Vitamin D, often referred to as the "sunshine vitamin," plays a crucial role in overall health, beyond its well-known benefits for bone health. Sun exposure is the primary natural source of Vitamin D, but it can also be obtained from certain foods and supplements. Recent research highlights its significant role in preventing colorectal cancer (CRC) and improving patient outcomes through immune modulation, inflammation reduction, and cancer cell apoptosis. Additionally, adequate Vitamin D levels have been linked to slower aging, as evidenced by its ability to preserve telomere length, and may even protect against cognitive decline. Given the widespread prevalence of Vitamin D deficiency, particularly in indoor-dominant lifestyles, ensuring adequate intake through sunlight, diet, or supplementation is essential for maintaining health and preventing chronic diseases.
As winter settles in, there's often an inexplicable urge to find a sunny spot and stretch out like a cat basking in the light. The sense of comfort is hard to describe—it's as if the warmth isn’t just reaching the skin but also soothing the emotions. While soaking in the sun, you might even find yourself imagining, “I’m engaging in photosynthesis.” It’s as though staying in the sunlight just a bit longer will recharge you completely.
In reality, the desire to bask in the sun isn't entirely driven by impulse; it could be a direct signal from your body: "You’re low on Vitamin D, go soak up some rays." According to various studies, Vitamin D deficiency has become a widespread, hidden problem. It’s estimated that globally, around 30%-80% of people have serum 25-hydroxyvitamin D [25(OH)D] levels below the normal range of 20 ng/mL. This means that one in every seven people may be deficient in Vitamin D.

As the so-called "sunshine vitamin," Vitamin D is most naturally and economically obtained through exposure to sunlight. When our skin is exposed to the rare winter sunlight, the dormant 7-dehydrocholesterol in the skin undergoes a series of chemical transformations, eventually converting into biologically active Vitamin D3.
Vitamin D and Cancer Prevention: Lowering Colorectal Cancer Risks
When people think of Vitamin D, they often associate it with "bone health" and "calcium absorption." However, this "sunshine vitamin" has many underrated health benefits. A recent study published in Nutrients revealed that maintaining adequate Vitamin D levels could significantly lower the risk of colorectal cancer (CRC) and improve patient outcomes through mechanisms such as modulating immune responses, suppressing inflammation, promoting cancer cell apoptosis, and regulating gut microbiota.

DOI:10.3390/nu17081351
The research team conducted an extensive systematic review, analyzing 50 clinical studies across PubMed, Web of Science, and Scopus databases, involving over 1.3 million participants. The analysis confirmed that serum 25(OH)D levels are closely linked to survival outcomes in colorectal cancer patients.
For example, a study of 206 stage I-III colorectal cancer patients showed that those with higher free 25(OH)D levels (>0.01-0.02 pg/mL) had significantly better 5-year survival rates than those with lower levels. Additionally, in patients with liver metastasis, having 25(OH)D levels above 20 ng/mL was associated with longer survival and reduced recurrence.
However, the risks of Vitamin D deficiency are clear. The DACHS cohort study, which involved 2,910 colorectal cancer patients, found that those with serum Vitamin D levels below 30 nmol/L had a 78% higher risk of death compared to those with higher levels.
In the general population, adequate Vitamin D levels also contribute to better health outcomes.
For instance, a cohort study from Australia found that when 25(OH)D levels were below 50 nmol/L, the risk of developing colon and rectal cancer significantly increased. Conversely, when 25(OH)D levels exceeded 75 nmol/L, not only did the risk of colorectal cancer-related death decrease, but the risk of death from cardiovascular disease and other cancers also dropped.
Can We Rely on Diet to Boost Vitamin D?
Since sunlight exposure isn’t always an option, especially in winter, it’s worth considering whether we can obtain sufficient Vitamin D from food. Foods like fatty fish (salmon, mackerel, sardines) are excellent sources, with 100g of salmon providing about 600-1000 IU of Vitamin D. Other good sources include egg yolks (about 40 IU per egg) and animal liver (e.g., chicken liver, which contains 50 IU per 100g).
Interestingly, the same review also highlighted that dietary Vitamin D intake plays a crucial role in CRC prevention and improving patient prognosis. For example, a meta-analysis of 31 studies showed that people in the highest Vitamin D intake group (around 80 ng/mL) had a 25% lower risk of colorectal cancer compared to those in the lowest intake group (around 10 ng/mL).

Large-scale meta-analyses also found that supplementing with 500-2000 IU of Vitamin D per day was associated with a 13% reduction in colorectal cancer risk. Even a daily increase of 100 IU in Vitamin D intake could reduce the risk of colorectal cancer by about 4%.
However, it's important to note that more is not always better. The recommended tolerable upper intake level (UL) for healthy adults is typically 4,000 IU per day. Excessive Vitamin D intake can lead to risks like hypercalcemia and kidney stones.
Why Does Vitamin D Have Anti-Cancer Powers?
Researchers believe that Vitamin D’s ability to prevent cancer can be attributed to its role in activating and coordinating the immune system (enhancing T-cell function, regulating inflammatory cytokines), as well as its direct action on cancer cells, suppressing their growth, invasion, and promoting apoptosis. Furthermore, Vitamin D can inhibit tumor angiogenesis, cutting off the tumor’s nutrient supply.
Vitamin D: A Defender Against Aging
Beyond cancer prevention, Vitamin D has also been linked to anti-aging benefits. A study published in The American Journal of Clinical Nutrition found that daily supplementation with 2,000 IU of Vitamin D3 over four years significantly slowed the rate of leukocyte telomere shortening in middle-aged and elderly individuals. Telomeres are protective caps at the ends of chromosomes that prevent them from deteriorating or sticking together. With each cell division, telomeres shorten, and when they become too short, cells can no longer divide, leading to aging or cell death.

DOI:10.1016/j.ajcnut.2025.05.003
The study found that over four years, Vitamin D3 supplementation preserved an average of 140 base pairs of telomere length, which is equivalent to delaying biological aging by about three years. In contrast, Omega-3 supplementation did not show a significant impact on telomere length.
Researchers suggest that Vitamin D may help maintain telomere length by enhancing telomerase activity, a "longevity enzyme" that extends telomeres. It may also reduce oxidative stress and protect telomeres from damage. Additionally, Vitamin D may influence aging pathways by regulating cell signaling pathways like NF-κB and PI3K/Akt.

Effects of different supplements on changes in leukocyte telomere length (LTL) over time
Vitamin D for Brain Health: A Shield Against Cognitive Decline
Vitamin D may also protect brain health. A study from the University of Calgary found that adequate Vitamin D levels could help delay age-related cognitive decline, with supplementation linked to a 40% reduction in the risk of dementia.

DOI:10.1002/dad2.12404
In an era dominated by "indoor culture," with increasing sedentary lifestyles and decreasing time spent outdoors, it’s essential to be mindful of Vitamin D deficiency. For those who spend most of their time indoors or have limited sun exposure, ensuring adequate Vitamin D levels—whether through sunlight, diet, or supplements—could be a highly effective, cost-efficient strategy for improving overall health and preventing age-related diseases.

This article underscores the vital importance of Vitamin D, not only for maintaining bone health but also for its broader role in cancer prevention, aging, and even cognitive function. Whether through sunlight, diet, or supplements, maintaining sufficient Vitamin D levels should be a priority for everyone, especially in our modern, often indoor-centric lifestyles.