A Harvard study has found that a common daily vitamin may help slow the biological aging process, confirming earlier evidence that vitamin D supplements can help prevent telomeres from shortening.
Published in The American Journal of Clinical Nutrition in July 2025, the research showed that people who took 2,000 international units (IU) of vitamin D every day for four years had less than half the telomere shortening of those given placebo pills.
Telomeres, often compared to the plastic tips on shoelaces, act as protective caps on the ends of chromosomes. As people age, telomeres get shorter, a process linked to poor health and increased mortality.
“Vitamin D tamps down inflammation, a major cause of telomere shortening and chronic diseases of aging,” said lead author JoAnn Manson, professor of medicine at Harvard Medical School.
According to a Fox News report, Manson said that the findings “suggest a promising role for vitamin D in slowing a pathway for biological aging and age-related chronic disease.”
The randomized trial involved about 1,000 adults aged 50 and older, divided into two groups. One group took vitamin D supplements, while the other received placebo pills.
After four years, the vitamin D group not only showed less telomere shortening but also recorded fewer autoimmune diseases and reduced inflammation markers.
Manson noted that the dosage tested, 2,000 IU per day, was “very safe,” without side effects or adverse events.
However, she cautioned that very high doses, such as more than 10,000 IU daily, could raise calcium levels in the blood or urine and may be toxic. “More is not necessarily better,” she said.
Vitamin D can be obtained through fortified foods such as dairy, cereal, and orange juice, as well as natural sources like salmon, sardines, tuna, and wild mushrooms.
Sunlight exposure also allows the body to synthesize the vitamin, and Manson said going out for a 15-minute midday walk several times a week is typically enough.
Despite the promising results, Manson emphasized that supplements are not a substitute for healthy habits.
“Although it’s much easier to pop a pill than to be physically active outdoors and eat healthfully, these lifestyle factors will do more to keep you healthy and lower your risk of cardiovascular disease, cancer, type 2 diabetes and other chronic diseases of aging,” she said.
“Although the results are exciting, we believe that replication of these results in a separate randomized trial will be important before changing general guidelines for vitamin D intake,” Manson added.
