As way back, as the mid-1920s, Windaus et al. suggested that skin, when exposed to sunlight, produced the active form of Vitamin D (D3) from a cholesterol precursor – 7-dehydrocholesterol (Holick, 2016).
Synthesis in the skin is the major source (up to 90%) of Vitamin. D (VIT D) in humans. Lack of appreciation that sun exposure is the major source, is the primary reason why VIT D deficiency is now a worldwide epidemic! Very few foods contain enough VIT D; foods fortified with VIT D aren’t of much use either: often inadequate to satisfy even a child’s (let alone an adult’s) VIT D requirement!
What is Vitamin D? And, what does it do?
Vitamin D is a master hormone rather than just a vitamin per se – has multi-system functions. Few of these are:
- Calcium-bone metabolism, nerve impulse generation & conduction
- Functioning of the muscle, heart, pancreas and endocrine organs
- Strengthens immunity, fights infections including TB
- Plays a role in cancer prevention – lung, colon, and breast, to name a few
Vitamin D Deficiency
VIT D deficiency has a worldwide prevalence; it underpins the aetiology (cause) of several chronic metabolic-endocrine disorders, including obesity. Researchers define deficiency as 25(OH)-VIT D blood levels of < 20 mg/mL; blood level of 25-
hydroxyVit. D of >75 nmol/L, or 30 mg/ml is required for optimal health.
IsVIT D deficiency a problem? Yes indeed; because deficiency ofVIT D may increase the risk of (Source: Pereira-Santos M. et al., 2015):
- Osteoporosis (thinning of bones), and therefore increased risk of fractures,
- Osteoarthritis,
- Metabolic disease including obesity, diabetes, hypertension, and several autoimmune diseases/ disorders,
- Infections – VIT Ddeficiency compromises functioning of immunity and therefore, may increase risk of the individual to infections,
- Psychiatric disorders (mainly, depression), and
- Cancers – those exposed to more sunlight during their lifetimes are less likely to die of cancers. Cancer-related death rates decline as one moves toward the lower latitudes (between 37°N and 37°S)
VIT D Deficiency and Obesity
Obese individuals exhibit 35% greater prevalence of VIT D deficiency compared to leaner ones. It is suspected that obese individuals – owing to the social stigma attached with being obese – are more likely to reduce their exposure to sunlight, perform fewer outdoor activities and/or use clothes that cover much of the body; this limits the exposure to sun and consequently, hampers cutaneous (skin) VIT D synthesis.
How low VIT D levels favour Obesity
- Increased metabolic clearance of VIT D possibly with enhanced uptake by fat tissue (VIT D is fat-soluble), leaving little in the plasma to do its job
- Increased differentiation of pre-adipocytes (immature fat cells) into adipocytes (mature fat cells) capable of storing fat
- Increased inflow of calcium into fat cells, leading to increased fat production
- Increased secretion of parathyroid hormone, shown to be linked to obesity
Sources of VIT D
80-90% of VIT D present within the human body originates from skin synthesis where sunlight activation plays a key role; rest is supplied through food or supplementation (these sources absolutely pale in comparison to sunlight!).
Sunlight as a Source of VIT D
Synthesis in the skin – secondary to exposure to sunlight - remains the major source (up to 90%) of VIT D in humans. Lack of appreciation that sun exposure is the major source, is the primary reason why VIT D deficiency is now a worldwide epidemic! The recommendations for the avoidance of all sun exposure due to risk of skin cancer has put the world’s population at risk of VIT D deficiency.
Daily solar exposure – to maintain physiologically effective serum levels – 15 minutes in summer and 20 minutes in early fall or late spring is recommended; coloured people require twice as long.Interestingly, from November to March, in countries north of 37° latitude regions, no amount of solar exposure is sufficient!
In individuals who tend to burn easily / tan poorly, exposure to sun should not exceed 20 minutes per day; exposure longer than 20 minutes does not further increase VIT D synthesis but could increase risk of skin cancer!
Food Sources of VIT D
Few foods contain VIT D; foods fortified with VIT D are also an alternative.
- Fatty Fish: tuna, mackerel, salmon, sardines (and caviar), fish Oils: Cod liver oil
- Eggs (yolk), milk, cheeses, fortified dairy products and orange juices
- Mushrooms
Pharma Sources
Oral VIT D3 supplementation rather than solar exposure, should be used by fair-skinned, sun-sensitive individuals.
As mentioned earlier, blood level of 25-hydroxyVit. D of >75 nmol/L, or 30 mg/ml is required for optimal health. In the absence of adequate sun exposure, supplementation with 800-1000 IU VIT D/day is needed to achieve the above levels. Pharmaceutical form of VIT D in the US is VIT D2; in Canada, Europe, Japan and India, VIT D3.
Most of us have low vitamin D levels. A recently published study shows that, this is likely due to a genetic predisposition and that low vitamin D may not necessarily be a problem; low VIT D or low calcium intake are not associated with increased fracture risk, say the authors. In short, for most of us, there’s no need for VIT D-calcium supplementation! Also, do not forget that VIT D- Calcium supplementation has no/minimal benefits but may pose health risks!
Take Home Messages
- Lack of physiological blood levels of VIT D can cause health problems
- Almost 90% of VIT D present in the human body come from synthesis in the skin after exposure to sunlight
- Sunlight exposure remains your best bet to keep VIT D levels within physiological ranges to avoid diseases or disorders
- Sunlight exposure of 15 minutes in summer and 20 minutes in early fall or late spring is recommended; coloured people require to be twice as long in the sun.
References
- Catherine Shore-lorenti et al, Clinical Endocrinology (2014) 81, 799-811
- Garland et al., Am J Public Health. 2006 Feb;96(2):252-61
- Nathaniel Mead, Environmental Health Perspectives. April 2008; 11(4)
- Pereira-Santos et al., Obesity Reviews. April 2015; 16(4):341–349
- Michael M. Holick, Anticancer Res. 2016 Mar;36(3):1345-56
- Michael F Holick and Tai C Chen, Am J ClinNutr 2008;87(suppl): 1080S-6S.
- Katerina Trajanoska et al., BMJ. 29th. Aug., 2018
About The Author
Dr. Deepak specialises in Sports & Exercise Medicine. He has previously worked in the UK (with top athletes, as well as reputed personalities from the entertainment industry..
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