Not so sure: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6121423/
There is some opinion that many benefits attributed to vitamin d supplements may apply only in individuals who have a deficiency from normal levels.
While this research paper doesn't draw conclusion one way or another whether Vitamin D helps against cold/influenza-virus, my person opinion is that getting enough Vitamin D is beneficial in general and certainly doesn't hurt you.
If anyone wants to dive deep into Vitamin D benefits, the paper I quoted above had the following conclusions:
Higher levels of vitamin D are strongly associated with prevention of falling and fractures from falling; lower incidence of cancer or cancer mortality; lower mortality from cardiovascular disease; fewer symptoms of colds or influenza; prevention of both types of diabetes, multiple sclerosis, chronic back pain, depression, and possibly autism.
Take a look at these Clinical Research & Epidemiological Studies as well:
- Vitamin D and calcium supplementation reduces cancer risk:
2007 - American Journal of Clinical Nutrition — 6 pages
This 4-year, population-based, double-blind, randomized placebo-controlled trial showed that the relative risk (RR) of all forms of cancer was reduced to 0.232 in women receiving 1100 IU of daily Vitamin D plus calcium compared to the placebo control group. (Please see the study for all details.)- Prospective Study of Predictors of Vitamin D Status and Cancer in Men:
2006 - Journal of the National Cancer Institute — 9 pages
This is a statistical study based upon a cohort of 51,529 medical professionals aged 40-75 in 1986. From their analysis, an increase of 25 nmol/L (which is 10 ng/ml, the more commonly used units) of predicted serum 25-hydroxy Vitamin D, resulted in a 17% reduction in total cancer incidence, a 29% reduction in total cancer mortality, and a 45% reduction in digestive-system cancer mortality. (Please see the study for all details.)- Prevalence of Cardiovascular Risk Factors & Serum Vitamin D Levels in the U.S.:
2007 - Archives of Internal Medicine / American Medical Association — 7 pages
This study divided 7186 male and 7902 female adults, 20 years or older, into four equal-size groups based upon their measured serum Vitamin D levels. Comparing the lowest serum group (the lowest quartile) to the highest (the highest quartile), those with the lowest serum Vitamin D levels had 1.3 times the occurrence of hypertension (high blood pressure), 1.98 times the occurrence of diabetes mellitus, 2.29 times the occurrence of obesity, and 1.47 times the occurrence of high serum triglyceride levels. (Please see the study for all details.)- 25-Hydroxyvitamin D and Risk of Myocardial Infarction in Men:
2008 - Archives of Internal Medicine / American Medical Association — 7 pages
This prospective statistical study examined 18,225 male health professionals aged 40 to 75 years who were free of cardiovascular disease at the time of blood collection. During 10 years of follow-up, 454 men developed nonfatal myocardial infarction (MI, heart attack) or fatal coronary heart disease. Compared with twice the number of age- and smoking status-matched control subjects, men deficient in Vitamin D (≤15 ng/mL) were 2.42 times more at risk for heart attack than those with sufficient serum Vitamin D (≥30 ng/mL). (Please see the study for all details.)- Predictors of Vitamin D Status and Cancer Incidence in Men:
2006 - Journal of the National Cancer Institute — 9 pages
This is a report on a “prospective statistical study” meaning that they took an existing medical database of information and analyzed it carefully for statistically significant relationships of a specific nature. In this case, they were examining the relationship, if any, between serum (blood) levels of Vitamin D and incidents of cancer and cancer mortality in men. What they found was that every increase of 25 nmol/L (that's 10 ug/mL on the scale most often used) of serum Vitamin D was associated with a 17% reduction in total cancer incidence, a 29% reduction in total cancer mortality, and a 45% reduction in digestive-system cancer mortality. The study concluded, in addition to the above, that the Vitamin D supplementation required to achieve a 25 nmol/L (10 mg/mL) increment in Vitamin D serum levels was approximately 1500 IU/day. (Please see the study for all details.)- Low Vitamin D Status Despite Abundant Sun Exposure:
2007 - The Journal of Clinical Endocrinology & Metabolism — 6 pages
An odd thing that Vitamin D researchers occasionally encounter is surprisingly low serum Vitamin D levels in people having plenty of exposure to the sun. This study examined students of the University of Hawaii and patrons of a Hawaiian skateboard shop who are regularly in the sun. (Please see the study for all details.)