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Less time in the sun may increase colorectal cancer risk

Spending less time in the sun means less exposure to UVB light, which may be linked to an increased risk of developing colorectal cancer, according to a recent study.

Colorectal cancer is the third most diagnosed cancer worldwide and is associated with high morbidity and mortality rates. There were two million new cases of colorectal cancer reported worldwide in 2018. The global burden of colorectal cancer is expected to increase by 60% to 2.2 million new cases and 1.1 million deaths by 2030.

Risk factors for colorectal cancer include increased age, obesity, a family history of colorectal cancer, a sedentary lifestyle, and a low-fiber, high-fat, and high-meat diet. Inadequate levels of vitamin D has also been identified as a potential risk factor in the development of colorectal cancer.

A recent study showed that most newly diagnosed colorectal cancer patients had insufficient levels of serum vitamin D.1 An intake of 1000 IU of vitamin D per day is associated with a 50% reduced risk of colorectal cancer.2

Vitamin D has a range of health benefits

Vitamin D is a fat-soluble vitamin, which is produced by the body when exposed to ultraviolet B (UVB) radiation in sunlight. UVB light of wavelengths between 280 and 315 nm from sunlight acts on 7-dehydrocholesterol in the skin, which is spontaneously converted into cholecalciferol. The activation process of vitamin D involves two conversion steps: into calcidiol in the liver and then into calcitriol in the kidneys.

Most people don’t get enough vitamin D from the sun, which means it might be necessary to get it from foods or nutritional supplements. Although vitamin D has limited dietary sources, it can be found in fatty fish (including salmon, tuna, and mackerel), egg yolks, cheese, and mushrooms.

Vitamin D is important in the regulation of calcium and phosphate in the body. These nutrients are essential for the health of bones, muscles, and teeth. However, it is extremely common to be vitamin D-deficient. Symptoms of vitamin D deficiency include muscle weakness, pain, fatigue, and depression.

Relationship between UVB–vitamin D–colorectal cancer

A research study 3 published in BMC Public Health investigated the age-dependent relationship between UVB exposure and colorectal cancer. Researchers at the University of California, San Diego, USA used data from 186 countries to explore possible associations between UVB light and rates of colorectal cancer for different age groups.

The results of the study showed that lower UVB exposure from the sun was significantly correlated with higher rates of colorectal cancer across all age groups. The inverse relationship between UVB exposure and colorectal cancer rates was stronger in older age groups (above 45 years old). This relationship was significant even after taking other factors into consideration, such as skin pigmentation, life expectancy, and smoking.

The researchers proposed that lower UVB exposure from the sun leads to reduced levels of vitamin D, which results in an increased risk of colorectal cancer. Previous studies have established a link between vitamin D deficiency and colorectal cancer risk. For instance, a small study from the Women’s Health Study found a significant inverse association between vitamin D levels and colorectal cancer risk. The researchers discovered that higher serum vitamin D was linked to lower mortality of patients with colorectal cancer.

Dr Raphael Cuomo, a co-author of the study, stated, “Differences in UVB light accounted for a large amount of the variation we saw in colorectal cancer rates, especially for people over age 45. Although this is still preliminary evidence, it may be that older individuals, in particular, may reduce their risk of colorectal cancer by correcting deficiencies in vitamin D.” 4

The association between UVB–vitamin D–cancer needs further investigation. Additional factors that may affect UVB exposure and vitamin D levels that were not included in the study include vitamin D supplements, clothing, and air pollution.

This study supports the need for national and global public health programs to prevent vitamin D deficiency. Future studies are required to identify the effect of vitamin D supplementation on different cancer types. It would also be beneficial to study the impact of chronic vitamin D deficiency on colorectal cancer development, particularly in regions with lower UVB exposure.


1.Savoie M, et al. (2019). Vitamin D levels in patients with colorectal cancer before and after treatment initiation. Journal of Gastrointestinal Cancer, 50(4), 769-779. Retrieved from: 

2. Holick M. (2008). Vitamin D and sunlight: strategies for cancer prevention and other health benefits. Clinical Journal of the American Society of Nephrology, 3(5), 1548-1554. Retrieved from: 

3. Purushothaman V, et al. (2021). Could age increase the strength of inverse association between ultraviolet B exposure and colorectal cancer? BMC Public Health, 21(1238). Retrieved from:

4. Lower exposure to UVB light may increase colorectal cancer risk (2021). EurekAlert! Retrieved from:

Image by Николай Оберемченко from Pixabay 2.



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