Sunlight, Supplements, and Your Health: Vitamin D

Researcher: Dishita Ghai

Research Editor: Sahij Sidhu As winter approaches and days get shorter, it is common for vitamin D levels to drop. Here’s how vitamin D impacts our health and why it’s important to make sure you’re getting enough.

What is Vitamin D?

Vitamin D is a micronutrient and hormone important for our health (Sanlier & Guney-Coskun, 2022). We have all heard about staying out in the sun to get some vitamin D or eating foods rich in it. But the vitamin D we get from these sources is not active right away. It needs to be converted inside our body before it can do its job. 

Most of our vitamin D actually comes from our skin (Benedik, 2022). When ultraviolet B (UVB) rays from sunlight hit the skin, they convert a molecule called 7-dehydrocholesterol into vitamin D₃, also known as cholecalciferol (Benedik, 2022; Goltzman, 2018). We can also get vitamin D from food in the form of vitamin D₃ (from animal sources) and vitamin D₂ or ergocalciferol (from plant sources), both of which are absorbed in the intestine.

Once in the bloodstream, these forms of vitamin D travel to the liver, where they are transformed into calcidiol, also called 25-hydroxy vitamin D (Benedik, 2022). The next stop is the kidney, where calcidol is converted to the bioactive form called calcitriol. Now, vitamin D is ready to get to work. 

Figure 1

Overview of Vitamin D Synthesis and Activation.

Ultraviolet B (UVB) rays from sunlight convert 7-dehydrocholesterol in the skin into vitamin D₃, which then undergoes conversion in the liver and kidney to its active form, calcitriol. Dietary sources such as mushrooms and meat also contribute to vitamin D intake. Adapted from Mostafa and Hegazy (2015).

Vitamin D’s Role in Our Body

Vitamin D serves many important functions in our body, ranging from bone and muscle health to immune function (National Institutes of Health, 2025). Here are some key roles of Vitamin D:

• Increased Calcium and Phosphate Levels

Vitamin D increases the blood levels of calcium and phosphate by increasing uptake from the intestine and reabsorption from the kidneys (Goltzman, 2018; DeLuca, 2004). In cases of severe calcium deficiency, vitamin D can stimulate the bones to release calcium into the bloodstream (DeLuca, 2004).  

• Bone Health 

Vitamin D promotes bone health by increasing calcium and phosphate absorption, which are required for bone mineralization (Goltzman, 2018; DeLuca, 2004). Moreover, vitamin D also plays a key role in bone turnover by impacting bone formation and breakdown (Goltzman; 2018).

• Immune Function 

Vitamin D can boost the innate immune system and reduce the adaptive immune response (Sanlier & Guney-Coskun, 2022). It reduces inflammatory activity while promoting natural immunity by turning immune cells monocytes into macrophages and increasing anti-microbial cathelicidin molecules.  

Vitamin D deficiency is linked to several health issues such as osteomalacia, osteoporosis, muscle weakness and atrophy, cardiovascular disease, asthma, multiple sclerosis, and many more (Sahota, 2014).

Figure 2

Food Sources of Vitamin D.

Examples of foods that naturally contain or are fortified with vitamin D include salmon, eggs, milk, and cheese. Image from Canva.

Where can you get Vitamin D from?

Vitamin D might be easy to overlook, but getting enough is an important part of maintaining good health. Here are some simple ways to keep your levels up: 

1. Safe levels of sun exposure. 

Spending time in the sun allows the body to naturally produce vitamin D. One recommendation is to spend 5 to 30 minutes in the sun between 10 a.m. and 4 p.m. (National Institutes of Sciences, 2025). However, it is important to be safe and take precautions like limiting time in direct sunlight during peak hours and using sunscreen to reduce the risk of skin conditions, such as cancer. 

2. Eat Vitamin D-rich foods. 

Add foods that naturally contain or are fortified with vitamin D to your meals. Animal sources high in vitamin D include fatty fish, fish liver oils, and egg yolks, while plant-based options include mushrooms and dark chocolate (Benedik, 2022). In Canada, milk, yogurt, margarine, and plant-based milks are also fortified with vitamin D (Health Canada, 2025).

3. Consider supplements. 

Oftentimes, sunlight and food aren’t enough to meet your daily needs. That is when supplements can help. Health Canada (2022) recommends a daily dose of 400 IU (international units) if you don’t get enough from your diet. However, it is easy to take too much vitamin D from supplements, which can lead to toxicity and health issues (Benedik, 2022; National Institutes of Health, 2025). If you are thinking about supplements and want to know more, talk to your healthcare provider to find the right amount for you.

References

Benedik, E. (2022). Sources of vitamin D for humans. International Journal for Vitamin and Nutrition Research, 92(2), 118–125. https://doi.org/10.1024/0300-9831/a000733

DeLuca, H. F. (2004). Overview of general physiologic features and functions of Vitamin D. The American Journal of Clinical Nutrition, 80(6), 1689S-1696S. https://doi.org/10.1093/ajcn/80.6.1689s

Goltzman, D. (2018). Functions of vitamin D in Bone. Histochemistry and Cell Biology, 149(4), 305–312. https://doi.org/10.1007/s00418-018-1648-y

Health Canada. (2022, May 2). Vitamin D. Government of Canada. https://www.canada.ca/en/health-canada/services/nutrients/vitamin-d.html

Health Canada. (2025, September 3). Fortified foods. Government of Canada. https://www.canada.ca/en/health-canada/services/fortified-food/canadas-approach.html

Mostafa, W. Z., & Hegazy, R. A. (2015). Vitamin D and the skin: Focus on a complex relationship: A Review. Journal of Advanced Research, 6(6), 793–804. https://doi.org/10.1016/j.jare.2014.01.011

National Institutes of Health. (2025, June 27). Vitamin D: Fact sheet for health professionals. U.S. Department of Health and Human Services. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

Sahota, O. (2014). Understanding vitamin D deficiency. Age and Ageing, 43(5), 589–591. https://doi.org/10.1093/ageing/afu104

Sanlier, N., & Guney-Coskun, M. (2022). Vitamin D, the immune system, and its relationship with diseases. Egyptian Pediatric Association Gazette, 70(1), 39. https://doi.org/10.1186/s43054-022-00135-w