Vitamin D

​Autumn is starting and it's time to think about vitamin D. I tell you more about this vitamin not quite like the others.

Most Europeans and North Americans lack vitamin D, more than 75% of Americans and French people are vitamin D deficient. On top of it health authorities underestimate daily needs.
To make things worse, in France, health authorities have lowered the nutritional advices (ANC) intake  levels from 400 IU (10 µg) before 2000 to 200 IU (5 µg) now, while the European Food Agency (EFSA) recommends a daily intake of 600 IU (15 µg) of vitamin D for everyone.
Deficits are associated with a variety of health problems such as an increased risk of fracture, diabetes  or Parkinson's disease....
Women are also reported to be more likely to have vitamin D deficiency than men.
Elderlies are also a population at risk as with age, the precursor of vitamin D3, necessary for the synthesis of vitamin D by the action of UVBs, decreases. 

Why is vitamin D deficiency so common?

Latitude, time of the year, age, sex, skin colour, clothing and gender are all factors that affect the ability to produce vitamin D after exposure to the sun. 
Depending on the latitude, the intensity of ultraviolet rays varies. The intensity is the highest at the equator and decreases with increasing latitude. In mid- and high latitudes as in Europe, radiations have more ground to cover.
Vitamin D synthesis varies in the same way according to the seasons. During the winter months, the radiation is less intense and of shorter duration, which explains why vitamin D synthesis is more difficult during the winter months in many countries of the Northern Hemisphere.  If you are north of 37 degrees above the equator, or south of 37 degrees below the equator, you will not be able to produce vitamin D from the sun in winter at all.

https://grassrootshealth.net/wp-content/uploads/2017/09/Sunshine-Calendar-768x594.png

​Sun and vitamin D

​In summer, synthesis of vitamin D by the skin covers for two thirds of the intake. Exposure to the sun should be from March onwards at a rate of 15 to 30 minutes per day. 

​What is vitamin D?

Vitamin D is not a vitamin like any other, as it cannot be provided only by food. The sun is an essential element in the synthesis of vitamin D.
In the end, vitamin D should rather be considered as a prohormone, as it influences metabolism, cellular function and gene expression.
Vitamin D obtained either by the sun or by food or supplements is biologically inert and must undergo two transformations (hydroxylations) in the body to become active. The first occurs in the liver and converts vitamin D into 25-hydroxy vitamin D 25(OH)D (calcidiol), the storage form. The second occurs when it is transported to the kidney  to be transformed into 1,25-dihydroxy vitamin D 1.25(OH)2D (calcitriol) which is the active form.

​It has recently been found that vitamin D, in addition to being delivered to the liver for conversion to 25(OH) D, is also used directly by all tissues in the body. Many of these tissues (breast, colon, prostate and brain) can convert vitamin D into the active form within the tissues themselves. It is through this process that vitamin D can help cells fight infections, diseases and autoimmune disorders.

​The different forms of vitamin D

There are several variations of vitamin D.

• Vitamin D3 (cholecalciferol) made by the skin from a cholesterol derivative via ultraviolet light. As a supplement, it is of animal origin either extracted from fish liver or by exposing lanolin to UVB radiation.
• Vitamin D2 (ergocalciferol) present in plants (cereals, mushrooms, yeasts)

​Foods that are a source of vitamin D

​Vitamin D is mainly found in fish and dairy products specially if they are fortified.
There are no vegetables or fruits rich in vitamin D.

​Who is at risk of deficit?

In what situations can we  be at risk of deficit?
​ 

• have little or no exposure to the sun
• use sunscreen (UVB protection index >15)
• wear covering clothing
• spend all the time indoors.
• live in a city or region that is experiencing air pollution
• have pigmented skin (tanning, naturally dark or black skin)
• be over 70 years old
• live in northern Europe, the United States and Canada
• be pregnant
• be overweight (obese)
• expose yourself behind a window
• take medication (corticosteroids...)
• suffer from certain diseases (renal and hepatic insufficiency, hyperthyroidism, celiac disease, etc.)​
• eat little or no oily fish

​ How is a deficit measured?

A doctor will prescribe a blood test to evaluate your vitamin D level in the storage form 25 (OH) D, some home test kits are also available.
The indicated level can be in ng/mL (nanograms per millilitre), nmol/l (nanomoles per litre).
The dosages are made in IU (international units) or µg (micrograms) depending on the country.
 ​1 IU = 0,025 µg (÷ 40)
1 µg = 40 UI (x 40)
 
10 ng/mL = 25 nmol/L (x 2,5)
25 nmol/L = 10 ng/mL (÷ 2,5)

The currently recognized reference value levels of 25(oH)D are as follows:

​Vitamin D requirements

The recommended vitamin D nutritional intakes (ANC) for adults in France are 200 IU/day (or 5 µg/day) since 2001. These contributions appear to be underestimated, particularly in winter. Not considering any pathology and according to data in the international literature, the recommended daily intake (RDI) is 800 to 1000 IU/day. The RDA (recommended daily allowance) for children, adolescents and adults is 600UI/ day and 800UI/ day for seniors.
To compare, other organizations have different values ,the Vitamin D Council recommends a consumption of 5,000 IU/day while the Endocrine society recommends a consumption of 1,500-2,000 IU/day.

comparison between the different recommendations

​Vitamin D and health

1.25 (OH) 2D, the active form of vitamin D, regulates many genes and is involved in several metabolic pathways.

• role on bone health (stimulates intestinal absorption of calcium) 
• essential in the intestinal absorption of calcium and phosphorus
• role on the development of osteoporosis in postmenopausal women and the elderly
• ​modulator role in the functioning of the immune system
• involved in conditions such as hypertension, coronary atherosclerosis and diabetes.
• likely role on cell degeneration
• role on cancer prevention considered and studied
• association between vitamin D consumption and tooth decay prevention
• role in preventing falls, especially among the elderly
• role in treatment of hypoparathyroidism, psoriasis and rickets.

​Vitamin D and medicines

​Anti-epileptics, anti-convulsants, glucocorticosteroids and AIDS drugs can cause vitamin D deficiency in the long term.
​As this type of drug is often prescribed in the long term, supplementation is essential, according to the Endocrine Society.

​Toxicity and maximum doses

​The American Food and Nutrition Board indicates a maximum dose of 4000 IU/day/100µg from 9 years of age.
However, several researchers consider this Tolerable Upper Intake Level (UL) to be far too conservative and based on a misinterpretation of the available data and suggest an UL of 10,000 IU based on clinical trials.

Sources:
https://grassrootshealth.net/blog/daily-dosing/
https://www.vitamindcouncil.org/i-tested-my-vitamin-d-level-what-do-my-results-mean/
lanutrition.fr dossier vitamin D automne 2016
http://www.sarahbest.com/vitamin-d-and-the-sun/
http://drholick.com
https://www.revmed.ch/RMS/2011/RMS-319/Vitamine-D-actualite-et-recommandations
https://www.vitamined.net/besoin-journalier-vitamine-d/
https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

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