The "sunshine vitamin" has a range of surprising benefits.
Vitamin D is a hot topic currently, with a raft of studies proclaiming its benefits for a variety of serious conditions.
Conversely, other recent studies have been more cautious, questioning its perceived usefulness for treating some illnesses.
Vitamin D is a nutrient that is synthesized in our skin when it is exposed to sunlight, and it is also present in some foods.
Sunlight is the best source of vitamin D, but in the winter months, the National Institutes of Health (NIH) recommend topping up vitamin D levels by eating vitamin D-containing foods each day. These include oily fish, fortified milk, beef liver, egg yolks, mushrooms, and fortified breakfast cereals.
What does vitamin D do?
Scientists know that vitamin D is essential for many aspects of maintaining good health and that deficiency is linked with problems for both physical and mental health.
Increasingly though, studies are also suggesting that vitamin D might have protective benefits against heart failure, diabetes, cancer, respiratory tract infections, autoimmune disease, and even hair loss.
A surprisingly large number of people have insufficient levels of vitamin D. For instance, according to one study, more than 40 percent of adults in the United States are deficient. Because of its prevalence, it is important to determine what the public health implications of this epidemic might be.
Symptoms of vitamin D deficiency can vary between individuals, but they typically include pain in the joints, muscles, or bones; fatigue; breathing problems; and low mood or seasonal affective disorder (SAD).
Below, we run through a number of intriguing recent studies that investigate associations between vitamin D and an assortment of illnesses.
Vitamin D and heart failure
Several studies have suggested that vitamin D could offer protective benefits against cardiovascular illness, but scientists have yet to pinpoint what mechanisms are driving this association.
Recently, though, Medical News Today reported on a study that used a mouse model to investigate how a type of vitamin D, called 1,25-dihydroxyvitamin D3, affects heart cells. In particular, the researchers looked at the cells responsible for developing scar tissue following a heart attack, called cardiac colony-forming unit fibroblasts (cCFU-Fs).
cCFU-Fs are an important area of study because, when heart tissue is scarred, the heart has a harder time pumping blood, which can lead to heart failure.
The researchers behind the study found that vitamin D inhibited the action of cCFU-Fs, which prevented scar tissue from building around the hearts of the mice in the study, potentially preventing blockages in the cardiovascular system.
"With further study," wrote the authors, "vitamin D could prove to be an exciting, low-cost addition to current treatments, and we hope to progress these findings into clinical trials for humans."
Vitamin D and cancer
Breast cancer and bowel cancer have both been linked with cases of vitamin D deficiency in recent studies. One of these analyzed data from two randomized clinical trials and a prospective cohort study.
The researchers found that high levels of vitamin D were inversely associated with risk of breast cancer among women who were cancer-free at baseline.
Studies suggest that vitamin D impacts breast cancer risk.
According to the study results, the higher the levels of vitamin D, the lower the risk of breast cancer.
Although a link between vitamin D deficiency and colorectal cancer has previously been reported, not all studies have been able to replicate these findings. A new, large-scale study attempted to settle this by drawing on data from three continents, including 5,700 colorectal cancer cases and 7,100 controls.
The researchers calculated that people whose levels of vitamin D fall below those specified in the current guidelines have a 31 percent increased risk of developing bowel cancer. By contrast, those with vitamin D levels above the current recommended levels were 22 percent less likely to develop this cancer.
Vitamin D and belly fat
The study authors reported that having excess belly fat was linked with lower levels of vitamin D:
"[T]he strong relationship between increasing amounts of abdominal fat and lower levels of vitamin D suggests that individuals with larger waistlines are at a greater risk of developing deficiency and should consider having their vitamin D levels checked."
However, the study was not able to prove whether a deficiency in vitamin D causes fat to be stored around the belly, or if having belly fat somehow contributes to a deficiency in vitamin D. The researchers say that future studies will attempt to determine cause and effect in this relationship.
Vitamin D and Alzheimer's disease
A systematic review from researchers in Australia recently attempted to settle the debate surrounding vitamin D's ability to protect against Alzheimer's. The systematic review analyzed more than 70 studies looking at the association.
They concluded that there was no significant association between vitamin D deficiency and risk of Alzheimer's.
Intriguingly, the authors did suggest that — based on their systematic review — there may be an association between exposure to the sun's ultraviolet rays and protection against multiple sclerosis, Parkinson's disease, and Alzheimer's, but that this may be independent of vitamin D production.
The authors said that further studies would be needed to confirm these links and identify the mechanism responsible for such associations.
Vitamin D and chronic pain
Over the years, some scientists have theorized that low levels of vitamin D might cause or worsen chronic pain.
So, in 2015, a group of scientists set out to collate existing evidence to examine the relationship.
Could vitamin D ease chronic pain?
The resulting Cochrane review, updated in 2015, explains that:
"Observational and circumstantial evidence suggests that there may be a role for vitamin D deficiency in the etiology of chronic painful conditions." The team scrutinized the findings from a number of studies.
Following the analysis, they concluded that the available scientific evidence is not strong enough to support a connection between vitamin D deficiency and chronic pain.
The authors write, "Based on this evidence, a large beneficial effect of vitamin D across different chronic painful conditions is unlikely. Whether vitamin D can have beneficial effects in specific chronic painful conditions needs further investigation."
So, as ever, more work will be needed to finally close the lid on this interaction.
We hope this article has enhanced your understanding of the latest scientific thinking around this fascinating chemical. Please remember, however, that over-exposure to sunlight — especially the hot, midday sun — can result in skin damage and increase risk of skin cancer.