In this Nexus:
💊 What is vitamin D?
☀️ How vitamin D is produced in the skin
🤷♀️ My own vitamin D deficiency
🦴 Vitamin D important as a hormone
♾️ Vitamin D and longevity
🚩 Vitamin D deficiency
📖 Recommended daily intake of vitamin D
🚫 The more – the better? NO!
💡 Tips for optimal vitamin D levels
Food supplement enthusiasts will appreciate this Nexus edition, as I consider Vitamin D to be essential for consumption almost all-year-round, especially for those living in regions with distinct seasonal changes, as we have it here in the Northern Europe. Vitamin D is unique because it acts more like a hormone than a traditional nutrient. It is not only essential for calcium and phosphorus metabolism, and plays a pivotal role in maintaining bone health, but also it supports various physiological processes. In this Nexus I discuss vitamin D ant its’ physiological functions, how vitamin D is produced in the skin, what are the main signs of vitamin D deficiency, the relationship between vitamin D levels and longevity, and tips on how to effectively increase your vitamin D levels.
What is Vitamin D?
Vitamin D refers to a group of fat-soluble secosteroids that include two major forms:
- Vitamin D2 – ergocalciferol
- Vitamin D3 – cholecalciferol
While vitamin D2 is derived from plant sources and fortified foods, D3 is synthesized in the skin from cholesterol when exposed to ultraviolet B rays and can also be obtained from animal sources like fish, eggs, and dairy.
Both forms are biologically inactive and must undergo two hydroxylation reactions, first in the liver to form 25-hydroxyvitamin, and then in the kidneys to form the active hormone, 1,25-dihydroxyvitamin D, or calcitriol. This active form binds to vitamin D receptors present in various tissues, influencing a wide range of physiological functions.
How Vitamin D is Produced in the Skin
When skin is exposed to sunlight, specifically ultraviolet B radiation, 7-dehydrocholesterol in the skin absorbs the radiation and converts to previtamin D3. This compound undergoes what is called a thermal isomerization process to become vitamin D3 as shown below (1).
Under ideal conditions (clear skies, direct sun exposure, and no sunscreen), a fair-skinned person can produce between 10,000 to 25,000 IU of vitamin D within 20 to 30 minutes of midday sun exposure in the summer. It is important to mention that after a certain point the skin reaches a “vitamin D limit” and excess UV exposure will not increase production further. The efficiency of this synthesis during the day depends on several factors, including:
- Latitude and season. People living closer to the equator can produce more vitamin D year-round, while those farther from the equator can produce significantly less in winter due to the sun’s angle.
- Time of day. UVB rays, which are responsible for vitamin D production, are strongest between 10 a.m. and 3 p.m. during sunny days.
- Skin type. People with lighter skin can produce vitamin D more quickly compared to those with darker skin due to differences in melanin levels.
- Age. As people age, their skin’s ability to produce vitamin D decreases. Thus. older people will produce less vitamin D.
- Sun exposure. The amount of skin exposed to sunlight directly impacts vitamin D synthesis. More exposed skin (e.g., arms, legs) results in more production.
Under ideal conditions (clear skies, direct sun exposure, and no sunscreen), a fair-skinned person can produce between 10,000 to 25,000 IU of vitamin D within 20 to 30 minutes of midday sun exposure in the summer. It is important to mention that after a certain point the skin reaches a “vitamin D limit” and excess UV exposure will not increase production further.
My Own Vitamin D Deficiency
With each article I try to share something personal, from my own experience. And yes, I have been fighting vitamin D deficiency for quite some time now, I believe more than 2 years (at least as far as I knew about it). The worst deficiency symptom I was feeling was frequent infections or common cold during the cold season.
The optimal vitamin D level for adults is between 100 and 150 nmol/L (40 to 60 ng/mL).
Mine was 69 nmol/L… And I was a bit naïve, thinking that in my geographical zone (Lithuania, Northern Europe), plus summer holiday in Southern countries I can produce enough Vitamin D during summer (using sunscreen) for the whole year. I checked my levels before and after the summer, and surprise surprise – there was no difference! This means that when you already have a deficiency, the efforts to eliminate it needs to be serious, and not just some sun. After various tries of both medicinal products and supplements of vitamin D, I have found the one that works for me! While I don’t promote any brands, I can tell that it is a vitamin D in an olive oil, used as a spray to the mouth. And for the first time as far as I check, my vitamin D levels are in a normal range! 131 nmol/L!
What Does Science Say?
Vitamin D Important as a Hormone
As mentioned in the intro, vitamin D is more like a hormone, and less like a regular nutrient, as it has many crucial functions in our body:
Bone Health and Calcium Homeostasis
The major function of vitamin D is to maintain serum calcium concentrations within the physiologically acceptable range. It accomplishes this by increasing intestinal calcium absorption. In a vitamin D-deficient state, the intestine typically absorbs 10-15% of dietary calcium. In a vitamin D-sufficient state, 30% typically is absorbed from the diet; as much as 60-80% can be absorbed during periods of growth and pregnancy or lactation, with increased demand for calcium (2).
Vitamin D deficiency results in abnormalities in both calcium and phosphorus metabolism. Adequate vitamin D levels are necessary to prevent bone disorders such as rickets, osteomalacia, and osteoporosis.
Immune System Regulation
Vitamin D can modulate the innate (the one that we had when we were born) and adaptive (the one that we gained through our lifetime) immune responses. Vitamin D receptor is present on immune cells (B cells, T cells and antigen presenting cells) and these immunologic cells are all are capable of synthesizing the active vitamin D metabolite. Therefore, vitamin D has the capability of acting directly in the immune system (3).
Vitamin D enhances the pathogen-fighting effects of monocytes and macrophages and reduces the risk of chronic inflammation, which is implicated in autoimmune diseases. Vitamin D also influences the production of antimicrobial peptides, such as cathelicidins and defensins, which help the body combat infections. Deficiency in vitamin D is associated with increased autoimmunity as well as an increased susceptibility to infection (3).
Muscle Function
Vitamin D plays a key role in muscle health because there are many vitamin D receptors in muscle tissue. The active form of vitamin D, calcitriol, affects muscles in two main ways. First, it works through receptors in the cell’s nucleus, where it can turn certain genes on or off, leading to muscle cell growth, repair, and preventing cell death. Second, it acts through receptors on the cell membrane, where it quickly improves muscle contraction by helping regulate calcium levels (4).
Vitamin D also reduces a protein called myostatin, a protein which limits muscle growth, while increasing other factors like follistatin and insulin-like growth factor-2 that promote muscle strength and size, especially in type II muscle fibers. These fast-twitch fibers are responsible for quick, powerful movements like sprinting or jumping. A lack of vitamin D can cause these fibers to shrink, leading to weaker muscles, a problem seen more often in older people. Thus, vitamin D deficiency has been associated with muscle weakness, particularly in older adults, leading to a higher risk of falls and fractures.
Cardiovascular Health
Emerging evidence suggests that vitamin D plays a role in cardiovascular health. Vitamin D regulates blood pressure by acting on endothelial cells and smooth muscle cells. Its deficiency has been associated with various cardiovascular risk factors and appears to be linked to a higher mortality and incidence of cardiovascular disease. Several mechanisms have been proposed relating vitamin D deficiency to CV risk factors such as renin-angiotensin-aldosterone system activation, abnormal nitric oxide regulation, oxidative stress or altered inflammatory pathways (5).
Cell Growth and Differentiation
Vitamin D is involved in cell growth regulation, differentiation, and apoptosis. Its active form can inhibit the proliferation of cancer cells, supporting the theory that adequate vitamin D levels may lower the risk of certain cancers, including colorectal, breast, and prostate cancer (6).
As for the visual lovers, physiological functions of vitamin D are presented in the picture below (7):
Vitamin D and Longevity
Recent research has shifted from viewing vitamin D solely as a bone health nutrient to recognizing its potential benefits for overall health and longevity. By modulating inflammation and supporting immune function, vitamin D may reduce the risk of chronic illnesses that commonly occur with aging, such as cardiovascular disease, diabetes, and neurodegenerative disorders. Thus, maintaining adequate vitamin D levels is an important factor for prolonging healthspan and longevity.
Vitamin D deficiency has been strongly associated with various health outcomes, including all-cause mortality (8). A meta-analysis published in prestigious Biomedical Journal aimed to investigate the association between serum vitamin D concentrations and mortality in a large consortium of cohort studies. It was found that people in the lowest 20% of vitamin D levels in the studied group had a 1.6 times higher risk of dying from any cause compared to those in the top 20%. For cardiovascular deaths, the risk was 1.4 times higher in people without a history of heart disease and 1.65 times higher in those who had heart disease before. For cancer deaths, people with a history of cancer had a 1.7 times higher risk of dying from cancer. However, there was no increased risk of cancer death for people who had no prior history of cancer (9).
Direct and indirect actions of Vitamin D on longevity and age related diseases are shown below (10):
Aging reduces vitamin D production in skin. There is a decrease in the concentration of 7-dehydrocholesterol in the epidermis in old compared with young individuals and a reduced response to UV light, resulting in a 50% decrease in the formation of previtamin D3 (11). The following are effects of age on vitamin D and calcium metabolism (12):
- Decreased calcium absorption
- Intestinal resistance of calcium absorption to circulating vitamin D
- Decreased vitamin D receptors
- Decreased renal production of vitamin D by the aging kidney
- Decreased skin production of vitamin D
- Substrate deficiency of vitamin D
Vitamin D Deficiency
Vitamin D deficiency is a global concern, with estimates suggesting that over a billion people worldwide have insufficient levels. Vitamin D deficiency (serum 25-hydroxyvitamin D < 50 nmol/L or 20 ng/ml) is associated with unfavorable skeletal outcomes, including fractures and bone loss. A Vitamin D level of >50 nmol/L or 20 ng/ml is, therefore, the primary treatment goal, although some data suggest a benefit for a higher threshold. Severe vitamin D deficiency with concentration below <30 nmol/L (or 12 ng/ml) dramatically increases the risk of excess mortality, infections, and many other diseases, and should be avoided whenever possible (13).
Reported prevalence rates of severe vitamin D deficiency, defined as levels below 30 nmol/L (12 ng/mL), are 5.9% in the United States (14), 7.4% in Canada (15), and 13% in Europe (16). Additionally, the prevalence of Vitamin D levels below 50 nmol/L (20 ng/mL) is estimated at 24% in the United States, 37% in Canada, and 40% in Europe. These rates may vary with age, with lower levels commonly observed in both children and older adults.
Symptoms of deficiency can vary depending on the severity but commonly include:
- Bone and muscle pain. Due to impaired bone mineralization, leading to conditions like osteomalacia (soft bones) in adults and rickets in children.
- Muscle weakness. Deficiency can cause muscle aches and weakness, increasing the risk of falls and fractures.
- Frequent infections. Weakened immune function may lead to more frequent respiratory and other infections.
- Mood changes and depression. There is an observed link between low vitamin D levels and mood disorders, though the exact mechanisms are still under investigation.
- Fatigue and general malaise. Persistent low energy levels may be a sign of vitamin D deficiency.
Symptoms and diseases associated with vitamin D deficiency are also shown below (17):
Recommended Daily Intake of Vitamin D
The recommended daily intake of Vitamin D varies by age, life stage, and health status. Here are some general guidelines based on age:
- Infants (0-12 months): 400 IU (10 mcg)
- Children (1-18 years): 600-1,000 IU (15-25 mcg)
- Adults (19-70 years): 600-800 IU (15-20 mcg)
- Older Adults (70+ years): 800-1,000 IU (20-25 mcg)
- Pregnant and breastfeeding women: 600-800 IU (15-20 mcg)
Some health organizations suggest higher doses, especially for those with low sun exposure, darker skin, or specific health conditions. For individuals at risk of vitamin D deficiency or those already with one, the doses are much higher, as shown in the table below (18):
The More – The Better? NO!
This is not the case that the more you will consume vitamin D – the better it will be for you. Vitamin D overdose can lead to a condition called hypercalcemia, where there is too much calcium in the blood. Long-term excessive intake of vitamin D can cause serious complications, including kidney damage and cardiovascular issues (19).
The tolerable upper intake level for adults is generally set at 4,000 IU (100 mcg) per day, but toxicity usually occurs with prolonged use of much higher doses such as over 10,000 IU daily. Therefore, always consult with a healthcare provider before taking high doses of vitamin D, because you can cause more harm than good to your body.
Symptoms of vitamin D overdose can include:
- Nausea and vomiting
- Loss of appetite
- Constipation or diarrhea
- Frequent urination
- Increased thirst
- Weakness and fatigue
- Confusion or disorientation
- Bone pain
- Kidney problems, including the formation of kidney stones
- Calcium deposits in soft tissues (e.g., heart, lungs, kidneys)
Tips for Optimal Vitamin D Levels
I think I have proved throughout the article the undeniable importance of vitamin D to our overall health. And I will end this Nexus edition with some tips on increasing and keeping you vitamin D levels in an optimal range:
- Do a blood test. You won’t know if you need vitamin D supplementation or not before you do the test. I suggest to test it twice a year, especially if you are in a higher risk group to have deficiency or if you live quite north. A reminder, the optimal vitamin D level for adults is between 100 and 150 nmol/L (40 to 60 ng/mL).
- Food sources high in vitamin D. Include food in your diet high in vitamin D such as cod liver oil, salmon (wild-caught), mackerel, herring, sardines and tuna (canned in oil), swordfish, trout, egg yolk, fortified milk.
- Choose the right form and dosage. Vitamin D3 is more effective than vitamin D2. Typical daily doses range from 600-800 IU for adults, but higher doses are needed for those who are deficient. If you have severe deficiency, consult with your physician regarding treatment.
- Choose the right formulation. I cant repeat it more times – VITAMIN D IS FAT-SOLUBLE, this means that it should be used either in capsules filled with oil containing vitamin D, or in a liquid form, most frequently olive oil. NEVER USE TABLETS. It will be poorly absorbed and the effect will be minimal.
- Consider combination with calcium. Vitamin D and calcium work synergistically. When used with vitamin D, calcium will be beneficial to ensure adequate calcium intake through diet or supplements to support bone health.
- Timing and absorption. Since vitamin D is fat-soluble, taking it with a meal that contains healthy fats can enhance absorption. Additionally, taking it at the same time each day can improve consistency.
- Monitor and adjust. Periodically recheck your vitamin D levels, especially if you are taking high doses. Excessive vitamin D can lead to hypercalcemia, which can cause complications such as kidney stones and vascular calcification.
Hope you have enjoyed this Nexus edition on a very important vitamin in our body. At least I really enjoyed writing about it! More on longevity, healthspan and health optimization coming soon! Stay tuned and do not hesitate to propose your own topic, I will be glad to write about it!
