Adults who took 2,000 IU of vitamin D3 daily lost less telomere length over four years than those on placebo. The difference worked out to about three years of slowed biological aging.
That is the finding of a Harvard-led randomized trial published in The American Journal of Clinical Nutrition in May 2025.
The results come from a substudy of VITAL, a randomized, double-blind, placebo-controlled trial run out of Brigham and Women's Hospital in Boston. It enrolled women aged 55 and older and men aged 50 and older across the United States.
"VITAL is the first large-scale and long-term randomized trial to show that vitamin D supplements protect telomeres," said JoAnn Manson, MD, principal investigator and chief of the Division of Preventive Medicine at Brigham and Women's Hospital.
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What telomeres actually do
Telomeres are repetitive DNA sequences that cap the ends of chromosomes and keep them from fraying or fusing. Every time a cell divides, its telomeres get a little shorter. When they become too short, the cell can no longer divide and eventually dies.
Because telomeres shrink with age and with chronic stress or inflammation, researchers use telomere length in white blood cells as a proxy for biological aging. Shorter telomeres have been linked to a higher risk of age-related conditions.
How the study was designed
The telomere substudy followed 1,054 participants, with telomere length measured at the start, after two years and after four years. Participants received 2,000 IU of vitamin D3 daily, 1 gram of marine omega-3s daily, both, or placebo.
Vitamin D3 significantly reduced telomere shortening compared with placebo. The size of the effect worked out to preventing the equivalent of nearly three years of aging over the four-year period.
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Omega-3 supplementation, by contrast, showed no meaningful effect on telomere length.
"Our findings suggest that targeted vitamin D supplementation may be a promising strategy to counter a biological aging process," said Haidong Zhu, first author of the study and a molecular geneticist at the Medical College of Georgia at Augusta University.
Where the dose fits in the US guidelines
The 2,000 IU daily dose used in the trial is well above the standard Recommended Dietary Allowance for vitamin D, which is 600 IU for adults 19 to 70 and 800 IU for those over 70.
That is still below the Tolerable Upper Intake Level of 4,000 IU per day set for adults.
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Telomere length is a validated marker of cellular aging, not a direct measure of lifespan. The VITAL researchers looked at one biological signal in one older-adult population.
Anyone considering a higher-dose supplement should talk it over with a doctor first, particularly when other medications, kidney conditions or calcium disorders are in the picture.
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