We all know Vitamin D is important for bone health. It may play a role in a number of other medical conditions too. Low vitamin D levels have been associated with conditions like COPD, hypertension, migraine headaches, and Parkinson’s disease. Despite this, there has been a lot of controversy regarding the effectiveness of vitamin D treatment.

The Pros and Cons of Vitamin D

Before you invest in supplements, look at the data. Meta-analyses and systematic reviews pool together data from multiple studies. To be as objective as possible, that is what we will use as our best evidence below.

1 — Asthma

Diagnosis Life summary:
A systemic review and meta-analysis in Lancet Respiratory Medicine (https://doi.org/10.1016/S2213-2600(17)30306-5) included 8 randomized controlled trials involving 1,078 people. Vitamin D did not decrease the number of exacerbations someone had overall but it decreased their severity. Specifically, those flare-ups were less likely to require treatment with steroids. The benefit was most pronounced in people who had low 25(OH)D levels (< 25 nmol/L).

2 — Atopic dermatitis

Diagnosis Life summary:
A systemic review and meta-analysis in Nutrition (https://doi.org/10.1016/j.nut.2016.01.023) looked at 9 randomized control studies. People treated with vitamin D had less severe atopic dermatitis overall. Another analysis in Nutrients (https://doi.org/10.3390/nu8120789) looked at 11 observational studies and randomized trials. Study participants were noted to have lower serum 25(OH)D levels overall, most pronounced in the pediatric population. 25(OH)D supplementation decreased the severity of atopic dermatitis by 6 points on the SCORAD index. Yet another review in Nutrients (https://doi.org/10.3390/nu11081854) included 16 studies with both children and adults. It again showed a tendency towards low vitamin D levels in people with atopic dermatitis (mean difference -14 nmol/L) when compared to healthy controls. SCORAD scores decreased by as much as 11 points, which was clinically significant. The treatment effect was most pronounced when people were treated with 1500–1600 IU/daily for at least 3 months.

3 — Cancer

Diagnosis Life summary – PROS:
One meta-analysis in the Annals of Oncology (https://doi.org/10.1093/annonc/mdz059) looked at 10 randomized control trials involving 6,537 people. 25(OH)D supplementation did not decrease the number of cancer cases but it did decrease the number of deaths from cancer, especially for people who supplemented daily. A systemic review and meta-analysis in BMJ (DOI:10.1136/bmj.l4673) included 50 trials involving 74,655 study participants. Vitamin D supplementation statistically significantly reduced the cancer deaths by as much as 15%. Vitamin D3 appeared to be more effective than D2.

Diagnosis Life summary – CONS:
A systemic review and meta-analysis in The American Journal of Clinical Nutrition (https://doi.org/10.1093/ajcn/nqx047) included 30 studies with 18,808 participants. The researchers found no statistically significant differences in cancer rates or cancer-related deaths for people treated with 25(OH)D. A randomized placebo-controlled trial in the New England Journal of Medicine (https://doi.org/10.1056/NEJMoa1809944) recruited nearly 26,000 adults over 50 to 25(OH)D supplementation or placebo. Over 5 years, 1,617 people were diagnosed with cancer (739 from the treatment group). There was no statistically significant decrease in breast, colon, or prostate cancer in the treatment group

4 — Cardiovascular disease

Diagnosis Life summary:
A randomized placebo-controlled trial in the New England Journal of Medicine (https://doi.org/10.1056/NEJMoa1809944) recruited nearly 26,000 adults over 50. They treated with 25(OH)D supplementation or placebo. Over 5 years, 805 people had a cardiac event (396 from the treatment group). There was no statistically significant decrease in heart attacks, stroke, or cardiac-related deaths. A systemic review and meta-analysis in BMJ (https://doi.org/10.1136/bmj.l4673) included 50 trials involving 74,655 study participants. Again, there was no notable decrease in cardiovascular with vitamin D supplementation.

5 — COPD

Diagnosis Life summary:
A systemic review and meta-analysis in the International Journal of Chronic Obstructive Pulmonary Disease (https://doi.org/10.2147/COPD.S101382) looked at 21 studies involving nearly 12,000 people. Vitamin D deficiency was 77% more common in people who had COPD and 2.8-times more common in people with severe disease. It was not, however, associated with flare-ups of the condition.

6 — Dementia

Diagnosis Life summary:
A systemic review and meta-analysis in BMC Geriatrics (https://doi.org/10.1186/s12877-016-0405-0) took a close look at 6 studies involving 18,974 adults. Researchers found that dementia risk was higher in people with 25(OH)D deficiency at levels < 25 nmol/L than people with levels ≥50 nmol/L. “Translated into absolute numbers, 28 (at least 10 but up to 50 more) out of 1000 people with serious vitamin D deficiency would develop dementia compared with 1000 people with sufficient D levels over 18.03 years.”

7 — Diabetes

Diagnosis Life summary:
People theorize that vitamin D decreases insulin resistance and improves glycemic control. A systemic review and meta-analysis in Nutrients (https://doi.org/10.3390/nu10030375) pooled data from 20 studies. Altogether, 2,703 study participants who had diabetes received 25(OH)D treatment for at least 8 weeks. People who had low levels at the start of their respective studies tended to have better fasting blood sugars after treatment. So did people who were not obese or who were of Middle Eastern decent. An analysis of 29 studies (n=3,324) in the Journal of Diabetes Complications (https://doi.org/10.1016/j.jdiacomp.2017.04.019), however, found no significant effect in fasting blood sugar. Instead, they found a modest decrease in HbA1C with 25(OH)D supplementation. People with low levels benefited most.

8 — Hypertension

Diagnosis Life summary:
A prospective study and meta-analysis in PLOS One (https://doi.org/10.1371/journal.pone.0174298) focused on a link between vitamin D and hypertension. After 2 years, 42.6% of 2,456 adults in the prospective study developed hypertension. At first look, it appeared that people with low levels were 22% more likely to have high blood pressure but this finding was not statistically significant when confounding factors were taken into consideration. A subsequent meta-analysis of 7 studies suggested a 23% increase in incident hypertension for people with vitamin D deficiency. Would vitamin supplements improve blood pressure control? We don’t know … yet.

9 — Migraine

Diagnosis Life summary:
A literature review in Neurological Sciences (https://doi.org/10.1007/s10072-019-04021-z) found a high prevalence of vitamin D deficiency in people who suffer from migraines, as high as 45 to 100% depending on the study! Lower levels were associated with increased headache frequency. With this in mind, treatment with 25(OH)D at doses of 1000–4000 IU per day could potentially decrease the number of headaches.

10 — Parkinson’s disease

Diagnosis Life summary:
A systemic review and meta-analysis in Medical Science Monitor (https://doi.org/10.12659/MSM.912840) looked at 8 studies. The researchers noted a high prevalence of vitamin D deficiency (<20 ng/mL) and insufficiency (<30 ng/mL) in people who had Parkinson’s disease. Not only that, people with less than 15 minutes of sun exposure per week had higher odds for Parkinson’s disease too. This implies taking 25(OH)D could decrease the risk for Parkinson’s disease or improve symptoms. Unfortunately, the available data did not show improved motor function for people treated with 25(OH)D. More studies are needed to see if treatment could decrease the risk of developing the condition in the first place.

11 — Upper respiratory tract infections (URIs)

Diagnosis Life summary:
A systemic review and meta-analysis in BMJ (https://doi.org/10.1136/bmj.i6583) looked at 25 randomized control trials involving 11,321 people. Vitamin D supplementation decreased the risk for URIs by as much as 12% although it did not necessarily decrease the severity of URI symptoms when they happened. People with baseline serum levels < 25 nmol/L were most likely to benefit from supplementation. It may not be a cure for the common cold but it’s a start!

Teaching Point: 25(OH)-Vitamin D is the active form of vitamin D.