Vitamin d is a fat-soluble vitamin that is
important for bone health.
Vitamin d is completely different from most
other vitamins.
In fact, when your skin is exposed to the sun,
it is a steroid hormone produced by cholesterol.
For this reason, vitamin d is often referred
to as the "sunshine vitamin."
However, sun exposure rarely provides enough
vitamin d, so it must be taken from supplements or diets.
However, only a few foods contain large
amounts of this important vitamin, and deficiencies are common (one, 2, 3).
In fact, approximately 41.6% of the U.S. the
population is deficient (4).
This article explains everything you need to
know about vitamin d.
What is vitamin d?
Vitamin d is a fat-soluble vitamin, meaning it
dissolves in fats and oils and can be stored in the body for a long time.
There are two main forms of diet (5):
- Vitamin D3
(cholecalciferol). Found
in some animal foods, such as fatty fish and egg yolks.
- Vitamin D2
(ergocalciferol). Found
in some plants, mushrooms, and yeasts.
In both cases, D3 (cholecalciferol) appears to
be almost twice as effective at increasing vitamin D levels in the blood
(ergocalciferol) (6, 7).
Summary: Vitamin D is a fat-soluble vitamin that your
body can store for a long time. In the two main forms— D2 and D3 — the latter
is more effective at increasing vitamin D levels in the blood.
What does it do in your body?
Vitamin D needs to go through two
transformation steps to become active (eight, 9).
First, it converts to calcined glycol or 25(OH)D
in your liver. This is the stored form of the vitamin.
Second, it is mainly converted into calcitriol
or 1,25(OH)2D in your kidneys. This is the active steroid hormone form of
vitamin d.
Calcitriol interacts with the Vitamin D
Receptor (VDR), which is present in almost every cell in your body. （10, 11）。
When the active form of vitamin D binds to
this receptor, it turns genes on or off, causing changes in the cell. This is
similar to how most other steroid hormones work (12, 13).
Vitamin D affects a variety of cells
associated with bone health. For example, it can promote the absorption of
calcium and phosphorus by the intestines (14).
However, scientists have recently discovered
that it also plays a role in other areas of health, such as immune function and
cancer prevention (15).
Summary: Vitamin D is converted to calcium, the stored
form of the vitamin, and then to calcitriol, the active steroid form.
Calcitriol binds to intracellular vitamin D receptors, turning genes on or off.
Sunlight is an effective way to get vitamin d
When the skin is exposed to ultraviolet B
(UVB) from the sun, cholesterol in the skin produces vitamin D (16).
If you live in a sunny area, sunbathing a few
times a week may be able to get all the vitamin d you need.
Remember that you need to expose a large part
of your body. If you only expose your face and hands, you will produce less
vitamin d.
In addition, if you stay behind glass or use
sunscreen, you will produce less vitamin d — or none at all (17).
However, when staying in the sun for a long
time, you should make sure to use sunscreen. Sunlight is healthy, but sunburn
can cause premature skin aging and increase the risk of skin cancer (18, 19).
If you stay in the sun for an extended period,
consider not using sunscreen for the first 10-30 minutes (depending on your
sensitivity to sunlight) and applying it before starting to burn.
Because vitamin d is stored in the body for
weeks or months at a time, you may only need occasional sun exposure to
maintain adequate blood levels.
That said, if you live in an area with
insufficient sunshine, getting vitamin d from food or supplements is an
absolute necessity — especially in the winter.
Summary: Sunlight is an effective way to get vitamin D,
but sunscreen can hinder its production. While safe sunbathing can help you get
enough levels, many people cannot enjoy the sun for most of the year.
Best food source
Here are some of the best food sources for
vitamin D3 content (20):
|
food |
quantity |
% RDI |
|
cod liver oil, 1 tablespoon (15 ml) |
1,360 IU / 34 mcg |
227% |
|
salmon, cooked, 3 OZ (85 g) |
447 IU / 11 mcg |
75% |
|
tuna, canned water, 3 OZ (85 g) |
154 IU / 4 mcg |
26% |
|
beef liver, cooked, 3 OZ (85 g) |
42 IU / 1 mcg |
7% |
|
1 LARGE WHOLE EGG (EGG YOLK CONTAINS D) |
41 IU / 1 mcg |
7% |
|
1 sardine, canned in oil and drained |
23 IU / 0.6 mcg |
4% |
While fatty fish such as salmon, mackerel,
swordfish, trout, tuna, and sardines are good sources, you have to eat them
almost every day to get them full.
The only excellent dietary source of vitamin d
is cod liver oil – for example, which contains more than twice the daily
reference intake (RDI) in a tablespoon (15 ml).
Keep in mind that dairy products and grains
often contain vitamin d (21).
Some rare mushrooms also contain vitamin d,
and egg yolks contain small amounts.
Summary: Cod liver oil is the best source of vitamin D3. Fatty fish
is also a good source, but you have to eat it regularly to get enough of it.
Deficiency symptoms
Vitamin d deficiency is one of the most common
nutrient deficiencies.
Some people are at greater risk than others.
in the united states, 41.6 percent of the total population is deficient,
although the situation is worse for minorities—82.1 percent and 69.2 percent
are black and Hispanic," respectively (4).
In addition, older adults are at a much
greater risk of nutritional deficiency (22).
People with certain diseases are also likely
to be deficient. One study showed that 96% of heart attack patients had low
vitamin D levels (23).
Overall, vitamin d deficiency is a silent
epidemic. The symptoms are usually subtle and can take years or decades to
surface.
The most famous symptom of vitamin d
deficiency is rickets, a bone disorder commonly found in children in developing
countries.
Thanks to the addition of vitamin D to certain
foods, rickets has been largely eliminated from Western countries. （24）。
Deficiencies have also been linked to
osteoporosis, decreased mineral density, and increased risk of falls and
fractures in older adults (25).
In addition, studies have shown that people
with low vitamin D levels have a much higher risk of autoimmune diseases such
as heart disease, diabetes (types 1 and 2), cancer, dementia, and multiple
sclerosis. （26）。
Finally, vitamin D deficiency has been linked
to shortened life expectancy (27, 28, 29).
That said, it's unclear whether deficiency
causes these diseases, or whether people with low levels are more likely to
develop them.
Summary: Vitamin D deficiency has been linked to a variety of
health problems and shortened life expectancy.
Potential health benefits
Here are some of the potential benefits of
vitamin d:
- Reduces the risk of
osteoporosis, falls, and fractures. Higher doses of vitamin D help prevent osteoporosis,
falls, and fractures in older adults. （30）。
- Better strength. Vitamin D can increase physical strength in the upper
and lower extremities (31).
- Cancer prevention. Vitamin D may help prevent cancer. One study noted that
1,100 IU per day – along with calcium – reduced cancer risk by 60% (32, 33).
- Depression management. Studies have shown that vitamin D may relieve symptoms
in patients with clinical depression (34).
- Reduces the risk of developing
type 1 diabetes. One
infant-specific study showed that 2,000 IU of vitamin D per day reduced
the risk of type 1 diabetes by 78% (35).
- Increase mortality. Several studies have shown that vitamin D may reduce
people's risk of dying during the study period, suggesting that it may
help prolong life (36, 37).
However, many of these results are
preliminary. According to a recent review, more evidence is needed to confirm
many of these benefits (38).
Summary: Research shows that vitamin D may have many benefits for
cancer, bone health, mental health, and autoimmune diseases. However, more
research is needed.
How much should you eat?
The only way to know if you are deficient (and
therefore need a supplement) is to measure your blood levels.
Your healthcare provider will measure the
stored form of vitamin D, called calcifediol. Anything below 12 ng/ml is considered
insufficient, while anything above 20 ng/ml is considered sufficient.
The RDI for vitamin D is as follows (39):
- 400 IU (10 mcg): Infant, 0-12 months
- 600 IU (15 mcg): children and adults, 1-70 years
- 800 IU (20 mcg): Elderly and pregnant or lactating women
Although the adequacy of the measurement is 20
ng/ml, many health experts believe that people should set the concentration in
the blood above 30 ng/ml for optimal health and disease prevention (40).
In addition, many believe that the recommended
intake is too low and that people need more to reach optimal blood levels (41).
According to the National College of Medicine,
the safety limit is 4,000 IU (100 micrograms) per day (42).
Vitamin d3 supplements appear to be more
effective than d2 supplements at raising vitamin d levels. D3 capsules are
available in most supermarkets and health food stores, as well as online.
Summary: The RDI for vitamin D is 400 IU (10 mcg) for
infants, 600 IU (15 mcg) for children and adults, and 800 IU (20 mcg)
for older adults and pregnant or lactating women.
Optimize your other nutrients
It is important to remember that nutrients
usually do not function in isolation.
Many of them are interdependent, and
increasing your intake of one nutrient may increase your need for another.
Some researchers claim that fat-soluble
vitamins work synergistically and that it is critical to optimize vitamin a and
K intake while supplementing with vitamin D3 (43, 44).
This is especially important for vitamin K2,
another fat-soluble vitamin that most people do not consume enough (45).
Magnesium— another important mineral often
lacking in modern diets — may also be important for vitamin D function (46, 47).
Summary: There is evidence that vitamin D works with
magnesium and vitamins A and K to promote health.
What happens if you take too much?
Vitamin d is easy to overdose on, which is a
myth.
Vitamin D toxicity is very rare and can only
occur when you take very high doses for long periods (48).
The main symptoms of intoxication include
confusion, inattention, drowsiness, depression, vomiting, abdominal pain,
constipation, and high blood pressure. (49）。
Summary: Vitamin D toxicity is very rare. Symptoms include
confusion, drowsiness, depression, constipation, and high blood pressure.
Conclusion
Vitamin d is a fat-soluble vitamin that is
important for bone health.
For people who are low in this nutrient,
increasing intake can also reduce depression and improve strength.
When exposed to the sun, your skin produces
vitamin d. foods such as fatty fish, fish oil, and liver also contain vitamin
d, as well as certain fortified foods and supplements.
Deficiencies are common due to limited sun
exposure and a small number of abundant dietary sources.
If you do not often bask in the sun and rarely
eat fatty fish, you may consider supplementing.
Getting enough vitamin d can go a long way
toward promoting health.