follow-up; these diseases were confirmed by extensive medical record review. Cox proportional hazard models were used to test the effects of vitamin D and omega 3 fatty acids on autoimmune disease incidence. The primary endpoint was all incident autoimmune diseases confirmed by medical record review: rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease, psoriasis, and all others.
Participants were followed for a median of 5.3 years. 18 046 self-identified as non-Hispanic white, 5106 as black, and 2152 as other racial and ethnic groups. The mean age was 67.1 years. For the vitamin D arm, 123 participants in the treatment group and 155 in the placebo group had a confirmed autoimmune disease (hazard ratio 0.78, 95% confidence interval 0.61 to 0.99, P=0.05). In the omega 3 fatty acids arm, 130 participants in the treatment group and 148 in the placebo group had a confirmed autoimmune disease (0.85, 0.67 to 1.08, P=0.19). Compared with the reference arm (vitamin D placebo and omega 3 fatty acid placebo; 88 with confirmed autoimmune disease), 63 participants who received vitamin D and omega 3 fatty acids (0.69, 0.49 to 0.96), 60 who received only vitamin D (0.68, 0.48 to 0.94), and 67 who received only omega 3 fatty acids (0.74, 0.54 to 1.03) had confirmed autoimmune disease.
The authors concluded that vitamin D supplementation for five years, with or without omega 3 fatty acids, reduced autoimmune disease by 22%, while omega 3 fatty acid supplementation with or without vitamin D reduced the autoimmune disease rate by 15% (not statistically significant). Both treatment arms showed larger effects than the reference arm (vitamin D placebo and omega 3 fatty acid placebo).
This is the best trial of dietary supplements that I have seen for a very long time. Yet, the authors caution: Because participants were older adults, the results might not generalize to autoimmune diseases that primarily have their onset in younger people. However, the pathogenesis of many of the specific autoimmune diseases observed (eg, rheumatoid arthritis and psoriasis) is similar in younger adults. The trial tested only one dose and formulation of each supplement. The relatively low number of participants with a confirmed diagnosis of most individual diseases, and the challenge of confirming diagnosis of autoimmune thyroid disease based on medical records, limited statistical power to detect an effect on individual disease outcomes and subgroups of a priori interest. Given the latency of autoimmune disease onset, longer follow-up could be informative, and participants are being followed in an open label extension study.
As regular readers know, I am not easily impressed – but today I am.