Vitamin D: Understanding This Essential Nutrient and Its Food Sources
What Is Vitamin D?
Vitamin D is often called a vitamin, but it is more accurately described as a prohormone. Unlike most vitamins, which must be obtained entirely from food, vitamin D can be synthesized in the skin when it is exposed to ultraviolet B (UVB) radiation from sunlight. This unique characteristic makes vitamin D different from all other nutrients.
Vitamin D exists in two main forms. Vitamin D2 (ergocalciferol) is found in some plant foods and fungi. Vitamin D3 (cholecalciferol) is found in animal foods and is also the form produced in human skin. Research suggests that vitamin D3 is more effective at raising and maintaining blood levels of vitamin D than D2.
Both forms are biologically inactive when first consumed or produced. They must be converted in the body to become active. The liver first converts vitamin D to calcidiol (25-hydroxyvitamin D), which is the form measured in blood tests. The kidneys then convert calcidiol to calcitriol (1,25-dihydroxyvitamin D), which is the active hormone form.
Why Vitamin D Matters
Vitamin D plays essential roles in many body systems.
Calcium and bone metabolism
The most well-known function of vitamin D is regulating calcium and phosphorus absorption from the intestines. Without adequate vitamin D, the body can only absorb 10–15% of dietary calcium. With sufficient vitamin D, absorption increases to 30–40%. This makes vitamin D essential for building and maintaining strong bones and teeth. Severe deficiency causes rickets in children and osteomalacia (soft bones) in adults.
Immune function
Vitamin D receptors are found on most immune cells, including T cells, B cells, and macrophages. Vitamin D helps regulate both the innate and adaptive immune responses. It supports the body’s defense against pathogens while also helping to prevent excessive inflammatory responses. Research has linked vitamin D deficiency to increased susceptibility to infections and autoimmune conditions.
Muscle function
Vitamin D is important for muscle strength and function. Deficiency is associated with muscle weakness, particularly in older adults. This contributes to the increased risk of falls seen in people with low vitamin D levels.
Mental health
Vitamin D receptors are present in areas of the brain involved in mood regulation. Research has found associations between low vitamin D levels and depression, though the relationship is complex and not fully understood. Vitamin D is involved in the synthesis of serotonin, a neurotransmitter that affects mood.
Cardiovascular health
Vitamin D plays a role in regulating blood pressure and supporting cardiovascular function. Deficiency has been associated with increased risk of hypertension and cardiovascular disease, though supplementation studies have shown mixed results.
Cell growth regulation
Vitamin D influences cell growth, differentiation, and death (apoptosis). This has led to research into its potential role in cancer prevention, though results remain inconclusive.
The Global Problem of Vitamin D Deficiency
Vitamin D deficiency is one of the most common nutritional deficiencies worldwide. It affects people in all regions, including sunny countries. Several factors contribute to this problem.
Limited sun exposure
Modern lifestyles mean that most people spend the majority of their time indoors. When outdoors, many people use sunscreen or cover their skin. People living at latitudes above approximately 35° North or below 35° South cannot produce vitamin D from sunlight during winter months because the sun angle is too low. This includes most of Europe, Canada, and the northern United States.
Skin pigmentation
Melanin in the skin reduces UVB absorption. People with darker skin require significantly longer sun exposure to produce the same amount of vitamin D as people with lighter skin.
Age
The ability to synthesize vitamin D in the skin decreases with age. Older adults also tend to spend less time outdoors and may have reduced dietary intake.
Body composition
Vitamin D is fat-soluble and can become sequestered in body fat. People with higher body fat percentages may have lower circulating vitamin D levels.
Limited dietary sources
Few foods naturally contain significant amounts of vitamin D. This makes it difficult to obtain adequate vitamin D from diet alone, particularly for people who avoid animal foods.
Dietary Sources of Vitamin D
Natural food sources of vitamin D are limited. The following foods provide meaningful amounts.
Fatty fish
Fatty fish are the best dietary sources of vitamin D3. Wild-caught fish generally contain more vitamin D than farmed fish.
Salmon (Salmo salar, Oncorhynchus species): A 100-gram serving of wild salmon can provide 600–1000 IU of vitamin D. Farmed salmon contains approximately 100–250 IU per 100 grams.
Herring (Clupea harengus): Fresh Atlantic herring provides approximately 200–400 IU per 100 grams. Pickled herring contains less.
Mackerel (Scomber scombrus): Provides approximately 350–400 IU per 100 grams.
Sardines (Sardina pilchardus): Canned sardines provide approximately 175–200 IU per 100 grams.
Tuna (Thunnus species): Canned light tuna provides approximately 150 IU per 100 grams. Fresh tuna contains more.
Cod liver oil
Cod liver oil is one of the most concentrated natural sources of vitamin D. One tablespoon (15 ml) provides approximately 1,360 IU. Cod liver oil also contains high levels of vitamin A, so intake should be limited to avoid vitamin A toxicity.
Egg yolks
Egg yolks contain vitamin D3, with levels varying based on the hens’ diet and sun exposure. Conventional eggs provide approximately 40 IU per yolk. Eggs from pasture-raised hens exposed to sunlight can contain 3–4 times more vitamin D.
Liver
Beef liver contains approximately 50 IU per 100 grams. Liver also provides other fat-soluble vitamins.
Mushrooms
Mushrooms are the only significant plant source of vitamin D. They produce vitamin D2 when exposed to UV light, similar to human skin. Wild mushrooms or mushrooms grown under UV light can contain significant amounts. Common cultivated mushrooms grown in darkness contain very little vitamin D.
Chanterelles (Cantharellus cibarius) and morels (Morchella species) that grow in sunlight can contain 200–1,200 IU per 100 grams. Commercially grown white button mushrooms typically contain less than 10 IU per 100 grams unless they have been UV-treated.
UV-treated mushrooms are now available in some markets. These can provide 400–800 IU per 100 grams.
Fortified foods
Many countries fortify certain foods with vitamin D. Common fortified foods include milk, plant-based milk alternatives, orange juice, breakfast cereals, and margarine. Fortification levels vary by country and product. Check product labels for specific amounts.
Factors Affecting Vitamin D Absorption
Several factors influence how well vitamin D is absorbed from food.
Dietary fat
Vitamin D is fat-soluble and requires dietary fat for optimal absorption. Taking vitamin D supplements or eating vitamin D-rich foods with a meal containing fat improves absorption significantly.
Gut health
Conditions affecting fat absorption, such as celiac disease, inflammatory bowel disease, or pancreatic insufficiency, can impair vitamin D absorption.
Magnesium status
Magnesium is required for the enzymes that convert vitamin D to its active form. Magnesium deficiency may impair vitamin D metabolism. Good sources of magnesium include dark leafy greens, nuts, seeds, and whole grains.
Vitamin K2
Vitamin K2 works synergistically with vitamin D in calcium metabolism. Vitamin D increases calcium absorption, while vitamin K2 helps direct calcium to bones and teeth rather than soft tissues. Food sources of vitamin K2 include fermented foods, egg yolks, and animal fats from grass-fed animals.
Sun Exposure and Vitamin D Production
For most people throughout human history, sunlight was the primary source of vitamin D. The skin produces vitamin D3 when exposed to UVB radiation. The amount produced depends on several factors: time of day, season, latitude, altitude, cloud cover, skin pigmentation, age, and the amount of skin exposed.
During summer months at latitudes between 35° North and 35° South, 10–30 minutes of midday sun exposure on bare arms and legs (without sunscreen) several times per week can produce adequate vitamin D for most light-skinned individuals. People with darker skin may need 3–6 times longer exposure.
At higher latitudes during winter months, UVB radiation is insufficient for vitamin D production regardless of time spent outdoors. During these months, dietary sources and supplements become essential.
It is important to balance sun exposure for vitamin D production with skin cancer risk. Brief, regular exposure is preferable to prolonged exposure or burning.
When Supplementation May Be Needed
Many people cannot obtain adequate vitamin D from sun exposure and diet alone. Supplementation may be appropriate for:
- People living at high latitudes, especially during winter
- People with limited sun exposure
- People with darker skin
- Older adults
- People with conditions affecting fat absorption
- People who are overweight or obese
- People following vegan or vegetarian diets
The recommended daily intake varies by country and organization. Many health authorities recommend 600–800 IU daily for adults, though some researchers suggest higher amounts (1000–4000 IU) may be optimal. Blood testing (serum 25-hydroxyvitamin D) can help determine individual needs.
Vitamin D3 supplements are generally preferred over D2 for their better efficacy at raising blood levels. Vitamin D3 derived from lichen is available for those following a vegan diet.
Vitamin D toxicity is possible with very high supplement doses over extended periods. Toxicity does not occur from sun exposure or dietary sources. The safe upper limit for adults is generally considered to be 4,000 IU daily, though some individuals may safely take higher doses under medical supervision.
Herbal Considerations
While herbs do not provide significant amounts of vitamin D, herbalists consider vitamin D status when working with clients. Adequate vitamin D supports the effectiveness of herbs used for immune support, bone health, and mood regulation.
Herbs traditionally used to support bone health, such as horsetail (Equisetum arvense) and nettle (Urtica dioica), provide minerals like silica and calcium that work alongside vitamin D. Herbs that support liver and kidney function may indirectly support vitamin D metabolism, as these organs are responsible for converting vitamin D to its active form.
For immune support during winter months, herbalists often recommend combining vitamin D-rich foods or supplements with immune-supporting herbs such as elderberry (Sambucus nigra), echinacea (Echinacea species), and astragalus (Astragalus membranaceus).
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