Natural Sources of Vitamin D3: What Nutritionists Don't Tell You
However, finding reliable natural sources of vitamin D3 can be challenging, especially when considering factors like absorption rates and dietary restrictions. This comprehensive guide explores everything from sunlight exposure to food sources, helping you understand not just what is a natural source of vitamin D3, but also how to combine it effectively with calcium and other nutrients for optimal absorption.
Whether you're a vegetarian seeking plant-based options or simply looking to boost your D3 levels naturally, we'll cover all the essential information you need to make informed choices about your vitamin D3 intake.
Understanding Vitamin D3 Basics
Vitamin D3, scientifically known as cholecalciferol, stands out as a unique nutrient that functions both as a vitamin and a hormone in your body 1. Unlike other vitamins, your body naturally produces vitamin D3 when sunlight strikes your skin, triggering an intricate synthesis process 1.
Why your body needs it
Your body primarily needs vitamin D3 for maintaining strong bones and overall health. Specifically, it helps your body absorb calcium in the gut and maintains proper calcium and phosphate levels in your blood 1. Furthermore, vitamin D3 serves multiple crucial functions:
- Supports muscle movement and strength
- Enables nerve communication between brain and body
- Strengthens immune system against bacteria and viruses
- Regulates cell growth and inflammation
- Maintains glucose metabolism
- Promotes heart health 2
Notably, without adequate vitamin D3, bones can become thin, brittle, or misshapen 1. In children, severe deficiency leads to rickets, while in adults, it causes osteomalacia - a condition marked by bone pain and muscle weakness 2.
Natural D3 vs supplements
When comparing natural and synthetic sources, research shows that natural vitamin D3 works more effectively with your body 3. Natural D3 from whole food sources comes bundled with complementary nutrients that enhance absorption 3. For instance, vitamin D3 in cod liver oil appears alongside vitamin A and omega-3 fatty acids, creating a synergistic effect 3.
Synthetic vitamin D3, primarily derived from sheep wool through chemical processes, lacks these additional beneficial compounds 3. Though chemically identical to natural D3, synthetic versions don't offer the same broad health benefits that whole food sources provide 3.
The absorption rate of both forms depends significantly on dietary fat presence, since vitamin D3 is fat-soluble 2. Consequently, taking vitamin D3 with a meal containing some fat improves its absorption 2. Blood levels of 50 nmol/L (20 ng/mL) or above indicate adequate vitamin D status for most people 2.
Understanding these basics helps explain why natural sources often prove more beneficial than isolated supplements. While supplements can help fill nutritional gaps, obtaining vitamin D3 from natural sources provides additional health-supporting nutrients that work together for better absorption and utilization in your body 3.
How Your Body Makes Vitamin D3
Your skin performs a remarkable chemical transformation when exposed to sunlight, acting as a natural vitamin D3 factory. The process begins when ultraviolet B (UVB) rays with wavelengths between 290-315 nanometers strike your skin 4. These rays interact with a protein called 7-dehydrocholesterol (7-DHC) in your skin's epidermis, primarily in the stratum basale and stratum spinosum layers 5.
The sunlight connection
The synthesis starts as UVB radiation breaks open the B ring of 7-DHC, forming previtamin D3 6. This molecule then undergoes isomerization to create vitamin D3. Your body has a sophisticated self-regulating mechanism - with continued UV exposure, additional products called lumisterol3 and tachysterol3 form, preventing vitamin D3 overproduction 7.
Your skin's melanin content plays a crucial role in this process. People with darker skin have larger, darker melanin pigments that protect against excess sunlight. Under those circumstances, they need longer sun exposure to produce the same amount of vitamin D3 as lighter-skinned individuals 8. According to research, darker-skinned individuals may require up to two hours of sun exposure to generate the same vitamin D3 levels that lighter-skinned people achieve in just 30 minutes 7.
Best time for sun exposure
The optimal time for vitamin D3 production occurs between 11:00 AM and 2:00 PM 4. During these hours, UVB rays reach maximum intensity, making vitamin D3 synthesis most efficient. Additionally, research indicates that exposing about one-third of your body surface area during these peak hours provides optimal results 9.
Several environmental factors influence vitamin D3 production:
- Latitude and Season: Areas farther from the equator receive fewer UVB rays due to atmospheric absorption 9
- Altitude: Higher elevations allow more UVB penetration, increasing vitamin D3 production 7
- Cloud Cover: Clouds reduce UVB radiation reaching the skin 10
- Air Pollution: Environmental pollutants can decrease UVB availability 4
As opposed to popular belief, brief sun exposure proves sufficient for vitamin D3 production. Research shows that exposing arms and legs to midday sun for 5-30 minutes twice weekly can meet vitamin D3 requirements 5. Together with proper timing, this approach helps maintain healthy vitamin D3 levels without risking excessive sun exposure.
Primarily, your body can store vitamin D in fat cells for months, ensuring availability during periods of limited sun exposure 8. This storage capacity helps maintain adequate levels even when daily sun exposure isn't possible.
Top Natural Food Sources of D3
Among natural food sources, fatty fish stands as nature's richest provider of vitamin D3. Rainbow trout leads the pack, delivering 645 IU (81% of daily value) in just 3 ounces 11. Sockeye salmon follows closely, offering 570 IU (71% of daily value) per serving 11.
Fish and seafood options
The vitamin D3 content varies significantly across different seafood varieties:
- Wild salmon: Contains up to 160% of daily value per 3.5-ounce serving 12
- Herring: Provides 214 IU per 3.5-ounce portion 12
- Sardines: Delivers 200 IU per typical can 11
- Cod liver oil: Offers 450 IU per teaspoon 12
Egg yolks and dairy
Egg yolks primarily contain vitamin D3 and its metabolite 25(OH)D3 10. A single large egg yolk provides 37 IU, essentially 5% of daily requirements 12. The vitamin D3 concentration in eggs can increase through enhanced chicken feed supplementation 1.
Dairy products generally contain varying amounts of naturally occurring vitamin D3. Fresh milk contains 0.3-1.0 µg/kg, primarily cream offers 3.7-10.8 µg/kg, moreover butter provides 5.9-14.1 µg/kg 1. Particularly interesting are hard cheeses, containing 2.0-18.1 µg/kg of vitamin D3 1.
Mushrooms and fortified foods
Mushrooms uniquely synthesize vitamin D2 when exposed to UV light 13. Alternatively, some commercially available mushrooms undergo intentional UV treatment, substantially increasing their vitamin D content 13. UV-treated portabella mushrooms contain 11.2 µg (446 IU) per 100g, compared to untreated ones with merely 0.3 µg (11 IU) per 100g 13.
Fortified foods serve as reliable vitamin D sources. Most U.S. milk supplies are fortified with approximately 3 mcg/cup (120 IU) 10. Plant-based alternatives, including soy, almond, and oat milk, match these fortification levels 10. Ready-to-eat breakfast cereals, orange juice, and yogurt often contain added vitamin D 10.
Wild mushrooms, such as chanterelles and morels, naturally contain higher vitamin D levels (5.2-5.3 µg per 100g) compared to commercially grown varieties 13. The vitamin D content in mushrooms remains stable even after months of storage, though it gradually diminishes over time 14.
Best Natural Sources for Vegetarians
Finding natural vitamin D3 sources on a vegetarian diet requires careful consideration, as most traditional D3-rich foods come from animal sources. Fortunately, science has uncovered several plant-based alternatives that can help maintain optimal D3 levels.
Plant-based D3 options
First of all, lichen emerges as a groundbreaking natural source of vitamin D3 for vegetarians. This unique plant species, consisting of a symbiotic relationship between fungus and algae, grows abundantly on mountainsides, rocks, and trees 15. Primarily, lichen provides a purer source of D3 compared to animal-derived alternatives, as it remains free from pesticides and other contaminants 15.
UV-exposed mushrooms stand out as another remarkable plant-based option. These fungi contain ergosterol, which converts into vitamin D2 when exposed to ultraviolet light 3. A half cup of UV-exposed white mushrooms contains approximately 366 IU of vitamin D 3. Subsequently, research confirms that the vitamin D2 present in mushrooms raises blood vitamin D levels as effectively as supplements 3.
Fortified alternatives
The food industry offers numerous fortified options for vegetarians seeking reliable D3 sources:
- Plant-based milks (soy, almond, rice, hemp, oat) - up to 100 IU (2.5 mcg) per cup 16
- Fortified cereals - ranging from 0.2 to 2.5 mcg (8 to 100 IU) per serving 17
- Fortified orange juice - up to 2.5 mcg (100 IU) per serving 16
- UV-treated tofu - approximately 20% of daily vitamin D requirements 3
Alternatively, vegetarians can boost their vitamin D absorption by consuming these foods with healthy fats. Beneficial fat sources include peanuts, nuts, seeds, avocado, and small amounts of vegetable oils 2. This combination enhances nutrient absorption, as vitamin D is fat-soluble 2.
For optimal results, public health authorities recommend supplementation between October and March 2. When choosing supplements, vegetarians should look specifically for D3 derived from lichen, as it offers superior bioavailability compared to D2 supplements 18. Most importantly, lichen-derived D3 provides an environmentally conscious choice, as it naturally occurs in large quantities and requires minimal processing 15.
Remember that fortification standards vary by country, making it crucial to check nutrition labels for specific vitamin D content 16. Fortified foods can contribute significantly to daily vitamin D intake, though they should be part of a broader strategy that includes sensible sun exposure and possibly supplementation during winter months.
Factors Affecting D3 Absorption
Several complex factors influence how effectively your body absorbs vitamin D3. Understanding these elements helps optimize absorption from both natural sources and supplements.
Role of dietary fats
The presence of dietary fat plays a fundamental role in vitamin D3 absorption. Indeed, research shows that taking vitamin D3 with a fat-containing meal increases absorption by 32% 19. Primarily, this occurs because vitamin D3 is fat-soluble, requiring dietary fat in the gut for proper absorption 20.
Alternatively, the type of fat consumed affects absorption rates. Studies indicate that monounsaturated fats enhance vitamin D3 absorption, whereas polyunsaturated fats might reduce its effectiveness 21. First thing to remember, oils with higher monounsaturated to polyunsaturated fat ratios, similar to olive oil, could increase vitamin D3 bioavailability 22.
Mineral interactions
The relationship between vitamin D3 and minerals creates a complex network of interactions. Magnesium emerges as a crucial player, as it assists in activating vitamin D3 in your body 23. In light of this connection, insufficient magnesium levels can impair vitamin D3 metabolism 23.
Similarly, vitamin D3 influences the absorption of several essential minerals:
- Zinc absorption increases through vitamin D3's effect on intestinal transport 24
- Iron levels show correlation with vitamin D3 status 5
- Copper concentrations demonstrate an inverse relationship with vitamin D3 in deficient individuals 5
Health conditions impact
Various health conditions can significantly affect vitamin D3 absorption. As a result of certain digestive disorders, absorption rates may drop dramatically:
- Celiac disease reduces absorption to 50% of normal levels 9
- Biliary obstruction decreases absorption to less than 28% 9
- Chronic pancreatitis lowers absorption to under 18% 9
- Crohn's disease, cystic fibrosis, and certain liver conditions impair absorption 25
Obesity presents a unique challenge, as vitamin D3 gets stored in fat tissue, removing it from bloodstream circulation 26. Furthermore, individuals who have undergone weight-loss surgery face increased risks of vitamin D3 malabsorption 4.
Certain medications can interfere with vitamin D3 absorption. Cholesterol-lowering drugs, anti-seizure medications, and weight-loss medications like orlistat can reduce vitamin D3 levels 27. Additionally, laxatives, steroids, and some tuberculosis medications may lower vitamin D3 absorption 4.
Combining D3 with Other Nutrients
The intricate relationship between vitamin D3 and other nutrients forms the foundation of optimal health benefits. Understanding these connections helps maximize the effectiveness of natural vitamin D3 sources.
The calcium connection
The partnership between vitamin D3 and calcium stands as fundamental for bone health. Primarily, vitamin D3 enables calcium absorption in your intestines, increasing the efficiency from a mere 10-15% to an impressive 30-40% 28. Without adequate vitamin D3, your body struggles to maintain proper calcium levels, potentially leading to various bone-related conditions.
Your parathyroid glands closely monitor this relationship. Alternatively, when vitamin D3 or calcium levels drop too low, these glands produce excess hormone, causing a condition called hyperparathyroidism 29. This compensation mechanism, although helpful short-term, can lead to bone weakening and increased fracture risk over time.
Research demonstrates that vitamin D3 and calcium work synergistically for:
- Bone mineralization and density maintenance
- Prevention of hypocalcemic tetany
- Protection against osteoporosis in older adults
- Muscle strength preservation 10
Importance of K2
The addition of vitamin K2 creates a powerful trio for optimal health. Vitamin D3 promotes the production of vitamin K-dependent proteins, which require K2 for proper carboxylation 30. This interaction proves crucial for both bone and cardiovascular health.
K2 serves as a natural director of calcium, ensuring it reaches the right destinations in your body. Besides directing calcium to bones, K2 prevents its accumulation in soft tissues and arteries 31. This mechanism becomes increasingly important as vitamin D3 enhances calcium absorption.
Clinical studies reveal that combining these nutrients yields superior results. For instance, postmenopausal Korean women taking vitamin D3 with K2 and calcium showed greater bone mineral density improvements compared to those taking vitamin D3 and calcium alone 30.
The synergistic benefits extend beyond bone health. K2 activates two essential proteins:
- Osteocalcin: Integrates calcium into bone structure
- Matrix GLA protein: Prevents arterial calcification 32
Certainly, vitamin D3 supplementation requires additional K2 intake to prevent long-term health risks 33. This combination helps regulate:
- Blood glucose metabolism
- Insulin secretion
- Cardiovascular function
- Bone metabolism 34
The optimal absorption of these nutrients depends on proper timing and combinations. Research indicates that while vitamin D3 and calcium need not be taken simultaneously, their combined presence in the body remains essential for maintaining bone density 35. Nevertheless, K2 supplementation becomes particularly important for individuals taking vitamin D3 supplements, as it helps prevent potential calcium-related complications 33.
Therefore, when seeking natural sources of vitamin D3, consider foods that provide these complementary nutrients. Fatty fish, for example, offers both vitamin D3 and K2, making it an excellent choice for maintaining this crucial nutrient balance.
Signs of Low Vitamin D3
Recognizing vitamin D3 deficiency early can prevent serious health complications. Research shows that many people remain unaware of their deficiency status, with studies indicating vitamin D deficiency affects 41.6% of adults in the United States 6.
Common symptoms
The body sends various signals when vitamin D3 levels drop too low. Initially, these symptoms might seem unrelated, making them easy to overlook. Clinical studies have identified several key indicators:
- Persistent fatigue: Research links vitamin D deficiency with increased fatigue symptoms, as demonstrated in a 2019 study of 480 older adults 6
- Sleep disruption: A 2020 study found that children with low vitamin D levels experienced poor sleep quality and shorter sleep duration 6
- Bone and muscle issues: Patients often report bone pain, particularly in the lower back, alongside muscle weakness or cramps 4
- Mood changes: Depression and mood alterations frequently accompany low vitamin D3 levels 4
- Impaired healing: Studies indicate slower wound healing after surgery or injury 6
- Frequent illness: A weakened immune system makes individuals more susceptible to infections 7
Primarily, these symptoms manifest differently in children and adults. In children, vitamin D deficiency might cause incorrect growth patterns and bone deformities 4. Ordinarily, adults experience more subtle signs, with many remaining asymptomatic until deficiency becomes severe.
Risk factors
Understanding risk factors helps identify individuals who might need additional natural vitamin D3 sources. Research has uncovered several key demographic and lifestyle factors that increase deficiency risk:
Demographic Factors African-American adults show the highest prevalence rate at 82.1%, followed by Hispanic adults at 62.9% 8. This elevated risk stems from increased melanin levels, which affect UV ray absorption and vitamin D3 production 8.
Health-Related Factors Body composition plays a crucial role, as 42.5% of vitamin D deficient individuals fall under the obesity category, compared to 31.1% of vitamin D sufficient individuals 8. Additionally, certain health conditions significantly impact vitamin D3 status:
- Chronic kidney or liver disease affects vitamin D metabolism 6
- Digestive disorders like Crohn's disease, ulcerative colitis, and celiac disease impair nutrient absorption 6
- Gastric bypass surgery creates challenges in vitamin D absorption 6
Lifestyle and Environmental Factors Daily habits and environmental exposure influence vitamin D3 levels significantly. Research reveals that among vitamin D deficient individuals, only 28.47% consumed milk products daily, contrasting with 42.13% of vitamin sufficient participants 8. Furthermore, education levels correlate with vitamin D status, as 57.80% of vitamin D sufficient individuals were college-educated compared to 53.41% of those deficient 8.
Interestingly, smoking status affects vitamin D levels, with statistically significant deficiency rates among both former and current smokers 8. This finding underscores the importance of lifestyle choices in maintaining adequate vitamin D3 levels.
Medical professionals recommend blood tests to check vitamin D status, particularly for individuals with multiple risk factors. Blood levels below 20 ng/mL indicate deficiency, while levels between 21-29 ng/mL suggest insufficiency 6. Regular monitoring becomes crucial for those with ongoing risk factors or persistent symptoms.
Testing and Monitoring D3 Levels
Blood testing provides definitive insight into your vitamin D3 status, moving beyond symptom assessment to precise measurement. The 25-hydroxy vitamin D test stands as the most accurate method for determining vitamin D levels in your body 36.
When to get tested
Routine vitamin D screening isn't recommended for everyone 4. Nonetheless, certain conditions and risk factors warrant testing:
- Diagnosed bone conditions including osteoporosis, osteomalacia, or osteopenia 37
- Chronic kidney disease stage III or greater 37
- Digestive disorders affecting nutrient absorption 38
- Long-term use of medications like anticonvulsants or glucocorticoids 37
- Recent weight loss surgery 38
- Age over 65, as both skin production and gut absorption decrease 38
Ordinarily, healthcare providers recommend testing if you experience persistent symptoms of deficiency or have multiple risk factors 1. Primarily, testing becomes crucial for individuals with limited sun exposure, malnutrition, or conditions affecting vitamin D metabolism 1.
Optimal ranges
The interpretation of vitamin D3 test results involves understanding several key ranges. Laboratory measurements typically report results in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L) 36. Current research establishes these reference ranges:
Deficiency Levels:
- Less than 30 nmol/L (12 ng/mL) indicates clear deficiency 36
- Between 30-50 nmol/L (12-20 ng/mL) suggests potential deficiency 36
Target Ranges:
- 50-125 nmol/L (20-50 ng/mL) represents normal levels 36
- 40-60 ng/mL proves optimal for most adults 39
Henceforth, after initiating vitamin D supplementation, follow-up testing should occur after three months to confirm improvement 13. Forthwith, testing frequency can be adjusted based on results:
- Twice yearly monitoring until reaching therapeutic goals 14
- Annual testing once achieving optimal ranges 14
Presently, experts acknowledge that true vitamin D deficiency below 12 ng/ml remains uncommon 40. Alternatively, seasonal variations significantly impact vitamin D levels, following a sinusoidal pattern that mirrors sunlight exposure 13. Winter measurements might not reflect year-round status, making timing considerations crucial for accurate assessment 13.
The Endocrine Society guidelines suggest maintaining levels above 30 ng/mL to ensure sufficiency 39. This recommendation accounts for assay variability and provides a safety margin above the minimum threshold 39. Most importantly, achieving blood levels above 30 ng/mL often requires daily intake exceeding 2,000 IU 39.
For individuals taking vitamin D supplements, a vitamin D challenge test can confirm proper absorption 39. This test involves measuring blood levels 12 hours after a single 50,000 IU dose, with approximately 80% typically absorbed through the lymphatic system 39.
Research utilizing National Health and Nutrition Examination Survey data reveals concerning statistics about vitamin D status in the United States. Almost 20% of the population shows values categorized as "at risk for inadequacy" (30-49 nmol/L or 12-19 ng/mL), while 5% fall into the "at risk for deficiency" category (< 30 nmol/L or 12 ng/mL) 39.
Conclusion
Natural vitamin D3 remains essential for optimal health, yet many people struggle to maintain adequate levels. Above all, understanding your body's vitamin D3 needs and knowing reliable natural sources helps prevent deficiency. Sunlight exposure during peak hours, combined with D3-rich foods like fatty fish, eggs, and UV-exposed mushrooms, provides a solid foundation for maintaining healthy vitamin D3 levels.
Certainly, vegetarians face unique challenges but can rely on lichen-based sources and fortified foods. The key lies in pairing vitamin D3 sources with healthy fats and complementary nutrients like calcium and K2, maximizing absorption and effectiveness.
Therefore, maintaining optimal vitamin D3 levels requires a comprehensive approach. Regular blood testing, especially for high-risk individuals, helps track vitamin D3 status and adjust intake accordingly. This proactive strategy, combined with natural D3 sources and proper nutrient combinations, supports long-term health and prevents deficiency-related complications.
References
[1] - https://medlineplus.gov/lab-tests/vitamin-d-test/
[2] - https://www.vegansociety.com/resources/nutrition-and-health/nutrients/vitamin-d-0
[3] - https://www.imaware.health/blog/vitamin-d-for-vegans?srsltid=AfmBOoqAGP1Mx1Z7VLBlw86-M1VwhTaAqla0Jcjbv_SX5b9gNHFAMCQF
[4] - https://my.clevelandclinic.org/health/diseases/15050-vitamin-d-vitamin-d-deficiency
[5] - https://www.sciencedirect.com/science/article/pii/S1018364722001938
[6] - https://www.healthline.com/nutrition/vitamin-d-deficiency-symptoms
[7] - https://www.nebraskamed.com/primary-care/9-vitamin-d-deficiency-symptoms-and-11-high-vitamin-d-foods
[8] - https://pmc.ncbi.nlm.nih.gov/articles/PMC6075634/
[9] - https://www.medicaljournals.se/acta/download/10.2340/00015555-0980/
[10] - https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
[11] - https://www.webmd.com/diet/foods-high-in-vitamin-d3
[12] - https://www.healthline.com/nutrition/9-foods-high-in-vitamin-d
[13] - https://pmc.ncbi.nlm.nih.gov/articles/PMC4494470/
[14] - https://www.southcarolinablues.com/web/public/brands/medicalpolicy/external-policies/vitamin-d-testing/
[15] - https://www.vegetology.com/en-us/blog/this-unheard-of-vitamin-d3-source-will-make-you-think-twice-about-your-supplements?srsltid=AfmBOooWrm1zRE1oSSrirO-dGg5C4-PCjdUbn8rZdndhPRFcRMi-3Jr2
[16] - https://www.healthline.com/nutrition/vitamin-d-foods-for-vegetarians
[17] - https://www.healthline.com/health/vegan-vitamin-d
[18] - https://www.vegetology.com/en-us/blog/this-unheard-of-vitamin-d3-source-will-make-you-think-twice-about-your-supplements?srsltid=AfmBOop7Uhp2_Q5HylubvDQfoSGJTS8keZn-0y952MbjWoa2IsMwMHrQ
[19] - https://www.jandonline.org/article/S2212-2672(14)01468-3/abstract
[20] - https://nutritionj.biomedcentral.com/articles/10.1186/1475-2891-9-65
[21] - https://www.nature.com/articles/nrendo.2011.150
[22] - https://academic.oup.com/jcem/article-abstract/96/10/3170/2834911
[23] - https://pmc.ncbi.nlm.nih.gov/articles/PMC6631968/
[24] - https://www.testmottagningen.se/en/artiklar/halsa-och-livsstil/sa-samverkar-d-vitamin-med-andra-naringsamnen/?srsltid=AfmBOopyIgroTevjf55R7xvuxw4tqFY8najk6KNAx33fkaPlMiQ0CgcT
[25] - https://www.health.harvard.edu/healthbeat/9-things-that-can-undermine-your-vitamin-d-level
[26] - https://www.health.harvard.edu/staying-healthy/taking-too-much-vitamin-d-can-cloud-its-benefits-and-create-health-risks
[27] - https://pmc.ncbi.nlm.nih.gov/articles/PMC5623087/
[28] - https://pmc.ncbi.nlm.nih.gov/articles/PMC2669834/
[29] - https://www.hopkinsmedicine.org/health/wellness-and-prevention/vitamin-d-and-calcium
[30] - https://pmc.ncbi.nlm.nih.gov/articles/PMC5613455/
[31] - https://www.healthline.com/nutrition/vitamin-d-and-vitamin-k
[32] - https://us.betteryou.com/pages/vitamin-d-and-vitamin-k
[33] - https://pmc.ncbi.nlm.nih.gov/articles/PMC7406600/
[34] - http://www.foodandnutritionjournal.org/volume10number3/studies-on-synergistic-interplay-of-vitamin-d-and-k-for-improving-bone-and-cardiovascular-health/
[35] - https://www.bonehealthandosteoporosis.org/patients/treatment/calciumvitamin-d/
[36] - https://www.healthline.com/health/25-hydroxy-vitamin-d-test
[37] - https://www.cms.gov/medicare-coverage-database/view/lcd.aspx?lcdid=34658&ver=34&
[38] - https://www.mountsinai.org/health-library/tests/25-hydroxy-vitamin-d-test
[39] - https://www.ccjm.org/content/89/3/154
[40] - https://www.health.harvard.edu/staying-healthy/should-i-get-my-vitamin-d-levels-checked