Vitamin D is often hailed as a vital nutrient for overall health and well-being. However, over 47% and 76% of people globally are still deficient or insufficient in vitamin D. Therefore, many question whether vitamin D supplementation is necessary to optimize health.
While some experts argue that individuals can meet their vitamin D needs through natural sunlight and a balanced diet, others believe supplementation is required to bridge the vitamin D gap. With conflicting guidance, examining the evidence and considering individualized circumstances is necessary before picking up a vitamin D supplement. This article will explore vitamin D roles, recommendations, and unique variables to deliberate when considering vitamin D supplementation.
What is Vitamin D?
Vitamin D is a unique nutrient that is considered both a fat-soluble vitamin naturally present in foods and a pre-hormone produced endogenously when UV light strikes the skin and triggers vitamin D synthesis. Vitamin D is critical in regulating and maintaining calcium and phosphate levels for normal bone growth and remodeling. However, although lesser known, vitamin D receptors are present in most tissues and cells in the body and have a wide range of biological actions pertinent to endocrine, immune, and metabolic functions.
Vitamin D can be found in two primary forms in foods and dietary supplements: vitamin D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D2 is found in plant food sources, like sun-exposed mushrooms. Vitamin D3 is found in some animal-based foods, such as the flesh of fatty fish (trout, salmon, tuna, mackerel, etc.), egg yolks, and fortified dairy products. Although both forms can raise serum vitamin D levels, research indicates that vitamin D3 increases serum levels to a greater extent and maintains these levels for more extended periods of time.
Since vitamin D is found in minimal quantities in foods, most people worldwide meet at least some of their needs through sunlight exposure or supplementation. UV-B radiation can penetrate unprotected skin to convert cutaneous 7-dehydrocholesterol into vitamin D3. Yet, the ability of sun exposure to produce vitamin D can vary based on the season, time of day, cloud cover, air pollution, skin melanin content, and sunscreen use. Supplementation is another method increasing in popularity, and supplements can be found in D2 (plant-based) or D3 (animal-based) forms.
Controversies in Vitamin D Criteria
A central controversy in vitamin D research is defining vitamin D standards and clinical cut-off points. The First International Conference on Controversies in Vitamin D was held in 2017 to address and resolve some of these debates and clarify areas of uncertainty. The reports from this conference suggest that serum total concentration of 25-hydroxyvitamin D (25(OH)D) is considered the best biomarker to define vitamin D status. According to the reports, most guidelines agree that 25(OH)D values below 30nmol/L (12ng/ml) are associated with an increased risk for bone disease and would be considered deficient.
However, not all governing bodies agree with these standards. For example, the United States Endocrine Society, the Institute of Medicine (IOM), and the World Health Organization (WHO) define vitamin D deficiency as a 25(OH)D concentration <50nmol/L (20ng/ml). Reference can be made to a recent paper published by Mason et al., which includes a table that defines up-to-date vitamin D cut-off points based on country for a deeper look at regional standards.
A major source of vitamin D for most humans is sun exposure to the skin. Evidence indicates that for those with light skin tones, ~10 minutes of daily sunlight exposure at lunchtime from March to September could sustain vitamin D levels throughout the year. However, considering the variety of factors that can impact or reduce a person’s skin production of vitamin D, such as skin tone and skin age, dietary intake is also recommended to ensure needs are being met. Below are dietary intake recommendations from the Endocrine Society Clinical Practice Guidelines.
- Infants and children: Ages 0-1 yr require at least 400 IU/d (IU=25ng) of vitamin D; children 1yr and older need at least 600 IU/d to maximize bone health.
- Adults: Ages 19-50 yr require at least 600 IU/d of vitamin D to maximize bone and muscle health.
- Older adults: Adults over 50 yr of age require 600-800 IU/d of vitamin D. It is unclear whether more is necessary to provide all the potential non-skeletal health benefits associated with vitamin D.
- Pregnant and lactating women: 600 IU/d of vitamin D
- Obese children and adults or those on certain medications (anticonvulsants, glucocorticoids, or others) may require two to three times more vitamin D for their age group to satisfy their body’s vitamin D requirements.
- Tolerable upper limits (UL) of vitamin D, which should not be exceeded without medical supervision, should be 1000 IU/d for infants up to 6 months, 1500 IU/d for infants from 6 months to 1 year, 2500 IU/d for children aged 1-3 years, 3000 IU/d for children aged 4-8 years, and 4000 IU/d for everyone over eight years. However, higher levels may be required to correct vitamin D deficiency and should be managed by a healthcare provider.
It’s important to note that there is no global consensus on the definition of vitamin D sufficiency. Furthermore, based on a variety of individual circumstances, vitamin D intake above baseline recommendations may be required to achieve sufficient serum 25(OH)D concentrations.
Vitamin D Deficiency
Several factors can contribute to vitamin D deficiency. Evidence cites the most common reason for low vitamin D levels is inadequate sun exposure, whether from geographic location, shift work, or skin protection from clothes covering or sunscreen use. Furthermore, other factors can reduce the skin’s production of vitamin D, such as aging skin or people with naturally dark skin tones who require at least three to five times longer sun exposure to make the same amount of vitamin D as a person with light skin.
Other causes for vitamin D deficiency include increased body fat levels, alterations to gut function such as fat malabsorption syndrome or bariatric surgery, or patients on various medications that enhance the breakdown of serum 25(OH)D.
The necessity of adequate vitamin D levels for bone health is undebated, as nutritional rickets and osteomalacia represent the most serious diseases of vitamin D deficiency. Yet, several other health conditions linked to low vitamin D levels should also be considered.
According to a 2023 review, vitamin D deficiency can also be linked to fracture risk, muscle atrophy, decreased immune function, increased risk of respiratory tract infection, and cardiometabolic disease. With so many actions, the signs and symptoms of low vitamin D levels can be hard to spot. Some symptoms that may be experienced with low vitamin D levels include fatigue, bone pain, muscle weakness, aches or cramps, and mood changes. Severe lack of vitamin D can lead to incorrect bone growth or bone and joint deformities.
Closing the Vitamin D Gap
With vitamin D intake and status being wildly variable among different populations, the best way to determine if supplementation would be beneficial would be to assess baseline levels. Vitamin D levels can be measured in the blood by assessing serum concentrations of 25(OH)D, which reflects vitamin D produced endogenously and obtained from foods and supplements.
Nutrient testing can be done at home or with a trusted healthcare provider. If testing reveals that 25(OH)D levels are adequate, keep up with what you’re currently doing. If testing shows that 25(OH)D levels are not ideal, it’s time to make a change.
Increasing vitamin D levels can be accomplished through safe and purposeful sunlight exposure or increased intake of food or supplement sources. The United States Endocrine Society recommends 1,500-2,000 IU daily if supplementing with vitamin D. However, suppose you are looking for a more personalized approach. Vitamin D calculators can guide you to the exact IUs needed to reach your target vitamin D levels.
Vitamin D is an important, inexpensive, and safe adjunct therapy for many conditions and stages of life. With the growing awareness of the high prevalence of vitamin D deficiency, vitamin D testing and vitamin D supplementation have increased in recent years. While it is evident that inadequate vitamin D is linked with several acute and chronic conditions, optimal vitamin D cut-off points are still up for debate, making it hard to establish firm recommendations.
The best way to determine if you are at risk for vitamin D deficiency is to test your baseline 25(OH)D levels. From there, you can determine if extra steps are required to close the vitamin D gap, improve health, and minimize disease risk.