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The Power of the Sunshine Vitamin: What You Need to Know About Vitamin D and Cancer

By Dr. Punam Rana MD MSc • Medical Oncologist & Mindfulness Educator

 
Vitamin D is unlike any other vitamin. While we can obtain it through food and supplements, our bodies also have the unique ability to produce it naturally through sunlight. But beyond its role in bone health, emerging evidence suggests vitamin D may have an important role in cancer prevention and treatment support.

What Is Vitamin D?

Vitamin D acts more like a hormone once activated in the body. When your skin is exposed to ultraviolet B (UVB) rays, it converts 7-dehydrocholesterol into vitamin D3, which is then converted by the liver and kidneys into calcitriol, its active form. Small amounts are also obtained through diet and fortified foods.

Why Do We Need Vitamin D?

  • Maintains strong bones by helping absorb calcium and phosphorus
  • Supports immune function and reduces inflammation
  • May help regulate cell growth and reduce cancer cell proliferation

Vitamin D rich foods like salmon, eggs, and dairy

A Global Deficiency

Vitamin D insufficiency affects nearly 50% of the global population. In Canada, where sunlight is limited for much of the year, studies estimate that between 75–97% of people may have inadequate levels.

Vitamin D and Cancer Prevention

Observational studies suggest a link between higher vitamin D levels and reduced risk of certain cancers, including:

  • Breast
  • Colorectal
  • Prostate
  • Pancreatic

A 2018 meta-analysis found lower colorectal cancer risk in those with higher vitamin D. However, randomized controlled trials (RCTs) like the VITAL trial did not show a decrease in cancer incidence, though some did show a reduction in cancer-related mortality.

Long-term studies—potentially 20 years or more—may be necessary to truly assess the impact of vitamin D on cancer prevention.

Vitamin D in Cancer Treatment

Vitamin D is not a cancer treatment, but may enhance conventional therapies:

The SUNSHINE trial found that colorectal cancer patients who received high-dose vitamin D (4000 IU daily) with chemotherapy had longer progression-free survival and reduced risk of disease progression compared to those who received a standard dose (400 IU).

Other studies suggest vitamin D may:

  • Improve quality of life during chemotherapy
  • Reduce treatment-related side effects



What’s the Right Dose?

The optimal dose of vitamin D varies based on factors like age, skin tone, sunlight exposure, and baseline levels. While the RDA is 600–800 IU for adults, higher doses (e.g., 4000 IU/day in the SUNSHINE trial) may be needed for additional benefits. Always consult a healthcare provider before starting high-dose supplements.

Sunlight Exposure

Aim for 10–30 minutes of sunlight exposure several times per week. Sunscreen should still be used to prevent skin damage.

Supplementation

Consider supplementing especially during winter months or if you are at risk of deficiency—but with medical guidance and periodic blood level checks to avoid toxicity.

Vitamin D supplements on a table

Final Thoughts

Vitamin D is essential for overall health—and it may play a powerful supporting role in cancer care. Whether through sunlight, diet, or supplements, ensuring adequate levels is a proactive step you can take. Speak with your doctor to determine the best approach for your unique needs.

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