Researchers suggest low vitamin D levels, mediated by low UVB exposure, may be responsible for many leukemia cases worldwide.
There were around 352,000 new cases of leukemia diagnosed worldwide in 2012, and last year, more than 54,000 cases of the cancer were diagnosed in the US alone.
While scientists are still unsure of the exact causes of leukemia, genetic and environmental factors are thought to play a role.
A number of studies have shown that vitamin D metabolites in the blood - known as 25-hydroxyvitamin D, or 25(OH)D, which is an indicator of the body's vitamin D levels - interact with acute myeloid leukemia (AML) cells. What is more, some studies have identified low vitamin D levels in patients with AML.
While vitamin D is found in some foods, including oily fish, cheese and egg yolks, it is present in small amounts. The body's best source of vitamin D is sunlight; ultraviolet B (UVB) radiation from the sun penetrates bare skin, inducing vitamin D synthesis.
In this latest study, coauthor Cedric Garland, adjunct professor in the Department of Family Medicine and Public Health at the University of California-San Diego, and colleagues set out to investigate whether low UVB exposure and low vitamin D levels are associated with leukemia risk.
Leukemia rates highest in countries farther from the equator
Garland and colleagues analyzed data from the International Agency for Cancer Research's (IARC) Global Cancer (GLOBOCAN) 2012 database.
Fast facts about leukemia
- Leukemia accounted for around 3.3% of all new cancer cases in the US last year
- Around 1.5% of men and women in the US will be diagnosed with leukemia at some point in their lives
- Around 58.5% of people with leukemia survive 5 years or more after diagnosis.
The team looked at the age-adjusted leukemia incidence rates for 172 countries, and the cloud cover-adjusted UVB irradiance for each country was assessed using data from the International Satellite Cloud Climatology Project.
The researchers found that individuals living in countries farther away from the equator, such as the US, Australia, New Zealand, Canada and Ireland, were at least twice as likely to have leukemia as people living in countries closes to the equator, such as Nigeria, Bolivia, Samoa and Madagascar.
The association remained after accounting for sex-specific life expectancy and altitude, according to the authors.
The team explains that individuals who live farther away from the equator are exposed to solar energy that has traveled farther through the Earth's atmosphere, which reduces the amount of UVB radiation that reaches the skin.
As such, the researchers say it is "plausible" that much of the leukemia burden across the globe is a result of low vitamin D levels caused by low UVB exposure.
"Skin photosynthesis accounts for a large proportion of 25(OH)D concentration. As a result, the inverse association between cloud-adjusted solar UVB exposure and incidence rates is likely to be mediated by circulating 25(OH)D, which is highly dependent on solar UVB irradiance," they explain, adding:
"Importantly, these results suggest that increased levels of UVB irradiance and vitamin D may help prevent development of leukemia."
While the team says some key strengths of their study are the inclusion of thousands of leukemia cases from hundreds of countries and the fact the findings are consistent with results from previous studies, they admit there are some limitations.
For example, they were unable to control for other factors that may fuel differences in leukemia risk between countries. "Some of these confounders may be very influential on risk for leukemia," they note.
Still, Garland and colleagues believe the association between low UVB exposure, low vitamin D levels and leukemia warrants further investigation.
While vitamin D deficiency has been linked to a number of health problems, a recent study reported by Medical News Today suggests higher monthly doses of the vitamin may increase the risk of falls among seniors.