Vitamin D is an essential fat-soluble vitamin. It’s sometimes called the “sunshine vitamin” because our skin makes it when exposed to the sun. It’s also the most common nutrient deficiency.
Like most vitamins, vitamin D has many functions in the body. It’s mostly known for its ability to help build strong bones. But, vitamin D is also important for a healthy immune system, digestive system, heart and mental health, blood sugar regulation, fertility, and resistance to cancer.
Vitamin D in the body
Vitamin D (calciferol) isn’t “active” in our bodies. To do its wonders, it first needs to be converted into the active form. This is a two-step process. First, the liver converts it into 25(OH)D (calcidiol). Then, that is converted into 1,25(OH)D (calcitriol) in the kidneys. It’s this third, calcitriol, form that’s active in the body.
Vitamin D acts like a hormone. That means it’s produced in one part of the body (the skin) and travels through to act on another part (the bones).
Because vitamin D is fat-soluble, when you have more than enough, it gets stored in the liver and isn’t flushed out in the urine like excesses of many other vitamins are.
Vitamin D for bones
Vitamin D is most known for its importance for bone health. Bones are alive and are constantly remodeling themselves. This means they, as all tissues, need a constant supply of nutrients.
Vitamin D helps the body absorb calcium more efficiently. And the mineral calcium is one of the major players to “mineralize” and strengthen our bones.
Vitamin D works with other hormones to ensure optimal levels of calcium in the blood. When it comes to calcium, the body always prioritizes the blood over the bones. This is because the blood transports calcium around the body for critical functions like contractions of the heart and muscles. This is why it’s more important to maintain the calcium levels in the blood over levels in the bone.
When there is enough calcium in the blood, any excess is stored in the bones. This is when the bones are mineralized and strengthened. When there isn’t enough calcium in the blood two things happen to raise this level. First, vitamin D stored in the liver is activated to help absorb more calcium from food. Second, the body removes calcium stored in the bones to raise levels in the blood.
When we don’t get enough vitamin D (and calcium) regularly, bones can become weak and brittle. In children, severe vitamin D deficiency can cause rickets, and in adults, it can cause osteomalacia. With less severe vitamin D (and/or calcium) “insufficiency” (as opposed to a more severe “deficiency”), osteoporosis can develop over the long term.
Having enough 25(OH)D in the blood is associated with higher bone density. Studies show that supplementing with vitamin D may reduce the risk of falls and bone fractures.
Vitamin D, the immune system, and inflammation
Several studies have shown a link between low levels of vitamin D and immune-related conditions like atopic dermatitis and rheumatoid arthritis. In the lab, vitamin D seems to have “anti-inflammatory” and “antioxidant” properties.
Vitamin D can reduce immune response and inflammatory markers. Some studies in people with immune conditions (e.g. cystic fibrosis, multiple sclerosis, & obesity), show that supplementing with vitamin D reduces some inflammatory markers in the blood, although not all studies agree.
Some researchers think vitamin D, due to its effects on the immune system, may also help with serious food allergies. A few small studies show that children with low vitamin D levels have an increased risk of food allergies. More research is needed.
Vitamin D and digestive diseases
Since vitamin D is fat-soluble, it’s absorbed along with fat in the diet. So, people who don’t eat or absorb enough fat are at risk of lower vitamin D levels. This can include people with many digestive issues such as celiac disease, inflammatory bowel diseases (IBD) like Crohn’s & colitis, as well as people who have had gastric bypass surgery.
Also, a healthy vitamin D status seems to go hand-in-hand with a healthy gut. For example, there is a link between sub-optimal vitamin D, gut microbiome status, gut inflammation, and diseases of the gut like IBD and colon cancer.
Vitamin D and cancer
It’s not just colon cancer that’s associated with low levels of vitamin D. Higher levels of vitamin D are associated with a lower risk for prostate, and breast cancers.
In the lab, cancer cells don’t seem to do as well when exposed to higher levels of vitamin D. They don’t divide or invade other tissues as well; and, they seem to die easier.
It’s unclear whether supplementing with vitamin D would reduce the risks of cancer in people.
Vitamin D and heart health
Several studies have linked low levels of vitamin D in the blood with heart disease.
Higher levels of vitamin D in the blood may reduce blood pressure and the risk of heart disease by a small amount.
Supplementing with vitamin D may help lower blood pressure slightly, but the evidence isn’t clear on how supplementing affects the risk of heart disease.
Vitamin D and blood sugar
Low vitamin D levels are associated with higher levels of insulin resistance in people without diabetes. It may also increase the risk of developing diabetes.
Supplementing with vitamin D may help improve blood sugar management in some people with diabetes.
Vitamin D for mental and brain health
Cells in key areas of the brain have “receptors” for vitamin D. Vitamin D also has a role in circadian rhythms and sleep, affects the growth of nerve cells, and impacts the developing brain.
There is growing evidence of the links between low blood levels of 25(OH)D and symptoms of depression.
Some studies also show a link between low vitamin D levels and increased risk of Alzheimer’s and Parkinson’s diseases.
Vitamin D and fertility
Vitamin D seems to help improve the motility and survival of sperm cells. Both too high and too low levels of vitamin D in the blood seem to be associated with infertility.
Forms of vitamin D
Many vitamins come in more than one form. With vitamin D, it comes in two different forms: D2 and D3. There are small differences in their chemical structure.
Vitamin D2 (ergocalciferol) is the plant-based form, while vitamin D3 (cholecalciferol) is from animals. Both forms can help rickets. At higher doses, however, vitamin D2 is less potent than vitamin D3.
Sources of vitamin D
There are three main sources of vitamin D – sun exposure, foods, and supplements.
Our skin contains “pre” vitamin D. When exposed to UV rays from the sun, this “pre-vitamin” is converted into vitamin D (calciferol). In fact, vitamin D levels decline in people throughout the winter. The problem is that too much UV radiation can contribute not only to skin cancer but also to dryness and other cosmetic changes in the skin over time.
Vitamin D is not naturally found in very many foods. The best sources include fatty fish and fish liver oils. Some are also found in beef liver, some cheeses, and egg yolks. Because these are animal sources, they are in the D3 form. Some are even already converted into 25(OH)D which is thought to be 5 times more potent than the regular D3 form.
Naturally occurring plant sources of vitamin D2 are some mushrooms that have been exposed to the sun. That’s about it.
Because it’s naturally found in so few foods, vitamin D is also added to certain foods. This is called “fortification.” In fact, fortified foods are the main source of dietary vitamin D in the US. Fortification of food with vitamin D can improve vitamin D status.
Some of these vitamin D fortified foods include milk, some orange juices, breakfast cereals, and yogurt. Check your labels to find out if yours has been fortified with vitamin D (it will be listed as an ingredient). You can also check which form of vitamin D was added: D2 or D3.
Vitamin D supplements come in both forms: D2 and D3. The plant-based D2 form is manufactured by exposing yeast to UV radiation. The animal-based D3 form is made from lanolin. If you are at risk for vitamin D deficiency, your health care provider can test your blood for levels of 25(OH)D and recommend a course of action specifically for you.
Vitamin D deficiency
Studies show that between 30-80% of people simply don’t get enough vitamin D. This deficiency is so common that some researchers have called it a “public health concern” and a “global problem.”
Vitamin D deficiency is when someone has less than 30 nmol/L of 25(OH)D in the blood. Ideally, you want at least 50 nmol/L (20 ng/L).
- <30 nmol/L = deficient
- 30-50 nmol/L = insufficient
- 50-80 nmol/L = optimal
Vitamin D deficiencies can happen when, over time, people are not getting enough safe sun exposure, or are not eating enough foods containing vitamin D. It can also happen if the vitamin D is not being absorbed very well, or if the kidneys have trouble converting the “pre-vitamin” D into the active form 1,25(OH)D. People who are more likely to be deficient in vitamin D include:
- Pregnant and lactating women, and breastfed infants;
- Older adults;
- People with limited sun exposure (including athletes who train indoors);
- People with darker skin;
- People with digestion issues that prevent proper absorption (e.g. inflammatory bowel disease, celiac disease, etc.);
- People with obesity; and,
- People who have undergone gastric bypass surgery.
How much vitamin D do we need?
For adequate blood levels of 25(OH)D, how much vitamin D do we need to get every day? To get enough vitamin D from the sun, a general rule is to get about 15–30 minutes of sun between 10:00 a.m. & 3:00 p.m. at least four to five times a week to the face, arms, legs, or back without sunscreen.
When it comes to vitamin D from foods and supplements, in Canada and the US, the Institute of Medicine (IOM) has set target daily amounts. This amount, called the “Recommended Dietary Allowance” (RDA), ensures that at least 97% of people get enough vitamin D every day. Those recommendations are:
- 10 mcg (400 IU) per day for infants under the age of one.
- 15 mcg (600 IU) per day for everyone aged 1-70 years old, including pregnant and lactating women.
- 20 mcg (800 IU) per day for everyone over the age of 70.
Vitamin D in foods and supplements may be measured in both mcg (micrograms) and/or IU (international units). The conversion factor is 40 IU = 1 mcg.
Vitamin D has many health-promoting roles in the body. Most of the evidence is for bone health, but it’s also associated with a healthy immune system, digestive system, heart and mental health, blood sugar regulation, fertility, and resistance to cancer.
Vitamin D is also the most common deficiency.
We can get vitamin D from sun exposure, some foods, and supplements.
The best way to know how much vitamin D you need is to have your blood tested if you’re at risk. If you don’t have a test or professional recommendation, following the label directions on your vitamin D supplements can be a safe way to get enough.
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