Current clinical practice reference ranges have been updated by the Institute of Medicine (IOM). Traditional levels defining vitamin D sufficiency have been based on the lowest threshold value for plasma (approximately 80 nmol/L or 32 ng/ml) that prevents secondary hyperparathyroidism, increased bone turnover, bone mineral loss, or seasonal variations in plasma parathyroid hormone.
The IOM review concluded that individuals are at risk of deficiency at serum levels below 30 nmol or 12 ng/ml and that practically all persons have sufficient serum levels at 50 nmol or 20 ng/ml.
The American Geriatric Society (AGS) published a new consensus statement on vitamin D and calcium supplementation for reduction of falls and fractures in adults 65 years and older and for high-risk populations with malabsorption syndromes, those using medications that accelerate vitamin D metabolism, the obese, and those with minimal sun exposure.
Vitamin D sufficiency is defined as levels at 75 nmol/L, or 30 ng/mL. Serum levels even higher at 90 to 100 nmol/L (36 to 40 ng/ml) are recommended by some. Optimal levels of 25(OH)D 3 have not been defined, and the measurement of serum levels lacks standardization and calibration.
A vitamin D deficiency may be due to inadequate dietary intake, inadequate exposure to sunlight, or malabsorption. Deficiency of vitamin D also can lead to secondary malabsorption of calcium. Calcium malabsorption occurs in chronic renal failure because renal hydroxylation is required to activate vitamin D, which promotes synthesis of a calcium-binding protein in intestinal absorptive cells.
A vitamin that can dissolve in fats and oils is called fat-soluble. Vitamins that are fat-soluble are nutrients that the body needs in small amounts to stay healthy and work the way it should. Fat-soluble vitamins are absorbed along with fats in the diet and are stored in the body's fatty tissue and in the liver.
Vitamin D is both a nutrient we eat and a hormone our bodies make. It is a fat-soluble vitamin that has long been known to help the body absorb and retain calcium and phosphorus; both are critical for building bone.
Vitamin D supplements are available in two forms: vitamin D2 (“ergocalciferol” or pre-vitamin D) and vitamin D3 (cholecalciferol). Both are also naturally occurring forms that are produced in the presence of the sun’s ultraviolet-B (UVB) rays, hence its nickname, “the sunshine vitamin,” but D2 is produced in plants and fungi and D3 in animals, including humans.
Vitamin D production in the skin is the primary natural source of vitamin D, but many people have insufficient levels because they live in places where sunlight is limited in winter, or because they have limited sun exposure due to being inside much of the time. Also, people with darker skin tend to have lower blood levels of vitamin D because the pigment (melanin) acts like a shade, reducing production of vitamin D (and also reducing damaging effects of sunlight on skin, including skin cancer).
Vitamin D toxicity is rare, but it has been reported in a few patients taking megadoses of vitamin D. Reported adverse effects include hypercalcemia, hyperphosphatemia, suppressed parathyroid-hormone levels, and hypercalciuria.
Vitamin D’s main job, according to the National Institutes of Health's Office of Dietary Supplements, is to promote calcium absorption, making it necessary for bone growth and bone remodeling (when mature bone tissue is removed and new bone tissue is formed). Because of that, a lack of vitamin D can lead to thin, brittle, or misshapen bones. But vitamin D offers a range of other benefits too, ranging from positives for both physical and mental health.
Vitamin D is famous for its bone-building and strengthening powers. Vitamin D promotes absorption of calcium in your gut, which ultimately allows for normal mineralization of your bones. Basically, the calcium that benefits your bones wouldn't be able to do its job without vitamin D. You need vitamin D for bone growth—and to prevent bones from becoming brittle. When teamed with calcium, it can help prevent osteoporosis, a disease that signifies that the density and quality of bone are reduced.
Along with its bone-building abilities, vitamin D is also influential in strengthening muscles. Lack of vitamin D in the body can increase the risk of having weak muscles, which in turn increases the risk of falls. This is especially important for the elderly. Vitamin D may help increase muscle strength thus preventing falls, which is a common problem that leads to substantial disability and death in older adults.
Vitamin D can also help build immunity. It can support the immune system by fighting off harmful bacteria and viruses. In fact, this role in possibly preventing infections has become a critical concern during COVID-19 pandemic, as researchers are interested in its potential role in infection outcomes. There is particular interest in its role in viral infections such as influenza and coronavirus. There is a study of 25 randomized control trials comparing vitamin D supplements to placebos, which found that vitamin D reduced the risk of acute respiratory infection with either daily or weekly vitamin D supplementation, particularly in individuals who were deficient in it. Studies indicate that high latitudes and winter season are risk factors for both low vitamin D, increased influenza, and other respiratory illness and adverse outcomes. We now are seeing a similar pattern with higher mortality rates in COVID-19 infections, though more research still needs to be done to determine whether the link is causal or merely a correlation.
Because vitamin D helps our body absorb calcium, it plays a crucial role in supporting oral health, lowering the risk of tooth decay and gum disease. A 2011 review in The Journal of the Tennessee Dental Association notes that while the research is scant, there's an "emerging hypothesis" that the vitamin is beneficial for oral health, due to its effect on bone metabolism and its ability to function as an anti-inflammatory agent and stimulate the production of anti-microbial peptides.
While studies are not conclusive, vitamin D may be helpful for preventing both type 1 and type 2 diabetes. One such study, published in 2006 in the journal Diabetes Care, found that while vitamin D on its own did not effectively lower the risk of an overabundance of sugar in the blood, a combined daily intake of >1,200 mg calcium and >800 IU vitamin D could effectively lower the risk of type 2 diabetes.
According to a 2019 review published in the journal of Current Protein & Peptide Science suggests that vitamin D may play a role in treatment of high blood pressure—one of the markers of cardiovascular disease. According to authors of the review, "even short-term vitamin D deficiency may directly raise BP [blood pressure] and promote target organ damage.” The researchers went on to add that, "due to the high correlation between vitamin D and hypertension, vitamin D supplementation therapy may be a new insight in the treatment of hypertension."
Obesity is a known risk factor for low vitamin D levels—which means more vitamin D may help with weight loss. One 2009 study in the British Journal of Nutrition found that, in overweight or obese women with low calcium levels, those who took a daily dose of calcium paired with vitamin D were more successful shedding pounds than those who took a placebo supplement, due to an "appetite-suppressing effect" of the combination.
The sun can brighten up your mood, and so can vitamin D. According to a 2017 review article in the journal Neuropsychology, researchers found "a significant relationship between depression and vitamin D deficiency." While they acknowledged that more research is needed to define the exact workings of it—such as, if low vitamin D levels are a cause or effect of depression—the authors recommend "screening for and treating vitamin D deficiency in subjects with depression" noting that it is an "easy, cost-effective and may improve depression outcome."
The National Cancer Institute's (NCI) provides some evidence that vitamin D may have cancer fighting powers. Evidence is increasing that vitamin D supplementation may improve cancer outcomes. The cancers for which the most human data are available are colorectal, breast, prostate, and pancreatic cancer.
The NCI specifically calls out a few reasons why researchers are interested in a link between vitamin D and a lowered risk of cancer. The organization points out that some research shows that incidence and death rates for certain cancers were lower among individuals living in southern latitudes, where levels of sunlight exposure are relatively high, than among those living at northern latitudes, though additional research needs to be done to find a specific causal or correlational link between more sunlight exposure and a lower risk of cancer.
More experimental evidence, per the NCI, shows, that cancer cells and of tumors in mice, vitamin D has been found to have several activities that might slow or prevent the development of cancer cells and tumors in mice, including promoting cellular differentiation, decreasing cancer cell growth, stimulating cell death (apoptosis), and reducing tumor blood vessel formation (angiogenesis).
Despite being readily available through sunlight, some foods, and supplementation, many Americans are still getting inadequate amounts of vitamin D—according to the most recent information from the Centers for Disease Control and Prevention.
If you feel you’re deficient or lacking in vitamin D intake, there are a few keyways you can up your daily dosage—staring with getting around 20 minutes of sunlight several times a week. The major cause of vitamin D deficiency is inadequate exposure to sunlight, an increasing feature in modern life. But remember: You still need to wear sunscreen whenever you step outside—even to get vitamin D.
Aside from the sun, you can also get extra vitamin D through a few (albeit very few) foods, like fatty fish (including salmon, tuna, mackerel, and sardines) and mushrooms (some of which are exposed to ultraviolet (UV) light to increase vitamin D levels).
Few foods are naturally rich in vitamin D3. The best sources are the flesh of fatty fish and fish liver oils. Smaller amounts are found in egg yolks, cheese, and beef liver. Certain edible mushrooms like maitake and shiitake provide vitiam D2 as well, and have also been used as medicine throughout history.
Other mushrooms that are too tough to eat have been used solely for medicinal purposes such as reishi. Plant chemicals and components in mushrooms may exert antioxidant, anti-inflammatory, and anticancer effects, but the exact mechanism is still unclear and an area of active research.
Animal and cell studies show that mushrooms can stimulate the activity of immune cells, macrophages, and free radicals that can stop the growth and spread of tumor cells and cause existing tumor cells to die. In addition, some commercially sold mushrooms contain higher amounts of D2 due to intentionally being exposed to high amounts of ultraviolet light. Many foods and supplements are fortified with vitamin D like dairy products and cereals.
Foods like milk, orange juice, yogurt, and breakfast cereals can also be fortified with vitamin D. And of course, you can always go the supplement route, in the form of vitamin D3.
As for when to take it, because vitamin is fat soluble, pairing it with your largest meal of the day is suggested.