Both a vitamin and a prohormone, the vitamin D is vital for healthy bones and teeth. Indeed, it plays a essential role in the metabolism of calcium in the body. It regulates the level of blood calcium by improving the intestinal absorption of this mineral, while minimizing its elimination by the urine. It also participates in the deposition and removal of calcium from bones, as needed by the body. “Calciferol”, one of the other names for vitamin D, comes from Latin and means “who carries calcium”.
Also read : Vitamins : Functions and resources
The benefits of Vitamin D
Vitamin D has also been called “anti-rash vitamin” because rickets, a growth disorder, is caused by vitamin D deficiency. In North America, as well as in several industrialized countries, small amounts of vitamin D milk and margarine to prevent this disease.
The vitamin D actually includes a set of fat-soluble substances that are sometimes called provitamins D. These provitamins include ergocalciferol (D2 – plant form) and cholecalciferol (D3 – animal form). The body partially transforms them into calcitriol (in hormonal form), the compound that generates most of the beneficial effects. Calcitriol also controls many genes that regulate, for example, cell proliferation and differentiation and insulin secretion. It is therefore becoming increasingly evident that vitamin D plays a role that goes far beyond the health of bones and teeth.
The body can directly synthesize the vitamin D through the skin, which is why it cannot be considered strictly or purely as a vitamin. Under the effect of ultraviolet rays from the sun (hence the name “sunshine vitamin”), our body produces cholecalciferol (vitamin D3) which after a passage to the liver then to the kidney takes its active form (calcitriol).
The vitamin D is a special case among nutrients and drugs. Indeed, it can accumulate in fats and the liver where it is stored. Depending on the needs of the body, it can be metabolized and put back into circulation.
Vitamin D dosage
The dosage of vitamin D is often given in international units (IU) rather than micrograms (µg). To find your way around, just know that 1 µg (1 millionth of a gram) is equivalent to 40 IU.
Prevention of osteoporosis
Osteoporosis Canada Recommendations
- For healthy people under 50:
a extra cost 400 IU to 1000 IU (10 to 25 µg) of vitamin D per day and a contribution food in calcium 1,200 mg, to be supplemented with a supplement, if necessary.
- For people aged 50 and over:
a extra cost from 800 IU to 2,000 IU (20 to 50 µg) of vitamin D per day and a contribution food in calcium 1,200 mg, to be supplemented with a supplement, if necessary.
Osteoporosis treatment (for people with osteoporosis)
Osteoporosis Canada Recommendations
- from 800 IU to 2,000 IU (20 to 50 µg) of vitamin D per day in the form of extra cost (the optimal dosage is determined by the attending physician) and a contribution food in calcium 1,200 mg, to be supplemented with a supplement, if necessary.
- The Canadian Cancer Society recommends a supplement of 1,000 IU (25 µg) per day of vitamin D in the fall and winter. Adults who are at higher risk for vitamin D deficiency should do the same all year round: the elderly, those with dark skin pigmentation, who do not go outdoors or who wear clothing covering the major part of their body.
|Several experts believe that the position of the Canadian Cancer Society, although it is an improvement over that of Health Canada, remains too conservative with regard to scientific evidence. Rather, they recommend a daily dosage of 2,000 IU to 4,000 IU of vitamin D3. In summer, the dose could be reduced, provided you expose yourself to the sun regularly (without sunscreen, but without sunburn).|
|Sun and vitamin D
It is estimated that sun exposure can provide 80% to 90% of the vitamin D required. A simple exposure (without sunscreen) of the hands, forearms and face for 10 to 15 minutes (at the latitude of Toronto) between 11 a.m. and 2 p.m., 2 or 3 times a week, would be enough to ensure an adequate intake for a healthy adult, from april to october approximately. This is an average: the exposure time required to get enough vitamin D also depends on the skin type, the intensity of the rays (UVB index), and the basal level of vitamin D in the blood. For example, people with dark skin, those who expose themselves before 11 a.m. or after 2 p.m., those who systematically use sunscreen and those who live in more northern latitudes must be exposed longer, or more often , to get an adequate supply of vitamin D. For example, people with dark skin must be exposed to 3 to 5 times longer than others.
Note. Since prolonged exposure to the sun can cause skin cancer, experts recommend the use of sunscreens with a sun protection index of at least 15 as soon as you are exposed for more than 15 minutes.
Recommended nutritional intake (RDA) for vitamin D
|Age||UI *||µg **|
|0 to 1 year||400 IU ***||10 µg ***|
|1 year to 70 years||600 IU||15 µg|
|over 70||800 IU||20 µg|
|Pregnant and breastfeeding women||600 IU||600 IU|
**1 microgram = 1 millionth of a gram
*** In the absence of sufficient scientific data, the IOM fixed, not a recommended nutritional intake (ANR), but an adequate intake (AS). Adequate vitamin D intake is based on the average intake observed in healthy North American babies.
Even if recommended nutritional intake in vitamin D was tripled in November 2010 by the American and Canadian authorities (table above), several researchers still find it much too low. They suggest increasing it to at least 2,000 IU to decrease the incidence of diabetes, cancer and cardiovascular disease. To learn more about this controversy, see the Vitamin D Research section.
How to take vitamin D supplements
Weekly or monthly dosage. To ensure adequate intake, it is not necessary to take one tablet every day. It is perfectly possible, as long as one chooses a liquid supplement, to take a weekly or even monthly dose. It is an increasingly common medical practice, especially among the elderly who already take a number of medications on a daily basis.
According to the Dr Reinhold Vieth of Mount Sinai Hospital in Toronto, however, vitamin D3 is better in a liquid form because it is not certain that tablets or capsules taken in large quantities dissolve properly. This would result in an insufficient dosage, part of the vitamin being eliminated in the stool before being metabolized. Vitamin D3 in liquid form is available over the counter in pharmacies and health food stores.
The Dr Vieth specifies that “several of these products, duly approved by Health Canada, are currently available. There are some that contain up to 1,000 IU per drop. By mixing them in the required amount with their food, it is easy to take a dose weekly (7 drops or 7,000 IU) or monthly (30 drops or 30,000 IU) “. It also matters, according to the Dr Vieth, to choose a supplement that contains only vitamin D, not a mixture of vitamin D and calcium or a multivitamin supplement, for example.
Studies published to date demonstrate the efficacy and safety of such a practice. In some trials, even annual doses of vitamin D without any side effects, but it seems to be more effective to stick to a bimonthly dosage (every 2 months) or less (monthly or weekly) in order to ensure a constant intake.
Food sources of vitamin D
For a more complete list of sources of vitamin D, see the Vitamin D Nutrient List.
|Salmon, grilled or poached||100 g (3 ½ oz)||600-920 IU|
|Canned salmon||100 g (3 ½ oz)||320-760 IU|
|Grilled bluefin tuna
Marinated Atlantic herring
|100 g (3 ½ oz)||280 IU|
|Grilled trout||100 g (3 ½ oz)||200-280 IU|
|Cow milk, 0% to 3.25% fat||250 ml (1 cup)||120 IU|
|Fortified soy beverage||250 ml (1 cup)||80 IU|
* 1 µg = 1 microgram = 40 IU
Vitamin D deficiency
Note. In this sheet, the term “blood vitamin D level” designates the level of 25 hydroxycholecalciferol (25 (OH) D) in the blood.
- Medical authorities in Canada and the United States have set blood levels for vitamin D adequate for good bone health at 50 nanomoles per liter (nmol / l). According to data collected by Health Canada, 90% of Canadians aged 6 to 79 reach this rate and only 4% are deficient.These data are however strongly contested by many experts (see the Research section). The most recent Canadian study has even established that 25% of the population has a blood vitamin D level below 50 nmol / l in winter. In people not taking a vitamin D supplement, 37% have a winter rate of less than 50 nmol / l. This proportion doubles for Canadians with dark skin. This study involved 5,306 people aged 6 to 79 living across Canada. Several surveys have been carried out in Quebec, Canada and Europe. The results clearly established that a significant proportion of the population living in northern latitudes is deficient in vitamin D, when the minimum blood level is set at 75 nmol / l.
- Too weak Sun exposure can cause a vitamin D deficiency. This is often the case in winter, at certain latitudes, or for people who do not regularly expose their skin to the sun for one reason or another (seriously ill, disabled, elderly , etc.).
- Over-scrupulously following advice on skin cancer by completely avoiding the sun or by consistently using sunscreen contributes to vitamin D deficiency in the population.
- The vegans, who do not consume meat, fish, eggs or dairy products, are at risk of deficiency because their diet provides little vitamin D.
- Certain intestinal absorption disorders, as well as certain diseases, such as cystic fibrosis (cystic fibrosis) or celiac disease, can lead to a deficit in vitamin D.
- It appears that the body’s ability to absorb or synthesize vitamin D decreases with age.
- Pediatricians often recommend supplementation for exclusively breastfed infants since the breastmilk contains very little vitamin D.
- For children, the deficiency in vitamin D causes the rickets. This is characterized by delays in motor development and growth, the failure of the fontanelles to close and weld, poor bone constitution and sleep disturbances. This disease corresponds to osteomalacia in adults. It is then manifested by bone demineralization, diarrhea, nervousness and burning sensations in the mouth and throat.
- According to recent recommendations from the Endocrine Society, people with obesity have increased needs for vitamin D because body fat sequesters some of the vitamin D, preventing it from circulating in the blood. An intake of 6,000 to 10,000 IU per day is recommended for obese adults.
Vitamin D history
During the industrial revolution (XIXe century), an epidemic of rickets hit the city of London. In 1918 Sir Edward Mellanby demonstrated that the disease was caused by a nutritional deficiency. In the 1920s, it was discovered that cod liver oil could counteract rickets.
In 1922, the Dr McCollum isolated the calciferol which we named later vitamin D. But it was not until 1924 that researchers from two American universities simultaneously discovered that the light of Sun was a source of vitamin D. Today, the rickets epidemic in London is attributed to the dense smog that covered the city at that time, depriving its inhabitants of the ultraviolet rays of the sun.
The first synthesis of the vitamin D was produced at Harvard in 1952 by R.B. Woodward, a scientist who received a Nobel Prize in 1965.
Vitamin D research
|Since the late 1990s, vitamin D has raised the greatest hopes for preventing, at minimal cost, the most deadly ailments in developed countries: cancer and cardiovascular diseases, in particular. The amount of work on vitamin D has skyrocketed in recent years. Between 2000 and 2012, the number of studies cited increased from 1,142 to 3,877. In comparison, the citations from studies on vitamins A or C remained more or less stable. However, recent studies, and in particular that published in 2014, report only modest effects, even zero, on these pathologies.
The most convincing results relate to the following conditions: rickets, hyperparathyroidism, psoriasis and osteoporosis.
On the other hand, results from randomized clinical studies are difficult to interpret because vitamin D is often combined with calcium. It is therefore difficult to distinguish the effects of vitamin D from those of calcium. In addition, many journals combine vitamin D with vitamin D analogs, making it difficult to draw conclusions.
Most of the studies are epidemiological studies. These studies, although numerous, only establish links and formulate hypotheses, and therefore do not constitute a evidence of any. Only tests on humans can establish a convincing cause and effect link, they say. In the meantime, they prefer to show “healthy skepticism”.
An opinion not shared by several experts whose opinions were also published in 2011, in several articles of a scientific journal specializing in public health. According to them, the current data are sufficient, because they all point in the same direction. For example, Edward Giovannucci, a world renowned American epidemiologist, considers that the public health authorities can no longer ignore the protective effects of vitamin D, especially since the deficiency is very widespread. Here are some of the other points raised by these experts to question the timidity of the recommendations established in 2010.
In June 2011, the Endrocine Society published its clinical recommendations for detecting and treating vitamin D deficiency. This panel of experts judges that to maintain an adequate blood level (75 nmol / l), daily supplementation of 1,500 to 2,000 IU is required for adults.
- Hypoparathyroidism. This disorder can be caused by the removal of the thyroid glands, causing hypocalcemia. A meta-analysis carried out from 4 randomized studies has shown that vitamin D (400 IU, once or twice a day orally), alone or in combination with calcium (0.5-1.5 g), prevents the risk of hypocalcemia caused by a thyroidectomy.
- Psoriasis. A vitamin D3 analogue – Dovonex® – is used to treat mild to moderate psoriasis. It can also be prescribed in combination with betamethasone (a corticosteroid) or vitamin D. Finally, it has been suggested that a combination of betamethasone and vitamin D is effective. The results also showed that the combination of vitamin analogs and steroids was the most effective treatment, compared to treatment alone with vitamin D, with a response rate of 35 to 86%, compared to a rate 4 to 40% response with vitamin D analogs. Some specialists suggest that vitamin D can be used as monotherapy or in combination with corticosteroids in the treatment of genital psoriasis.
- Rickets. Rickets is a condition caused by a vitamin D deficiency in children. The American Academy of Pediatrics recommends that babies receive at least 500 mL of vitamin D per day. Ergocalciferol or cholecalciferol are effective in treating rickets.
- Cavities. Many studies have reported an association between the consumption of vitamin D and the prevention of cavities. Studies have notably shown that vitamin D halves the risk of cavities. However, some of them are obsolete and with a weak research method.
- Fall prevention. Numerous clinical trials indicate that vitamin D, alone or in combination with calcium, prevents falls, especially in elderly people living in institutions. Vitamin D analogs such as alfacalcidol also appear to have a protective effect by increasing the absorption of calcium, which is also the case with the combination of cholecalciferol and calcium. The daily oral doses of vitamins used are 400-800 IU for vitamin D3 and 200-1,100 IU for vitamin D2, for a period of 3 to 6 months. If the studies are combined, the risk reduction can reach 30%. The authors of these studies suggest that the effectiveness of vitamin D is most pronounced in individuals with low vitamin levels. It should be noted, however, that other studies have not reported any preventive effect of vitamin D.
- Muscle pain and weakness. Although vitamin D deficiency is associated with muscle pain and weakness, current data cannot demonstrate with certainty that vitamin D supplementation has a preventive effect.
- Osteoporosis. Vitamin D and calcium are recommended in patients with osteoporosis, as well as in the prevention of osteoporosis following endocrine problems or a diet lacking in nutrients. Specialists therefore recommend a daily consumption of 1.2 g of calcium and 800-1,000 IU of vitamin D to prevent osteoporosis. A lot of research confirms the effectiveness of the association vitamin D/ calcium to prevent osteoporosis and slow its progression in people over 50, especially postmenopausal women. This is manifested by a slight increase in bone mineral density, often accompanied by a reduced risk of fracture. According to several authors, studies have clearly demonstrated that, to be effective, vitamin D supplementation must be accompanied by a supplement of calcium and be continuous.
According to other researchers, vitamin D (700 IU and more per day) alone has been proven to prevent fractures and the falls, regardless of calcium intake. Please note that Osteoporosis Canada recommends taking calcium preferably from food and supplementing if necessary with a supplement.
However, the results of a clinical study indicate that the preventive effect of vitamin D (400 to 7,000 IU) on fractures related to osteoporosis is hypothetical. Similarly, the preventive role of vitamin D (more than 400 IU per day for at least 6 months) in the preventive effects of osteoporosis in postmenopausal women does not seem clear, according to a meta-analysis of 25 randomized clinical studies.
- Epilepsy. A systematic review points out that vitamin D (200 IU per day) combined with calcium (390 mg) improves the well-being of patients with epilepsy.
- Cancer prevention. In vitro and animal studies have shown that vitamin D, more specifically the active hormonal form (calcitriol) has preventive effects against cancer and delays the progression cancerous tumors. On the other hand, a good number of studies indicate that living at a high latitude, therefore less sunny (40e parallel and more), is associated with an increased risk of several cancers. In addition, case-control studies have looked at the association between blood vitamin D levels and cancer risk. Here are the 3 forms of cancer that researchers have most often looked into: colorectal cancer, breast cancer and prostate cancer. Despite some encouraging results, no rigorous study allows to say that vitamin D has any effectiveness on this pathology.
Colorectal cancer. Colorectal cancer is the strongest link: low blood vitamin D levels are clearly associated with higher cancer risk, according to several meta-analyzes. Even very conservative organizations like the National Cancer Institute in the United States and the International Agency for Research on Cancer (IARC) of the World Health Organization recognize that this link is well documented.
However, a very large trial (Women Health Initiative – WHI, 36,282 women) was inconclusive: the participants took 1,000 mg of calcium and 400 IU of vitamin D and were followed for 7 years. After this period, a similar number of women had suffered from colorectal cancer.
Breast cancer. A link has been established by some researchers between a lower risk of breast cancer and a high intake of vitamin D, especially when it is more than 400 IU per day. Meta-analyzes of epidemiological studies, however, conclude that the link between a low vitamin D level and an increased risk of breast cancer is not clearly established. In addition, as in the case of colorectal cancer, the WHI study was not conclusive in terms of the preventive effect of vitamin D against breast cancer.
Prostate cancer. Meta-analyzes from epidemiological studies have not established a clear link between a low vitamin D level and an increased risk. One study found that vitamin D has no protective effect in prostate cancer, while two others report only mixed results. A meta-analysis published in 2014 covering 7 clinical trials did not report the effectiveness of a vitamin D intake – with or without calcium – on cancer.
Prevention of other diseases
Prevention of type 1 diabetes. Several case-control studies have shown that vitamin D supplementation during pregnancy and the early childhood is associated with a lower risk of this disease. And a recent clinical trial indicates that vitamin D supplementation (4,000 IU per day) has had a beneficial effect on blood sugar control in people with type 1 diabetes.
Prevention of type 2 diabetes. Vitamin D is thought to play an important role in this disease, as it influences insulin secretion and insulin resistance, as well as inflammation. The epidemiological data are numerous, but not always conclusive as regards a link between a low blood level of vitamin D and an increased incidence of type 2 diabetes. Clinical data are available, but they do not allow us to conclude. In fact, according to a systematic summary published in 2010, they come from trials with few subjects or trials whose primary objective was to verify the effect of vitamin D on bone health.
Prevention of certain autoimmune diseases. The active form of vitamin D has an immunomodulatory action. It could therefore play an important role in cases of autoimmune diseases, such as multiple sclerosis, rheumatoid arthritis and inflammatory bowel disease. This role is confirmed by epidemiological data.
Prevention of cardiovascular disorders. The vitamin D plays several important roles in cardiovascular health: reduction of inflammation, calcification of vessels, and blood pressure, for example. Several syntheses of epidemiological data conclude that there is a link between a low level of vitamin D in the blood and an increased risk of cardiovascular diseases. The authors disagree on the strength of this link, some consider it robust others, weaker.
In a placebo-controlled trial, taking 2,000 IU of vitamin D daily for 16 weeks reduced the arterial hardening (risk factor for hypertension, in particular) in adolescents of black color. A point that was confirmed by an observational study presented in April 2011 at the annual conference of the American College of Cardiology. The authors of this study, conducted with 554 subjects, found that a low intake of vitamin D was systematically associated with symptoms of vascular dysfunction. More specifically, the arteries lost their ability to dilate properly when the blood level of vitamin D dropped below 75 nmol / l. See our news Insufficient vitamin D intake is linked to hardening of the arteries, to find out more.
Link between vitamin D and metabolic syndrome is also drawing the attention of researchers.
Boosting immunity. In several recently published placebo-controlled clinical trials (2010 and 2011), vitamin D supplementation has:
- reduces the incidence of seasonal flu in children 6 to 15 years of age (1,200 IU per day for 4 months);
- slightly reduces the incidence of respiratory infections in young men (400 IU per day for 6 months);
- increased the immune response of subjects who were vaccinated against tetanus (2,000 IU per day during the 10 weeks preceding vaccination);
- reduces the number of relapses of pneumonia in children from 1 month to 36 months (a massive single dose of 100,000 IU).
Cognitive decline. In recent years, epidemiological studies in various countries have linked low blood vitamin D levels to an increased risk of cognitive decline in the elderly. A link with Alzheimer’s disease is also starting to emerge. A study published in 2014 demonstrated that low levels of vitamin D metabolites are associated with memory impairment and executive function. In addition, taking vitamin D could increase cognitive performance in older women. However, these results remain to be confirmed. A link with Alzheimer’s disease is also starting to emerge.
Asthma. In recent years, researchers have linked low vitamin D levels to asthma in children. A low level of this vitamin has also been associated with more frequent hospitalizations in asthmatic children followed for 4 years. However, more rigorous studies are needed to confirm these effects.