A recent randomized trial, organized at Harvard, suggests that they may do just that. The study found that taking a vitamin D pill daily protected “telomeres” from shortening. Telomeres form the protective ends of the long strands of gene-carrying contents (chromosomes) inside all cells. When we’re born, our telomeres are relatively long. As we go through life, and our cells keep dividing, the telomeres get shorter. Our lifestyle and inherited factors affect how rapidly our telomeres shorten (a measure of how rapidly we are aging). Studies have shown that (1) people with shorter telomeres are more likely to have poorer health and a shorter life, (2) higher blood levels of vitamin D are linked to longer telomeres, and (3) taking vitamin D raises blood levels of vitamin D. So, it would seem logical that taking a vitamin D supplement would increase vitamin D blood levels, which would slow the shortening of telomeres (and cell aging), which would potentially have health benefits. Like any beautiful theory, however, it still needs to be proved. Many randomized trials have disproved beautiful theories, particularly when it comes to vitamin supplements. This new randomized trial, published in the July 2025 of The American Journal of Clinical Nutrition, included about 1,000 people ages 50 or older who took daily vitamin D3 pills-2,000 international units (IU)—or daily placebo pills for at least four years. At the beginning of the study, and again two and four years later, the researchers measured the people’s telomere lengths. By the end, the people taking vitamin D showed less shortening of their telomeres compared with the other participants. Would this benefit continue with taking daily pills for longer than four years? We don’t know yet. Did vitamin D improve health and longevity? This and other randomized trials of vitamin D supplements have shown a reduction in the number of new autoimmune diseases and a reduction in markers of inflammation. While many scientists have suspected that vitamin D pills might reduce the risk of cancer, heart disease. stroke, diabetes, infectious diseases, and premature death, the results of randomized trials have yielded inconsistent results. Still, that raises the question: Should all people take vitamin D? People with osteoporosis or with vitamin D deficiency (determined by a blood test) definitely should. I also think taking it is a good idea for people who have autoimmune diseases or family members with autoimmune diseases. Even in the absence of a deficiency, taking daily vitamin D3 (2,000 IU) should be considered, for there is little risk in doing so, even in the absence of a vitamin D deficiency, osteoporosis, or an autoimmune disease. Benefits are possible in view of the reasonably persuasive evidence that the pills slow cellular aging and reduce inflammation—combined with the pills’ lack of side effects and low cost.
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