Vitamin D is an essential nutrient for people of all ages and with growing awareness of vitamin D’s health impacts, supplementation has increased in recent years. Yet, many individuals looking to optimize their vitamin D levels face a common dilemma: the uncertainty around supplement dosage and frequency. This article will explore the current evidence around vitamin D supplementation strategies to optimize efficacy.
What is Vitamin D?
Vitamin D is both a nutrient we eat and a hormone we can make endogenously through exposure to sunlight. The main role of vitamin D is to promote calcium absorption in the gut and maintain adequate serum calcium and phosphate concentrations to optimize bone development and maintenance. Vitamin D plays many other lesser-known roles in the body and is required for cellular growth, muscle and nerve function, and immune function.
Vitamin D is found in two primary forms: D2 (ergocalciferol), which comes from plant sources, and D3 (cholecalciferol), which comes from animal sources. When the skin produces vitamin D from sunlight, it is in the form of D3. Both forms are well absorbed and utilized, yet studies show that D3 increases serum vitamin D levels more effectively than D2. Vitamin D absorption in the gut is also enhanced with the concurrent presence of dietary fat.
Why is Vitamin D Important?
Other non-skeletal health outcomes associated with vitamin D include certain types of cancer, cardiovascular disease (CVD), depression and mood disorders, multiple sclerosis, and autoimmune diseases. Although observational studies see strong connections between vitamin D levels and the reported health outcomes, clinical trials are still inconclusive.
Recommended Levels of Vitamin D
Serum concentration of 25(OH)D is the primary indicator of vitamin D status, reflecting vitamin D produced endogenously and obtained from dietary intake. Concentrations are reported in nanomoles per liter (nmol/L) and nanograms per milliliter (ng/mL). While there is still controversy around vitamin D clinical cut-off points, there are generally accepted values to note. The United States Endocrine Society, the National and International Osteoporosis Foundation, and the American Geriatric Society define vitamin D deficiency as 25(OH)D of less than <30 ng/mL. Yet other commonly cited cut-off points include the following:
- Deficient: <30 nmol/L (<12 ng/mL)
- Insufficient: 30 to <50 nmol/L (12 to <20 ng/mL)
- Sufficient: >50 nmol/L (>20 ng/mL)
- High: >125 nmol/L (>50 ng/mL) is linked to potential adverse health effects.
Recommended Intake of Vitamin D
Vitamin D intake recommendations are provided in the Dietary Reference Intakes (DRIs). Recommended intakes are historically set to indicate daily intake levels required to avoid disease. Therefore, in the case of vitamin D, the recommended intake is established to mark the daily amounts needed to maintain bone health and normal calcium metabolism in healthy people. Vitamin D amounts can be listed as micrograms (mcg) or international units (IUs) and varies based on age. The Recommended Dietary Allowance (RDA) for vitamin D is 400-800 IUs (10-20 mcg), which should meet the needs of 97-98% of healthy individuals.
Yet, vitamin D intakes required to maintain optimal serum vitamin D concentrations will vary among individuals based on a wide range of factors such as age, skin tone, geographic location, and levels of obesity, among others. This is demonstrated in a 2019 systematic review and meta-analysis that found vitamin D supplementation needs to reach sufficient 25(OH)D concentrations varied among those who resided in Europe, North America, Asia, the Middle East, and Africa. Furthermore, many studies show that taking more than the RDA may be linked to greater health benefits. Evidence has demonstrated that the long-term dose to maintain average 25(OH)D serum concentrations of 30ng/ml (75nmol/L) is 800-1000 IU/d.
Vitamin D Sources
Vitamin D is found naturally in limited foods like fatty fish (trout, salmon, tuna, mackerel, etc.), beef liver, egg yolks, and mushrooms exposed to ultraviolet light. Fortified foods, such as dairy products, and plant-based alternatives, such as soy, almond, and oat milk, are often available within the U.S. More foods that may be vitamin D fortified include breakfast cereals and some orange juice, yogurt, and margarine brands.
Most people worldwide meet some of their vitamin D needs from sun exposure. UVB radiation with a wavelength of approximately 290-320 nanometers is required to penetrate uncovered skin to convert 7-dehydrocholesterol to pre-vitamin D3, which becomes vitamin D3. Experts suggest that 5-30 minutes of sun exposure to unprotected skin between 10 a.m. and 4 p.m. at least twice a week is required for sufficient vitamin D synthesis. However, several factors can affect the effectiveness of UVB exposure, such as the season, time of day, cloud cover, smog or pollution levels, skin melanin content, sunscreen, and age.
Furthermore, UV radiation is a carcinogen, and UV exposure can increase the risk of skin cancer. Therefore, providing specific guidelines on how much sun exposure is required and safe for everyone is problematic.
Since vitamin D food sources are limited and excessive exposure to UV radiation can increase the risk of sunburn or other health concerns, supplementation is a safe and practical way to achieve optimal vitamin D concentrations across all ages. However, vitamin D supplementation strategies vary across current literature, with some studies supplementing daily while others supplement weekly or monthly. Is there a superior supplementation strategy to optimize serum vitamin D levels?
Vitamin D Supplement Dosing Strategies
Vitamin D and 25(OH)D exhibit half-lives in the body of months and weeks, suggesting it is well suited to longer dose intervals if desired. Therefore, many have questioned whether daily dosing is required to achieve optimal vitamin D levels or if more prolonged periods can go between doses. Some patients prefer less frequent dosing, and data shows better adherence. This was demonstrated in a 2006 review that found that once-weekly dosing was associated with improved patient adherence to therapy compared to once-daily dosing.
A 2008 study set out to determine whether the daily, weekly, or monthly use of the same cumulative dose of vitamin D3 would produce similar biological responses. Patients were randomly allocated to one of three identical cumulative doses of vitamin D3, given as 1,500 IU daily, 10,500 IU once weekly, or 45,000 IU once every 28 days. Serum 25(OH)D concentrations changed significantly by day 7 in all groups.
While there were initial differences in serum 25(OH)D concentrations among the groups, there were no significant longer-term differences between the dosing regimens. The researchers concluded that if the cumulative vitamin D dose and the vehicle of administration are the same, then similar serum 25(OH)D concentrations will be attained over time.
A 2018 study compared whether a cumulative dose of vitamin D3 produces the same effects on the serum concentration of 25(OH)D if given daily or monthly. Sixty subjects were randomly assigned to receive 2,000 IU of vitamin D3 daily or 50,000 IU monthly, and serum 25(OH)D concentrations were measured at baseline to day 75. The researchers concluded that monthly administration of 50,000 IUs is an effective tool for rapidly normalizing 25(OH)D in deficient subjects. Still, daily administration of the same cumulative dose is similarly effective, but it takes longer to reach desirable levels.
Similar conclusions were found in a 2023 meta-analysis of randomized controlled trials. This study aimed to assess the effect of intermittent (weekly and monthly) vitamin D supplementation compared to daily supplementation on improving circulating 25(OH)D levels under similar dosage and duration conditions. After assessing 116 trials, it was determined that intermittent and daily vitamin D supplementation has similar efficacy in improving circulating 25(OH)D levels. Additionally, the researchers concluded that individuals should take at least 60,000 IU of vitamin D supplementation monthly (~2,000 IU daily) to achieve sufficient 25(OH)D concentrations. These recommendations align with the guidance given for adults by the United States Endocrine Society (1,500-2,000 IU per day).
Getting enough vitamin D is necessary for optimal health. Sunlight is an effective yet unreliable source, and vitamin D found naturally in foods is minimal. Therefore, supplementation is a safe and effective alternative.
Vitamin D supplements can be found in D2 (plant-based) or D3 (animal-based) forms. Still, evidence indicates that D3 is the more effective form. Current recommendations suggest consuming 400-800 IU per day; however, several studies show that higher levels (1,000-4,000 IU/day) may be needed for individuals to meet optimal serum vitamin D concentrations (>30 ng/mL or >75 nmol/L).
Supplement frequency can vary based on personal preference, and recent evidence indicates that higher monthly doses (60,000 IU) or daily doses (2,000 IU) are just as effective under similar dosage and duration conditions. Vitamin D calculators are available to determine how much you need to reach your target level if you seek an individualized dose.