Vitamin D is an essential nutrient that plays many important roles in the body. It helps regulate calcium and phosphate levels to maintain healthy bones. Vitamin D also supports immune function, cardiovascular health, and much more. Despite its importance, vitamin D deficiency is extremely common worldwide. An estimated 1 billion people have low levels of this critical vitamin. So why are so many people deficient in vitamin D? There are several potential reasons.
Limited sun exposure
Our bodies synthesize vitamin D from sunlight. UVB rays from the sun trigger vitamin D production in the skin. Just 15-30 minutes of midday sun exposure can satisfy vitamin D needs for lighter-skinned people. Those with darker skin need a bit more time in the sun. However, many people today spend less time outdoors. Some reasons for limited sun exposure include:
Working indoors
The shift towards more office and indoor jobs over the past century means less sun exposure for many people. Most individuals are stuck inside for much of the daytime, five days a week or more. Getting adequate sun exposure can be challenging with an indoor 9-to-5 office job.
Sunscreen use
Sunscreen is essential for skin cancer prevention. But it also blocks the UVB rays required for vitamin D synthesis. With proper use, just a single application can block over 95% of vitamin D production. More vigilant sunscreen use over the decades has likely contributed to lower vitamin D levels.
Pollution and clouds
Areas with high air pollution tend to have greater rates of vitamin D deficiency. Pollution clouds block some of the sun’s UVB rays. Overcast and cloudier climates also lead to less UVB exposure overall. Places like the Pacific Northwest have very high vitamin D deficiency rates, likely due to low sunlight exposure.
Living in northern latitudes
Above and below latitude lines of around 33°, vitamin D production drops off substantially during the winter. Places far from the equator don’t get enough UVB exposure for several months of the year. Nearly everyone living in Alaska, Canada, Scandinavia, and other northern regions is at high risk for wintertime vitamin D deficiency.
More time spent indoors
Children spend less time playing outdoors today compared to past generations. Most kids get only a few hours per week of outdoor play. Adults also tend to spend more leisure time inside browsing the internet, playing video games, watching TV, etc. Reduced outdoor time means less sun exposure and lower vitamin D levels.
Inadequate vitamin D from diet
While sun exposure is the main source, you can also get vitamin D from certain foods. Oily fish like salmon and mackerel contain decent amounts. Smaller amounts are present in egg yolks, certain mushrooms, and fortified foods like milk and cereal. However, it’s difficult to get enough dietary vitamin D to meet needs. Very few foods are naturally high in vitamin D, and fortified foods aren’t consumed frequently or consistently enough by most people.
Plant-based diets
Strict plant-based diets make it much more challenging to get adequate vitamin D. The nutrient is only found naturally in animal foods like fish, eggs, liver, and butter. While plant milks and cereals can be fortified, vitamin D intake remains low for vegans and vegetarians. Getting sufficient sun is critical for those avoiding animal foods.
Limited seafood intake
Fatty fish like salmon and mackerel are among the few natural food sources of vitamin D. But seafood consumption has declined over the decades in many countries. Cost issues, lack of availability, and concerns about contaminants have all contributed to reduced fish intake. With less vitamin D-rich seafood in people’s diets, deficiency risk increases.
Low consumption of fortified foods
In many countries like the United States and Canada, milk, cereal, orange juice and other foods are fortified with vitamin D. However, intake of these foods is often inadequate. For example, less than 15% of Americans eat the recommended three servings of fortified dairy per day. Daily consumption of fortified cereals is also low. Without adequate intake of fortified foods, it’s difficult to meet vitamin D needs through diet alone.
| Food | Vitamin D Content (IU per serving) |
|---|---|
| Wild salmon, 3.5 ounces | 600-1000 |
| Sardines, 1.75 ounces | 250 |
| Egg yolk | 20-25 |
| Fortified milk or plant milk, 1 cup | 100 |
| Fortified cereal, 3/4-1 cup | 40-50 |
| Shiitake mushrooms, cooked, 1/2 cup | 10 |
Obesity
Excess body fat causes vitamin D to become sequestered in fat cells. This makes it less bioavailable for use in other tissues and processes. Obese individuals require higher vitamin D intakes to reach optimal blood levels. But dietary sources and sun exposure don’t always provide enough. The high rates of obesity seen worldwide may be contributing to low vitamin D status.
Aging
As we age, our skin becomes less efficient at producing vitamin D from UVB sun exposure. The kidneys also become less able to convert vitamin D to its active form. Older adults are prone to spending more time indoors and eating poorer diets as well. For all these reasons, vitamin D requirements increase with age while vitamin D status tends to decrease.
Digestive disorders
Certain gastrointestinal conditions can impair vitamin D absorption. These include:
Celiac disease
This autoimmune disorder damages the small intestine, leading to impaired absorption of fat-soluble vitamins like D. Supplementation is often needed to correct deficiency.
Crohn’s disease
Chronic inflammation from Crohn’s disease can disrupt vitamin D absorption, especially in the small intestine. Correcting deficiencies is an important part of disease management.
Ulcerative colitis
Those with ulcerative colitis tend to have lower vitamin D levels. Absorption issues, use of corticosteroids, and less time spent outdoors all may contribute.
Gastric bypass surgery
Surgeries like gastric bypass often cause vitamin D deficiency due to reduced intestinal absorption area and changes in gut hormones involved in vitamin D metabolism.
Kidney and liver disease
Diseased kidneys have difficulty converting vitamin D to its active form. Various liver disorders can also imped vitamin D activation and metabolism. People with chronic kidney and liver disease usually need vitamin D supplementation to prevent severe deficiencies.
Certain medications
A variety of commonly prescribed drugs are linked to vitamin D deficiency, including:
Blood pressure medications
Thiazide diuretics like hydrochlorothiazide increase calcium excretion in urine, which raises vitamin D requirements. Loop diuretics like furosemide impair kidney function, reducing D activation.
Epilepsy drugs
Medications like phenytoin, phenobarbital, and carbamazepine speed up vitamin D breakdown and can cause deficiency.
Glucocorticoids
Corticosteroids like prednisone partially suppress vitamin D activity and increase loss of D from the body. Long-term use raises deficiency risk.
HIV medications
HIV drugs can lead to vitamin D deficiency through various mechanisms including impaired D activation in the liver and kidneys.
Weight loss drugs
Orlistat reduces intestinal absorption of vitamin D and other fat-soluble nutrients, sometimes significantly. Deficiencies may develop over long-term use.
Limited sun exposure in infants and breastfed babies
Newborns and young infants are at very high risk for vitamin D deficiency. They have lower vitamin D stores at birth and their vitamin D requirements per pound of body weight are extremely high. However, most infants get minimal sun exposure. When combined with low levels of vitamin D in breastmilk, severe deficiencies often develop in exclusively breastfed babies.
Not enough sun exposure
For cultural and safety reasons, parents try to minimize sun exposure in infants. However, this means babies miss out on UVB rays required to synthesize vitamin D.
Low vitamin D in breastmilk
Breastmilk contains only about 25 IU of vitamin D per liter. This meets less than 10% of an infant’s daily vitamin D requirement. Exclusively breastfed infants often become deficient unless given supplemental D.
No vitamin D supplements
Leading health organizations recommend all breastfed infants take 400 IU of supplemental vitamin D daily. However, many parents are unaware of this guideline. Not giving breastfed babies any D supplements puts them at high deficiency risk.
| Age | Vitamin D RDA (IU/day) |
|---|---|
| 0 – 6 months (breastfed) | 400 |
| 6 – 12 months (breast and/or formula fed) | 400 |
| 1 – 3 years | 600 |
| 4 – 8 years | 600 |
Genetic factors
Research shows genetics play a role in vitamin D status. Certain gene variants linked to vitamin D deficiency include:
Vitamin D binding protein
This protein binds and transports vitamin D in blood. Variants causing lower binding protein levels are linked to lower total D levels.
Vitamin D receptor
This receptor mediates vitamin D’s effects on target cells. Variants impairing receptor function may reduce vitamin D activity.
7-dehydrocholesterol reductase
This enzyme aids in vitamin D synthesis. Impaired enzyme function due to gene variants affects D production.
Cytochrome P450 enzymes
These liver enzymes help convert vitamin D to its active form. Variants reducing their activity can impair D activation.
While genes play a part, environmental factors like sun exposure and diet still have a stronger influence on vitamin D status. But genetic differences may partially explain why some people seem prone to deficiency despite adequate intakes and sun exposure.
Conclusion
Many different factors can cause low vitamin D levels. Limited sun exposure and inadequate dietary intake are the main reasons behind most cases of deficiency. Groups at highest risk include those living in northern regions, older adults, people with obesity, exclusively breastfed infants, and those with digestive disorders, kidney disease, or certain genetic traits. Given how common deficiency is, measuring vitamin D levels and addressing any shortfalls with supplementation is wise for most people today. Maintaining adequate vitamin D is crucial for supporting bone health and many other aspects of good health.