Vitamin D and Colon Cancer
If a low-cost vitamin could significantly reduce your risk of colorectal cancer, you’d expect it to be front and centre in every health campaign. But that’s not the case with vitamin D. Despite compelling data connecting low vitamin D levels with higher colorectal cancer risk, most people are still left in the dark – literally and figuratively.
Colorectal cancer rates are climbing, particularly in younger adults. At the same time, vitamin D deficiency is widespread, especially in countries like the UK where sunlight is limited for half the year. Coincidence? Increasingly, research suggests not.
One study involving over 12,000 people found that low blood levels of vitamin D were associated with a 31% higher risk of developing colorectal cancer. Another study linked high dietary vitamin D intake with a 25% lower risk. Data from the long-running Nurses' Health Study found that women with the highest vitamin D intake had a 58% lower risk. These are not small numbers.
Vitamin D works by binding to receptors throughout the body, including the colon, influencing how cells grow, divide, and respond to threats like inflammation or abnormal mutations. Preclinical studies show it can reduce inflammation, inhibit tumour growth, and support immune surveillance. In short, it helps keep the system in check.
So why isn’t this better known? Because the gold standard of evidence – randomised controlled trials (RCTs) – haven’t produced consistently strong results. The VITAL trial, which gave 2,000 IU/day of vitamin D to over 25,000 people, found no reduction in overall colorectal cancer incidence. But a meta-analysis of seven RCTs found a 30% improvement in survival rates among colorectal cancer patients taking vitamin D.
This tells us something important: vitamin D may not stop cancer from appearing in everyone, but it might help you survive it. That alone makes it worth taking seriously.
Still, questions remain. Does low vitamin D contribute to cancer development, or does cancer lower vitamin D levels? Does sunlight exposure play a bigger role than we think? These are legitimate unknowns. But while we wait for science to catch up, the public health advice is clear: get enough vitamin D.
For UK adults, that means 10 micrograms (400 IU) per day, especially during autumn and winter. For many people, that means taking a supplement, since the average diet only provides 2–4 micrograms per day. Sunlight in winter doesn’t deliver enough UVB to trigger vitamin D production, and those with darker skin, those who cover up, and those who spend most of their time indoors are even more at risk.
Signs of deficiency include fatigue, low mood, muscle weakness, bone pain, and in children, rickets. The long-term impact of chronic deficiency is even more serious, from osteoporosis to compromised immune response. And possibly, an increased cancer risk.
Plant-based eaters need to be especially mindful. Many fortified foods and supplements still use vitamin D3 derived from lanolin (from sheep’s wool). That makes them unsuitable for vegans. But there are alternatives: vitamin D2 (always vegan), and newer forms of vegan D3 derived from lichen, algae or UV-exposed mushrooms.
Companies like Kellogg’s still use animal-derived D3 in most of their cereals. It doesn’t change the taste or function of the product – just its accessibility. A simple swap to a vegan source would open their range to a growing market and align with public health advice that encourages supplementation.
Vitamin D is not a cure-all. But it is a low-risk, cost-effective tool with huge potential. It won’t replace screening or treatment, but it could complement them. And it’s something most people aren’t getting enough of.
Whether or not it ends up being the key to preventing colorectal cancer, ensuring adequate vitamin D is a smart move. It supports immunity, mental health, bone strength, and possibly cancer resistance. Ignoring that because the evidence isn’t perfect yet is a mistake.
And allowing companies to continue using animal-based D3 when vegan alternatives exist? That’s a choice. One they don’t have to keep making.
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