What is the Role of Vitamin D in Human Health?

Vitamin D, sometimes known as ‘the sunshine vitamin’, is a fat-soluble vitamin important for bone health, muscle function and the immune system^1-4.  Vitamin D is also being investigated for its role in protecting against some chronic diseases including cardiovascular disease and type-2 diabetes¹.

Vitamin D exists in two primary forms:

1. vitamin D2 (ergocalciferol) which is obtained from plant and fungi sources that have been exposed to UV light.
2. vitamin D3 (cholecalciferol), found in animal-based products such as fatty fish, eggs and liver³.

Both forms of vitamin D are biologically inactive when ingested and are absorbed in the small intestine.  They are transported to the liver and converted into 25-hydroxyvitamin D (25(OH)D), also known as calcifediol or calcidiol, followed by conversion in the kidneys into 1,25-dihydroxyvitamin D (1, 25(OH)2D), or calcitriol, which is the biologically active form used by the body (Figure 1)³.

Studies have shown that vitamin D3 leads to a greater increase of serum 25(OH)D than vitamin D2⁵.

The ‘sunshine vitamin’ gets its name from the fact that it is also produced in human skin from 7-dehydrocholesterol when exposed to sunlight, or more specifically, UVB rays.

Functions

Research shows that vitamin D plays a significant role in bone health, muscle health and the immune system:

1. 1. Bone health: Vitamin D is a critical regulator of calcium absorption.  In its active form, 1,25(OH)2D, it interacts with the vitamin D receptor (VDR) in the small intestine resulting in an increase in calcium and phosphate absorption².  However, calcium homeostasis is primarily regulated to maintain serum calcium within a narrow range for metabolic reasons with the parathyroid gland, bone, intestine and the kidney working together in this role⁴.  Chronic vitamin D deficiency that results in an increase in parathyroid hormone leads to increased bone resorption, compromising the structure of the skeleton and increasing the risk of fracture thus vitamin D sufficiency is important to optimise skeletal health⁴.
   2. Immune health: Vitamin D plays a crucial role in regulating both the innate and adaptive immune responses.  The expression of VDR in many different immune cells has been well demonstrated⁶.  It modulates the activity of immune cells such as B cells, T cells and antigen-presenting cells and promotes a balanced immune response.  Vitamin D deficiency is associated with an increased risk of hospitalisation for respiratory tract infections⁷ and supplementation has been shown to boost antigen-specific immunity in older adults with sub-optimal vitamin D status⁸.  Genetic variation in the VDR genes has also been linked to Vitamin D deficiency and the development of autoimmune disease⁶.
   3. Muscle function: The identification of a VDR in skeletal muscle cells, along with the strong association between vitamin D deficiency, muscle atrophy and sarcopenia, suggests an important role in muscle function⁹.  Proposed mechanisms include modulation of protein synthesis, mitochondrial metabolism and energy production, which may influence performance.  However, the effects of vitamin D3 supplementation on muscle mass, strength and physical performance remain debated, with conflicting findings.  Meta-analyses of randomised controlled trials in athletes have not shown conclusive benefits, highlighting the need for further research^10,11.  In contrast, supplementation has been associated with improved muscle strength in postmenopausal women¹².  Interpretation of trial results should consider whether populations have insufficient or sufficient vitamin D status and whether any supplementation used has increased 25(OH)D levels sufficiently as these factors may influence outcomes¹³.
   4. Mental health: Emerging evidence suggests a link between vitamin D and mental health – its neuroprotective properties may contribute to its role in mental wellbeing, reducing neuroinflammation, supporting serotonin synthesis and improving brain plasticity¹⁴.  Recent meta-analysis suggests no benefit of supplementation in healthy individuals¹⁵ but potential effects on depressive symptoms in those with major depressive disorder or with milder, clinically significant depressive symptoms¹⁴.  However, more high-quality research trials are needed.
   5. Other Potential Roles: Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular function and glucose metabolism¹⁶.  Ongoing research is also exploring the potential benefits on other health conditions including heart disease, diabetes and musculoskeletal diseases like multiple sclerosis¹⁷.

In European Union countries, approved health claims are available for the role of Vitamin D in supporting a wide range of functions including maintaining normal bones and teeth, muscle and immune function as well as supporting absorption of calcium and phosphorous subject to conditions.

In China, claims relating to the health of bones and teeth and the absorption and utilisation of calcium and phosphorus are also available for vitamin D containing foods subject to conditions.

Recommended Intakes

In some countries, national dietary reference tables can lag behind updated clinical practice or more recent Vitamin D supplementation policies.

• • United States:  The Institute of Medicine (IOM) Recommended Dietary Allowance (RDA) for vitamin D is 600 IU (15 micrograms) daily for adults aged 19–70 and 800 IU (20 micrograms) daily for adults over 70 years, assuming minimal sun exposure¹⁸.
  • Europe:  The European Food Safety Authority (EFSA) has set an adequate intake at 15 micrograms per day for healthy individuals over one year of age including pregnant and lactating women to ensure the majority of the population will achieve a serum 25(OH)D concentration near or above the target of 50 nmol/L¹⁹.
  • China: The Chinese Nutrition Society have set a daily Reference Nutrient intake of 400 IU (10 micrograms) for adults aged 18-50 and 600 IU (20 micrograms) for adults over 50 years²⁰.

Dietary Sources

Foods rich in vitamin D include oily fish (e.g., salmon, mackerel and sardines), egg yolks and offal.  However, sources are limited and there are high levels of inadequacy of vitamin D intake globally²¹.

In some countries, fortification of staple food (e.g., milk, margarine, cereals) is used to increase vitamin D intake but these policies differ by region and are often voluntary, leading to variable dietary contributions.

For this reason, local food composition data and values should be used when estimating intakes.  For example, milk is frequently quoted as a source of vitamin D but only when vitamin D fortified milk is available.

Liver is a source of vitamin D but is not widely consumed and is not recommended during pregnancy because of its high vitamin A content²².

Among plant-based options, mushrooms exposed to sunlight or UV radiation can supply vitamin D2.

Vitamin D – Beyond Food

The level of 25(OH)D in populations varies geographically due to latitude, skin pigmentation, sun exposure, diet and supplement use.

While endogenous production of vitamin D from sun exposure is also a potential source, recommendations to limit sun exposure to prevent skin cancer and limited sunlight in higher latitudes during winter mean that it cannot be relied upon as a source.

In many countries, vitamin D supplementation is recommended either during winter or throughout the year for more vulnerable populations (e.g. pregnant women, elderly, those with darker skin)^23-25.

The Endocrine Society also recommend supplementation for children aged 1 to 18 years and those aged 75 years and older as well as pregnant women and those with high-risk prediabetes²⁶.

Vitamin D Deficiency

The concentration of 25(OH)D in blood serum is currently the main indicator of vitamin D status as it reflects vitamin D produced through both sunlight and from the diet.  There is no universal agreement on the threshold for vitamin D ‘deficiency’.

However, there is widespread acknowledgement of vitamin D deficiency using the most conservative 25(OH)D threshold of < 25/30 nmol/L, in both low- and high-income countries¹.  Depending on the world region, the prevalence of serum 25(OH)D below this threshold  ranges from ~5 to 18% and 24 to 49% for levels below 50 nmol/L²¹.

Vitamin D deficiency in toddlers and young adults can cause rickets and slow brain development.  Deficiency in adults causes osteomalacia (brittle bones) increasing susceptibility to fractures.

Confirmed vitamin D deficiency is typically treated with high-dose supplements for a limited number of weeks followed by a maintenance dose²⁸.

Excess Intakes

It is important to remember that as a fat-soluble vitamin, vitamin D can be stored in the liver and excessive amounts should be avoided.

An upper daily limit of 100 micrograms from all sources has been set in the US by the IOM for those 9+ years¹⁸ while an upper daily limit of 50 micrograms for children and 100 micrograms for adults has been recommended by the EFSA²⁷.

In Closing

Ongoing research continues to explore vitamin D’s potential benefits further refining our understanding of its role in human health.  While vitamin D is essential, further research is needed to fully understand its role in various health conditions and to establish clear guidelines for supplementation.

This article was originally published on March 18, 2025 and updated on March 3, 2026.

It is easy to think that kids’ bodies are so adaptable, they can eat anything and grow up to be healthy adults.  Many parents remember a time when their kids only wanted to eat their favourite food rather than a balanced diet, but this is often short-lived.

Nutrients to Achieve Linear Growth

Linear growth occurs throughout childhood and adolescence and is mostly seen as increase in height and muscle mass.  There are some periods of growth during this time that are particularly rapid.

If critical periods of growth are affected, such as by not providing enough energy or building blocks required by the body to fuel this growth, permanent stunting or lifelong low bone mineral density can occur.

The consequences can include lifelong increased risk of overweight/obesity and bone fractures.

An average 2-year-old boy’s height typically doubles and weight increases 5 times that of his starting weight by 20 years of age (CDC Growth Chart for Boys Age 2 to 20 years).

Protein and calories are building blocks that support rapid linear growth of muscle and tissues and also support healthy body composition (lean muscle versus body fat).

Providing adequate calories allows protein to be used for growing body tissues.  Getting too many calories leads to overweight and obesity, which persists in adult years.

More than three out of four obese kids grow up to be obese adults (Freedman, 2001).

Protein needs to be adequate in quantity as well as quality by providing the nine essential amino acid building blocks.  Dairy and egg protein sources are gold standards when it comes to protein quality.

Generally, animal and soy are complete proteins, but plant proteins can provide a complete complement of amino acids when paired appropriately.

Children and teens restricting animal protein foods need to be extra vigilant to consume foods that will help them meet their essential amino acid needs, as well as other nutrients present in animal foods (like calcium, iron, zinc and choline).

Calcium and vitamin D support linear bone growth and build bone density.  Up to 90% of peak bone density is achieved by 18-20 years of age (National Institute of Health, 2015).

For the rest of a person’s life, the body relies on bone as a reserve to draw on in times of need.  This means that if children and teens don’t consume enough calcium and vitamin D during this time of peak bone growth, they are at a higher risk of osteoporosis because less has been stored in the bone for use in later years.

Iron is an essential part of red blood cells that deliver oxygen throughout the body, including active muscles and the brain.  As the body grows, blood volume expands to keep pace.  Moreover, for adolescent girls, menstruation more than doubles demand for iron to replace blood losses.

Zinc is a metabolic facilitator supporting numerous processes associated with growth, sexual maturation, and immunity (Institute of Medicine, 2001; Caulfield, 2004).  In many regions of the world, both iron and zinc intakes are inadequate due to limited availability of food sources of these nutrients.

This is typically the case when meat availability is limited, since plant sources of iron and zinc are not as available to be absorbed by the body due to factors such as dietary inhibitors, like phytate (a so-called ‘anti-nutrient’ that binds minerals in the digestive tract), found in many plants.

Nutrients to Achieve Cognitive and Sensory Development

As with linear growth, nutrients serve as both structural building blocks and as regulating factors in developing cognitive and sensory systems.  Key nutrients are highlighted here, but many other nutrients are essential for health.

Iodine is required to synthesize hormones in the thyroid gland that are especially important for the developing brain.

Zinc is an essential facilitator for dozens reactions that regulate growth, so it is no surprise that zinc is involved in cognitive development.  Supplementation in high-risk young children is not supported by evidence for improving cognitive outcomes, so preventing deficiency is key (Gogia, 2012).

Iron plays a direct role in the nervous system.  Moreover, iron-deficiency is associated with fatigue and impaired immunity, which may affect school attendance and ability to perform at school.

Emphasising dietary sources of iron, managing dietary inhibitors, and selective fortification combined with disease management continue to be public health priorities.

Brain cells are enriched in a specific long-chain omega-3 fatty acid called docosahexaenoic acid (DHA).  The benefits of supplemental DHA among children and teens is being actively researched.

The brain is not the only part of the nervous system developing in early age. Our senses are also developing.  When it comes to vision, vitamin A is essential (Institute of Medicine, 2001).

 

The carotenoids lutein and zeaxanthin are present in the retina of the eye and in brain tissue where they may serve as important antioxidant protection.

Putting Nutrients into a Food Perspective

With so many nutrients critical for healthy growth and development, it may seem impossible to balance getting enough of each nutrient without too many calories.

However, keep two simple principles in mind.

• Provide a variety of nutrient-rich foods while limiting high caloric density foods.  For more specifics, the American Academy of Pediatrics provides a feeding guide with respect to foods, portions and daily amounts (Daniels 2015).
• Tailor foods for children and adolescents to their specific needs.  Plant foods can be modified to improve protein quality or processed to remove anti-nutrients, making vitamins and minerals more easily absorbed by our bodies. For example, different plant proteins can be mixed to create complementary proteins which contain all of the essential amino acids.  Techniques such as soaking, cooking, or fermenting can reduce the amount of anti-nutrients in plant foods.  The types of nutrients and amount we put into foods targeted to kids should reflect what their bodies’ need.  In the age of consumers seeking short ingredient lists, it is important to make sure vitamins and minerals do not become collateral damage.
• Help kids develop food preferences for a variety of foods.  Childhood is a key time to develop food preferences beyond those they are born with.  It can take between 6 to 15 exposures before preference increases, but children exposed to a variety of foods early in life retain a preference for these foods over time (Ventura, 2013).  Read our blog ‘Turning Fussy Young Eaters info Foodies’ for more on helping kids learn good eating behaviors.

As experts working in the food industry, we can make a positive impact on kids health by focusing on balancing cost with delivering key nutrients in low to moderate calorie forms, while helping kids explore and expand food preferences.