Despite increasing scientific scrutiny of the benefits of vitamin D, there is still mystery and confusion about this essential nutrient. Part of the complexity is due to the fact that vitamin D is not simply a nutrient. It functions as a hormone and affects virtually every cell in the body.
Because the skin manufactures vitamin D only after exposure to sunlight, circulating levels often drop during winter months. That’s why grandmothers used to dose their families with cod liver oil rich in vitamin D as a winter tonic.
Were the Old Wives Right about Cod Liver Oil?
Science is still trying to figure out whether the grandmothers were right. As so often happens, the studies are inconclusive. Some controlled studies show that children given vitamin D supplements have fewer colds and other respiratory infections (Pediatrics, Sept. 2012). A randomized controlled trial of adults who had already suffered frequent respiratory tract infections found that vitamin D supplementation helped reduce the likelihood of such an infection (BMC Research Notes, Aug. 30, 2015).
There is also brand-new research showing that cold viruses reduce the activity of vitamin D receptors in lung epithelial cells, but vitamin D supplied as a supplement can decrease rhinovirus replication and release-a very important finding (Antiviral Research, Jan., 2017).
Negative Studies:
But there are a number of negative studies. One trial in adults examined whether 1000 IU daily of vitamin D prevented upper respiratory tract infections better than placebo and found it did not (Clinical Infectious Diseases, Nov. 15, 2013). Severe illness was less common among those taking vitamin D but overall there was no protection against colds and flu-like illnesses. More recently, investigators looked at vitamin D, colds and asthma exacerbations (American Journal of Respiratory and Critical Care Medicine, online, Nov. 5, 2015). In this study, more than 200 volunteers were randomized to receive placebo or vitamin D, and those getting vitamin D had no fewer or less severe colds.
Naturally High Levels of Vitamin D Appear Protective:
People who have higher levels of vitamin D circulating in their bloodstreams are less likely to experience other sorts of infections, however. One recent study found that overweight individuals undergoing gastric bypass surgery were three times more likely to suffer an infection following the surgery if their levels of vitamin D (25-hydroxyvitamin D) were below 30 ng/ml (JAMA Surgery, online Nov. 27, 2013). This is a significant concern, because obese people are much more likely to have low circulating levels of the vitamin (Journal of Epidemiology and Community Health, online, Nov. 6, 2013).
An epidemiological study found that Americans with 25-hydroxyvitamin D blood levels below 30 ng/ml had a 56 percent greater likelihood of coming down with pneumonia (PLoS One, Nov. 15, 2013). Similarly, an analysis of 347 children admitted to the hospital in Santiago de Compostela, Spain, showed that those with 25-hydroxyvitamin D levels below 20 were more likely to have severe lower respiratory tract infections (Journal of Pediatric Gastroenterology and Nutrition, online Oct. 9, 2015). However, among babies with respiratory syncytial virus (RSV), vitamin D status was unrelated to the severity of the infection (Journal of the Pediatric Infectious Diseases Society, Sep. 2015).
Low levels of this vitamin have also been linked to inflammation (including arthritis), cardiovascular disease, high cholesterol, diabetes, weight gain, infections, multiple sclerosis, depression or anxiety, cognitive decline and poor physical functioning (The Lancet Diabetes & Endocrinology, online Dec. 6, 2013). A study in India looked at supplementation with 2000 IU of vitamin D daily and found it was effective in eliminating asymptomatic bacterial vaginosis (Indian Journal of Medical Research, June, 2015).
Recent research has found how vitamin D modulates the immune system through its effects on white blood cells (monocytes). This suggests that the vitamin is indeed an important regulator of the immune response to infection (Nutrients, Dec. 12, 2016). Perhaps that explains why older adults in Colorado long-term care facilities were less likely to suffer from upper respiratory tract infections if they took high-dose vitamin D supplements (100,000 IU/month of vitamin D3) compared to placebo (Journal of the American Geriatric Society, online Nov. 16, 2016).
In addition, adequate levels of vitamin D are associated with a reduced risk of falls and broken bones and with better hand and leg strength (Journal of the American Geriatric Society, Nov., 2013). That connection did not hold up for the Colorado study cited just above though; the elderly individuals on high-dose vitamin D3 were more likely to fall during the study, though not more likely to break a bone.
Do You Need Vitamin D Supplements?
So why hasn’t all this research resulted in clear guidelines for how much vitamin D people should take in the winter to stay healthy? The studies on supplementation have not shown consistent results, but there are plausible explanations (Nutrients, June, 2015). For one thing, we don’t know exactly what level of 25-hydroxyvitamin D is best for warding off a specific disease in a particular patient. There are some genetic variations among people in terms of how vitamin D-related compounds are carried around the body and transferred into different tissues.
Some of these differences could help explain why the “dose interval” makes a difference (Journal of Clinical Endocrinology and Metabolism, Dec. 2013). Many of the studies showing that vitamin D supplementation makes no difference to health used very high doses given infrequently, say on a monthly or even six-month schedule. That might not be at all the same for the body as getting a much smaller amount of vitamin D on a steady, daily basis.
We have more details on the health benefits of vitamin D in our Guide to Vitamin D Deficiency. In it, we describe how to interpret tests of 25-hydroxyvitamin D as well as how to increase blood levels with supplements.
We will have to wait for better studies to determine the best regimen for supplementing vitamin D for prevention of cognitive decline as well as heart disease. In the meantime, though, it makes sense to prevent winter deficiency of this critical vitamin. When we spoke recently with Dr. David Perlmutter, he pointed out that there is no one “best” dose for vitamin D; the amount needed to achieve a healthy blood level is highly individual. But since it is possible to test for vitamin D in the blood, the supplement can be tailored for the patient.
