The only difference between vitamin D2 and D3 is their side-chain structure. Both forms need to undergo conversion by the liver and kidneys to become active vitamin D (calcitriol). Research suggests that vitamin D3 is more effective than vitamin D2 in raising serum 25(OH)D levels. Therefore, vitamin D3 might be the preferred choice for vitamin D supplementation.
Vitamin D is an essential nutrient required by all vertebrates, including humans. Apart from dietary intake, the body can also produce vitamin D through sunlight exposure. Its primary function is to maintain blood calcium and phosphate levels, thus supporting normal bone structure, muscle contractions, and cellular functions. It is an essential vitamin for overall human health [1].
Current Vitamin D Intake Status
Before 1890, it was noted that children who grew up in Europe along the coastal areas with abundant sunlight and fresh air had less delayed bone development and fewer cases of skeletal deformities compared to children living in urban areas [2]. However, the modern way of life, with increased indoor time and reduced sunlight exposure, has led to widespread vitamin D deficiency worldwide, becoming a global health issue. High-risk groups include the elderly and pregnant women [3].
What Is Vitamin D?
The unique role and sources of vitamin D set it apart from other vitamins. Vitamin D can be categorized into D2 and D3, which have different sources.
- Vitamin D2 Source: Ergocalciferol (vitamin D2) is derived from “ergosterol” through exposure to ultraviolet light. Ergosterol is a plant-based compound found in certain fungal sources [5].
- Vitamin D3 Source: Cholecalciferol (vitamin D3) is commonly extracted from animal source “lanolin.” It is formed from 7-dehydrocholesterol through the action of UVB radiation, eventually converting to active vitamin D (calcitriol).
Both vitamin D2 and D3 are not active forms of vitamin D. The only difference between them is their side-chain structure. They need to undergo conversion by the liver and kidneys to become active vitamin D (calcitriol) [6]. Research suggests that vitamin D3 is more effective than vitamin D2 in increasing serum 25(OH)D levels, making vitamin D3 a potential preferred choice for supplementation [7].
Issues of Vitamin D Deficiency
Vitamin D deficiency is associated with various acute and chronic diseases, including rickets, osteomalacia, childhood caries, periodontitis, autoimmune diseases, cardiovascular diseases, cancer, type 2 diabetes, and neurological disorders [4]. In case of vitamin D deficiency, improving intake through vitamin D supplementation and moderate sunlight exposure is recommended [3].
| Serum 25(OH)D concentration nmol/L | Health effects |
| 30-50 | Maximizes calcium absorption |
| <30 | Increased risk of rickets |
| <40 | Increased fracture risk |
| <50 | Osteomalacia risk |
Benefits of Vitamin D
1. Enhanced Calcium Absorption and Maintenance of Blood Calcium Balance
The primary function of vitamin D is to regulate the absorption of calcium in the intestines, maintaining bone mineralization and skeletal health. Without sufficient vitamin D, the body absorbs only 10% to 15% of dietary calcium. In contrast, when vitamin D levels are adequate, intestinal calcium absorption increases to 30% to 40%, underscoring its critical role in calcium absorption [8].
Vitamin D also plays a pivotal role in blood calcium balance by facilitating calcium absorption in the small intestine and mediating bone calcium release, thus maintaining overall blood calcium levels [9].
2. Promotion of Bone and Dental Health
Osteoporosis is often linked to inadequate calcium intake, but insufficient vitamin D intake can hinder calcium absorption, leading to osteoporosis [10]. A retrospective analysis demonstrated that supplementing with vitamin D significantly reduces the risk of hip fractures (by 18%) and non-vertebral fractures (by 12%) when taken together with calcium. Therefore, vitamin D supplementation is recommended alongside calcium intake to significantly decrease the risk of hip fractures [11].
The U.S. FDA also advises simultaneous consumption of vitamin D and calcium to reduce the risk of osteoporosis in elderly individuals and improve bone health in postmenopausal women [12]. Furthermore, vitamin D benefits oral health by not only promoting dental mineralization but also reducing oral inflammation, particularly in the elderly population, which is at high risk of calcium and vitamin D deficiency [13].
3. Immune System Support
Although the mechanisms behind vitamin D’s immune-regulatory effects are still under investigation, it is understood to modulate inflammation by regulating monocytes and T cells and suppressing antigen presentation by dendritic cells. As a result, vitamin D helps regulate the immune system [14]. Numerous studies have confirmed the beneficial impact of vitamin D on autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, and lupus erythematosus [15].
A clinical study examining the association between vitamin D intake and rheumatoid arthritis incidence found that higher vitamin D intake correlated with a lower risk of rheumatoid arthritis in elderly women [16].
4. Insulin Regulation and Diabetes Control
Research has shown that vitamin D deficiency increases the risk of both type 1 and type 2 diabetes. Vitamin D deficiency has been linked to impaired insulin synthesis and secretion, particularly significant in the onset of type 2 diabetes. Supplementing with vitamin D can improve insulin sensitivity in adults and aid in blood sugar control [17].
Clinical studies have indicated that vitamin D deficiency leads to reduced insulin secretion by pancreatic β-cells and lowered glucose tolerance. The Third National Health and Nutrition Examination Survey (1988-1994) revealed that lower vitamin D levels were associated with higher fasting blood glucose and higher post-oral glucose tolerance test glucose levels, indicating an adverse effect of vitamin D deficiency on blood sugar management [18].
5. Anticancer Effects
Recent research has discovered that vitamin D possesses anticancer properties, exerting various influences on cancer development and progression. Vitamin D inhibits the growth of cancer cells, while its deficiency promotes cancer cell proliferation [19].
A study investigating the link between vitamin D deficiency and colon cancer used two groups of animals for a three-month experiment. One group was fed a diet lacking vitamin D, while the other was fed a diet containing vitamin D. Both groups were injected with colon cancer cells (MC-26) and had their tumor sizes measured daily for 20 days. The results showed that the group supplemented with vitamin D exhibited significantly smaller tumors (a 40% reduction) compared to the vitamin D-deficient group, demonstrating that vitamin D deficiency indeed promotes colon cancer growth [20].
6. Prevention of Respiratory Infections
Vitamin D has a positive impact on the immune system and infection prevention. A retrospective study analyzed 25 studies (11,321 participants) to evaluate the overall impact of vitamin D supplementation on the risk of acute respiratory infections. The results showed that vitamin D supplementation significantly reduced the risk of acute respiratory infections, particularly in individuals who were vitamin D deficient or had never supplemented with vitamin D before [21].
Another clinical study found that vitamin D-deficient patients who received vitamin D supplementation had higher levels of TGF-β in their blood 28 days after receiving a flu vaccine, indicating an improved immune response in the group supplemented with vitamin D [22].
7. Relationship with COVID-19
Due to vitamin D’s role in mediating respiratory infections, it is believed to prevent inflammation and accelerate the recovery of lung tissues in COVID-19 patients [23]. Research has indicated a link between vitamin D nutritional status and COVID-19 mortality rates. Vitamin D deficiency might increase the risk of higher COVID-19 mortality rates. Vitamin D assists in the production of antimicrobial peptides in the upper respiratory tract, reducing the risk of viral infection and symptom development. Vitamin D may also help reduce post-SARS-CoV-2 infection inflammation [24].
Recommended Vitamin D Intake
According to the recommendations from the 8th edition of the National Health and Nutrition Examination Survey in Taiwan, the recommended daily intake of vitamin D for individuals can be found in the following table [25]:
| Ministry of Health and Welfare Chinese Dietary Reference Intakes (DRIs) | ||
| Age | Vitamin D intake | Maximum Intake |
| 0-1 years old | 400 IU (10 micrograms) | 1000 IU (25 micrograms) |
| 1-50 years old | 400 IU (5 micrograms) | 2000 IU (50 micrograms) |
| 51+ | 600 IU (15 micrograms) | |
| Pregnant and breastfeeding women | 400 IU (5 micrograms) | |
Which foods contain vitamin D?
Animal foods: oil-rich fish, high-fat fish, red meat, liver, milk.
Plant foods: brown mushrooms, shiitake mushrooms, fungus and other mushrooms that have been exposed to light.
| food/100g | Vitamin D Dosage |
| Wet black fungus | 1800-2000 IU |
| Brown mushroom, Maitake mushroom | 1000-1300IU |
| Mackerel | 800-1000 IU |
| Salmon | 600-900 IU |
| egg | 300IU |
| Milk (full fat) | 50IU |
The primary source of vitamin D in the human body (approximately 80-90%) is synthesized by our skin through exposure to sunlight. Moderate sun exposure of 10-30 minutes per day (depending on sunlight intensity and sunscreen use) can provide the basic required amount of vitamin D[26][27].
When should vitamin D be taken? How to effectively supplement vitamin D?
Vitamin D is a fat-soluble vitamin, so it is recommended to take it after a meal, and to choose the D3 form of supplementation, as it has better absorption and bioavailability[27]. Clinical studies have found that vitamin D3 is more effective in increasing the concentration of vitamin D in the blood compared to vitamin D2[28]. Therefore, when choosing a vitamin D supplement, opting for the D3 form is more beneficial for its effectiveness[11].
Further reading: How to Choose Vitamin D? Experts Highlight 4 Key Points, Recommendations and Comparison of Top 5 Vitamin D Brands
Is there an upper limit for vitamin D intake? Are there side effects from consuming too much?
According to the guidelines of the Health Promotion Administration, Ministry of Health and Welfare in our country, the upper limit of vitamin D intake is 2000 IU (50 micrograms) per day[25]. Consuming excessive amounts of vitamin D can lead to hypercalcemia, where there is an excessively high level of calcium in the blood. This phenomenon occurs because vitamin D promotes calcium absorption in the intestinal cells and regulates the reabsorption of bone minerals[26].
However, in general, under normal vitamin D supplementation, safety is assured[21]. The Mayo Clinic in the United States indicates that if you consume more than 4000 IU (100 micrograms) of vitamin D daily, potential side effects may include nausea, vomiting, loss of appetite, weight loss, constipation, heart rhythm issues, kidney stones, and kidney damage. However, this dose is double the recommended upper limit and falls into the category of extreme cases[29]. If you have concerns about the dosage of supplementation, it is advisable to consult with your doctor or a registered dietitian.
