This vitamin could slash colon cancer risk by half – but there’s a catch

The potential role of vitamin D in preventing and treating colorectal cancer (CRC) has attracted growing research interest – especially as CRC rates are rising, particularly among younger adults. This isn’t a new area of study. Low vitamin D levels have long been linked to a higher risk of developing colorectal cancer.

One large study involving over 12,000 participants found that people with low blood levels of vitamin D had a 31 per cent greater risk of developing CRC compared to those with higher levels. Similarly, another study reported a 25 per cent lower CRC risk among individuals with high dietary vitamin D intake.

Data from the Nurses’ Health Study – a long-term investigation of American nurses – showed that women with the highest vitamin D intake had a 58 per cent lower risk of developing colorectal cancer compared to those with the lowest intake.

Now, a review highlights vitamin D’s promise in colorectal cancer prevention and treatment – but also underscores the complexity and contradictions in current research.

While observational data, which follow people’s use of vitamin D, and mechanistic studies, to investigate how vitamin D works in the laboratory, suggest protective effects, this isn’t confirmed by larger trials.

In fact, randomised controlled trials (RCTs), in which some people receive vitamin D and others don’t, the gold standard by which treatments are judged, reveal inconsistent outcomes. This highlights the need for a balanced approach to its integration into public health strategies.

Vitamin D is synthesised in the skin in response to sunlight and exerts its biological effects through vitamin D receptors (VDRs)found throughout the body, including in colon tissue. When activated, these receptors help regulate gene activity related to inflammation, immune response and cell growth – processes central to cancer development and progression.

Preclinical studies have shown that the active form of vitamin D (calcitriol) can suppress inflammation, boost immune surveillance (the immune system’s ability to detect abnormal cells), inhibit tumour blood vessel growth and regulate cell division – a key factor in cancer development, as demonstrated in my recent research.

Epidemiological studies, which track health outcomes across large populations over time, consistently find that people with higher blood levels of vitamin D have a lower risk of developing CRC. This paints a hopeful picture, suggesting that something as simple as getting more vitamin D – via sun exposure, diet, or supplements – could lower cancer risk.

But the story gets more complicated.

Mixed results

When it comes to medical decision-making, randomised controlled trials (RCTs) are the gold standard. These studies randomly assign participants to receive either a treatment (like vitamin D) or a placebo, helping eliminate bias and isolate cause-and-effect relationships.

Unfortunately, RCTs on vitamin D and CRC have produced mixed results.

For example, the VITAL trial – a major RCT involving over 25,000 participants – found no significant reduction in overall colorectal cancer incidence with 2,000 IU/day of vitamin D supplementation over several years.

However, a meta-analysis of seven RCTs did show a 30 per cent improvement in CRC survival rates with vitamin D supplements, suggesting potential benefits later in the disease course rather than for prevention.

On the other hand, the Vitamin D/Calcium Polyp Prevention Trial found no reduction in the recurrence of adenomas (pre-cancerous growths) with supplementation, raising questions about who benefits most, and at what dosage.

Adding to the uncertainty is the question of causation. Does low vitamin D contribute to cancer development? Or does the onset of cancer reduce vitamin D levels in the body? It’s also possible that the observed benefits are partly due to increased sunlight exposure, which itself may have independent protective effects.

The big picture

These discrepancies highlight the importance of considering the “totality of evidence” – treating each study as one piece of a larger puzzle.

The biologic plausibility is there. Observational and mechanistic studies suggest a meaningful link between vitamin D and lower CRC risk. But the clinical evidence isn’t yet strong enough to recommend vitamin D as a standalone prevention or treatment strategy.

That said, maintaining sufficient vitamin D levels – at least 30 ng/mL – is a low-risk, cost-effective health measure. And when combined with other strategies like regular screening, a healthy diet, physical activity, and personalised care, vitamin D could still play a valuable role in overall cancer prevention.

Vitamin D is not a miracle cure – but it is part of a much broader picture. Its role in colorectal cancer is promising but still being defined. While it’s not time to rely on supplements alone, ensuring adequate vitamin D levels – through sun exposure, diet, or supplements – remains a smart choice for your health.

Colorectal cancer is a complex disease, and tackling it requires an equally nuanced approach. For now, that means focusing on evidence-based lifestyle changes, regular screenings, and staying informed as new research unfolds.

Justin Stebbing is a Professor of Biomedical Sciences at Anglia Ruskin University

This article was originally published by The Conversation and is republished under a Creative Commons licence. Read the original article