vitamin D

Foods Richest in vitamin D

Nutrient Amount DV% Rating
Salmon 511.43 128% Excellent
Sardines 175.09 44% Very Good
Cow's milk 62.22 16% Very Good
Tuna 92.99 23% Good
Eggs 43.50 11% Good
Mushrooms, Shiitake 20.30 5% Good
Salmon 1059.14 264.8% Excellent
Sardines 175.09 43.8% Very Good
Cow's milk, grass-fed 62.22 15.6% Very Good
Milk - Goat 124.44 31.1% Good
Mushrooms, Shiitake 17.40 4.3% Good
Eggs 26.50 6.6% Good
Milk 119.56 29.9% Very Good
Mushrooms - Shiitake 28.35 7.1% Good

About vitamin D

Basic description

Vitamin D is one of the most intensely studied yet widely debated nutrients in health research over the past decades. The research debate over vitamin D has focused partly on its roles in the body, and more recently on its optimal levels in the body and on the relationship of those levels to dietary intake.

Since vitamin D was first recommended as an essential nutrient for the U.S. public in 1943, recommendation levels for this nutrient have varied. Originally, recommended intake for vitamin D by the National Academy of Sciences was approximately 200 IU (5 micrograms of cholecalciferol). Over the years, this level had gradually been increased to 400 IU (10 micrograms of cholecalciferol) as the current Daily Value (DV) set forth by the U.S. Food and Drug Adminstration (FDA), and to current recommendation levels of 400-800 IU (10-20 micrograms of cholecalciferol) by the National Academy of Sciences. (At WHF, we use the DV of 400 IU as our recommended daily intake level.) However, this recommended daily intake level remains controversial for three basic reasons.

First is the long-known fact that human skin cells can make vitamin D from sunlight. When certain wavelengths of ultraviolet B (UVB) light from the sun land on our skin cells, a molecule in our skin cells called 7-dehydrocholesterol can be converted into a preliminary form of vitamin D called cholecalciferol. However, the exact amount of cholecalciferol that gets made is difficult to predict! The number of pigments in our skin cells, the strength of the UVB light, the overall health of our skin, and other factors impact this set of events. (One of these other factors, for example, involves use of sunscreen and general skin products containing UVB-blocking agents.) In other words, even though we know that our skin cells can make this preliminary form of vitamin D from sunlight, it is not easy for us to predict how much will get made.

Second is our knowledge that cholecalciferol from our skin cells is not the same as fully active vitamin D. Fully active vitamin D requires two additional steps. First is transfer of cholecalciferol in the bloodstream from our skins cells to our liver cells. This transfer is required in order for our liver cells to produce 25-hydroxycholecalciferol or 25(OH)D. Second is the transfer of 25(OH)D in the bloodstream from our liver cells to our kidney cells. This second transfer allows our kidney cells to take 25(OH)D and convert it into 1,25-dihydroxycholecalciferol, or 1,25(OH)D. It’s this more complicated form of vitamin D that is active as a regulator of certain immune system activities. In short: the role of our liver cells and kidney cells in creating fully active vitamin D adds further complications when researchers try to predict vitamin D status.

Finally, recommended daily intake of vitamin D is controversial because scientists aren’t certain about the relationship between blood levels of this vitamin and disease risk. Early studies on vitamin D and disease often focused on prevention of rickets (a disease involving bone formation related to deficiency of vitamin D and bone-related minerals). Recent studies on vitamin D and disease have focused on many health problems not specific to bone, including problems involving our immune, cardiovascular, and blood sugar regulating systems. As vitamin D research has expanded in scope, researchers have been less certain about optimal amounts of vitamin D necessary to prevent unwanted problems in these many body systems.

Of the WHF, we list one excellent, two very good, and three good sources of vitamin D. Needless to say, this is a much shorter list than we see with other nutrients and will present a challenge to the goal of meeting needs with diet alone. Luckily, however, unlike other nutrients, vitamin D is a nutrient that we have the opportunity to increase by increasing our exposure to sunlight, and for some people, this combination of diet-plus-sunlight might provide an acceptable amount of this vitamin. As we point out later in this profile, however, many people will want to consult with their healthcare provider when making decisions about vitamin D status.

Role in health support

Bone health

Vitamin D deficiency can lead to softening or malformation of bone. In children, this condition is called rickets. In adults, it is called osteomalacia.

The relationship between vitamin D and bone metabolism is more complicated than you might guess. As a hormone, vitamin D acts to increase calcium in the blood stream. The first two ways it accomplishes this are by increasing your ability to absorb calcium from foods and by reducing the amount of calcium you lose in the urine. The last way, however, is by pulling calcium from the bone to support your blood levels.

Obviously, if our goal is to promote strong bones, we don’t want to be pulling calcium from them into the blood stream. For this reason, vitamin D only functions as a bone builder when there is sufficient dietary calcium. Any bony fish, including sardines or canned salmon, would potentially be a rich source of both vitamin D and calcium. Check our calcium profile to learn more about dietary calcium.

Blood sugar control

Researchers have known for some time that the risk of high blood sugar and diabetes are higher in people with low vitamin D levels. More recent research has demonstrated that bringing these levels back up to normal can help reverse some of the risk. Although research has not been entirely consistent, it is becoming more accepted that vitamin D deficiency is a risk factor for developing diabetes.

Immunity

Vitamin D

Summary of food sources

The conversation about which foods contain vitamin D can be a bit challenging because the WHF list for vitamin D is quite short. Furthermore, one of the best dietary sources of vitamin D (milk) is only vitamin D-rich because of fortification that occurs at the time of processing. (While whole milk from grass-fed cows with plenty of time spent outdoors does contain vitamin D, we have not seen studies documenting predictable levels of vitamin D in non-fortified, grass-fed whole milks.) For all of these reasons, it requires very special effort for people who are reliant on foods alone (versus sun exposure) to reach their daily vitamin D needs.

It can be done, however. It is easiest to do if you like fish. Salmon, for instance, contains more than the Daily Value (DV) in just a single serving. Sardines contain over 40% of the DV, and tuna contains just under 25%.

Pasture-raised eggs are a good source of dietary vitamin D, with about 10% of the DV per egg. The vitamin D is concentrated in the yolk, so you’ll need to eat the whole egg to get it. Some mushroom species, including shiitakes, contain as much as 5% of the DV.

The biggest sources of vitamin D in the American diet are not whole natural foods, however, but fortified, processed foods. Virtually all commercial cow’s milk sold in the U.S. has been fortified for vitamin D in the amount of 100% DV per quart (meaning that each 8 ounce glass contains a little over one quarter of the DV). At one time, there had been a big problem with these fortification programs including too much or too little vitamin D, but recent surveys confirm they now contain a more predictable level. If you enjoy cow’s milk and do well with this food, it can make an outstanding contribution to your vitamin D intake. Our recommended form of cow’s milk is grass-fed, and if cows have had ample access to the outdoors and sunlight, their milk may contain vitamin D even if non-fortified. One additional note here: cholecalciferol is the form of vitamin D3 used in milk fortification.

If you regularly include and enjoy processed foods in your meal plan, we would also point out that fortified breakfast cereals and fortified juices can make a contribution to your vitamin D intake, since these foods are often fortified with vitamin D at various levels. However, we do not think that it makes sense to add processed foods to your meal plan if your primary goal is increasing your vitamin D intake. If you are concerned about your vitamin D intake level from whole, natural foods, we recommend that you consult with your healthcare provider and determine whether supplemental vitamin D makes sense, and in what amount.

Nutrient rating chart

Introduction to nutrient rating system chart

Read more background information and details of our rating system

WHF ranked as quality sources of
vitamin D

Food

Serving
Size

Cals

Amount
(IU)

DRI/DV
(%)

Nutrient
Density

World’s
Healthiest
Foods Rating

Salmon

4 oz

157.6

511.43

128

14.6

excellent

Sardines

3.20 oz

188.7

175.09

44

4.2

very good

Cow’s milk

4 oz

74.4

62.22

16

3.8

very good

Tuna

4 oz

147.4

92.99

23

2.8

good

Eggs

1 each

77.5

43.50

11

2.5

good

Mushrooms, Shiitake

0.50 cup

40.6

20.30

5

2.2

good

World’s Healthiest
Foods Rating

Rule

excellent

DRI/DV>=75% OR
Density>=7.6 AND DRI/DV>=10%

very good

DRI/DV>=50% OR
Density>=3.4 AND DRI/DV>=5%

good

DRI/DV>=25% OR
Density>=1.5 AND DRI/DV>=2.5%

Impact of cooking, storage and processing

Vitamin D is a very stable nutrient in foods. It will not be significantly damaged by most low-heat cooking techniques.

You can expect to lose a little more vitamin D from foods cooked in oil. (Of course, frying in oil at higher temperatures is not a cooking methond that we recommend at WHF.) Fried eggs lose about 20% of their vitamin D, compared to only 10% lost in the poaching process. Either way, however, you should expect most of the vitamin D to end up on your plate.

There is some confusion about how well vitamin D stands up to high-heat cooking techniques. One research group found that vitamin D in cheese baked at 450° F (232°C) for 10 minutes lost very little vitamin D. Another study reported that eggs baked at 325°F (163°C) for 40 minutes lost over half the original vitamin D content. One way to avoid potentially unwanted loss of vitamin D based on information from these mixed studies would be to avoid higher heats and lengthy baking times. We do just that in all of our recommended cooking methods at WHF.

Risk of dietary deficiency

The risk of dietary deficiency of vitamin D is substantial. In every age and gender group surveyed, average American diets fail to meet or exceed the Daily Value (DV) for vitamin D, even when supplements and fortified foods are included in the analysis. When we eliminate supplements, and look at dietary intake alone, we see that less than 5% of Americans meet the DV, and in many age groups it is less than 1%. Because fortified foods—foods containing vitamin D added during processing—make up 60% of our dietary vitamin D, eliminating them from dietary analysis would make this outcome even worse.

Because vitamin D can be obtained from the sun as well as from the diet, researchers have usually preferred to estimate deficiency from blood levels of the vitamin. According to a nationally representative random sample of Americans, 13% are deficient in vitamin D and another 30% have a marginal blood level.

Vitamin D deficiency also appears to be increasing, with rates of deficient blood levels tripling since the 1980s. This trend is probably related more to reduced sun exposure and widespread use of sunscreen than changes in dietary habits.

With most of the nutrients we discuss here at the WHF, it is very easy for us to construct a daily diet that easily meets your daily needs. A very small number of nutrients require some special focus on specific foods to maintain a good supply. Then, there’s vitamin D.

Vitamin D is a nutrient particularly dependent on specific foods and food groups. You’ll have to regularly consume foods from these groups to meet to your daily needs. From our perspective at WHF, a dietary approach to keeping vitamin D intake over the DV would typically focus on routine fish intake—especially higher-fat fish like salmon. Other whole foods that would be logical to consider include eggs, mushrooms, grass-fed cow’s milk, or whole food-based products that have been fortified with vitamin D (for example, D-fortified grass-fed milk, grass-fed yogurt or cheese).

If this nutrient is of special concern to you, we recommend that you consult with your healthcare provider for help in determining your vitamin D needs and the best approach for meeting them. Laboratory testing for vitamin D blood levels and gene testing for vitamin D metabolism are widely available from many healthcare providers.

Other circumstances that might contribute to deficiency

Unlike most other nutrients, there is more to the story here than dietary intake of vitamin D. The adequacy of our dietary intake of vitamin D is substantially related to our natural sunlight exposure. Unfortunately, variability in where we live and the tone of our skin make it impossible to give a single and clear recommendation about how much sun exposure is required.

The further north you are, the less likely you are to meet your vitamin D needs. In the continental U.S., if you live north of the 37th parallel (roughly where San Francisco, California and Richmond, Virginia are), you should expect to make little vitamin D from natural sunlight during the winter months. In addition, the farther north you go, the less vitamin D you are likely to make from winter sunlight partly due to longer periods of wintertime. The darker your skin, the less efficiently you produce vitamin D. For this reason, African-Americans have on average about half the blood level of vitamin D when compared to ethnicities with lighter skin tones.

Sunscreen interferes with the production of vitamin D in the skin. Researchers have been debating about how deep this reduction is. One research group found that application of SPF8 sunscreen as recommended (which most people fail to do) completely blocked any production of vitamin D with sun exposure.

We have seen calculators online that use your skin tone and latitude to predict the amount of sun exposure you’ll need to achieve sufficient levels of vitamin D without dietary intake. As long as you understand that these provide very rough estimates, we support their use to help you determine your vitamin D needs.

Relationship with other nutrients

As described above, vitamin D and calcium are very closely related in activity. Deficiency of either can lead to impaired bone formation, and deficiency of both in tandem is a common public health problem due to the amount of processed and unhealthy food in the American diet. As described above, there is even reason to believe that vitamin D in the absence of adequate calcium could cause you to lose bone by increasing the rate of bone loss.

Vitamin D also appears to slightly increase the absorption of magnesium in the intestine, but not to nearly the same degree that we see it increase calcium absorption. In fact, because calcium and magnesium compete with each other for absorption, we are concerned about magnesium deficiency as a potential consequence of widespread medical treatments focusing on calcium and vitamin D supplementation.

Vitamin D and vitamin K work together to help keep the rate of bone production and breakdown in balance. Low vitamin K levels are only starting to be understood as a risk for bone problems, so our knowledge in this area is much less complete compared to vitamin D.

Risk of dietary toxicity

The Tolerable Upper Intake Level (UL) for vitamin D is 4000 IU for adults. Given that it is a struggle for many people to reach the 400 IU Daily Value, it appears very difficult to regularly go above the UL from diet alone. Using our top vitamin D food as an example, you’d have to eat just under 2 pounds of salmon per day to be at the UL.

We would like to make one additional note on dietary toxicity of vitamin D and the UL of 4,000 IU. In certain clinical situations, it is clear to us that some individuals may need to exceed the dietary UL via vitamin D supplementation in order to promote optimal health. Once again, if you have a particular concern about vitamin D and your own health, we encourage you to meet with your healthcare provider to determine the best steps to take.

Disease checklist

Public health recommendations

In 2010, the National Academy of Sciences updated the Dietary Reference Intakes (DRI) for vitamin D. This DRI update included a set of Recommended Dietary Allowances (RDA) which are summarized in the chart below. Note that the recommendations for infants under one year are Adequate Intake (AI) standards. The RDAs and AIs are as follows:

These RDA recommendations are meant to prevent symptoms related to deficiency even in those with minimal sunlight exposure.

The DRI update also included a Tolerable Upper Intake Limit (UL) of 4000 IU for vitamin D.

A Daily Value (DV) of 400 IU (10 micrograms of cholecalciferol) is the standard you will see on food and supplement labels. It is also the daily recommended amount that we use for all of our calculations at WHF, including our food rating system.

Description

Up until the mid-1990’s, the answer to this question would have been fairly simple: vitamin D is a fat-soluble vitamin needed to prevent a bone disease in children called “rickets.” Previous studies dating all the way back to the early 1800’s had determined that cod liver oil could help to prevent and cure particular problems with bone development in children. In the early 1900’s, a compound called “fat-soluble factor D” was isolated from cod liver oil, and this factor turned out to be the vitamin that we now refer to as “vitamin D.” Scientific investigation of rickets helped establish the role of sunlight in providing us with vitamin D, and it also helped establish the role of vitamin D in bone health.

Beginning in the mid-1990’s, however, our understanding of vitamin D began to change in a dramatic way. It would not be an exaggeration to say that the last 15 years have brought a revolution in our understanding of this vitamin! We now know that vitamin D is not simply a fat-soluble vitamin needed for healthy bones it’s also a hormone. When a substance functions like a vitamin, it participates in and regulates our metabolism, allowing it to function properly. And that is exactly what vitamin D does: it helps to regulate our bone development, our muscle function, our immune function, our insulin activity, our calcium balance, and our phosphorus balance. Just like for estrogen and cortisol, there are receptors for vitamin D (called VDRs, or vitamin D receptors) on the cell membranes of most cell types in the body. Instead of serving a very limited metabolic role in relationship to bone health, vitamin D plays a sweeping role in many diverse aspects of our health according to research conducted over the past 15 years.

There are two basic types of vitamin D. Ergosterol is the basic building block of vitamin D in plants. Cholesterol is the basic building block of vitamin D in humans. When ultraviolet light from the sun hits the leaf of a plant, ergosterol is converted into ergocalciferol, or vitamin D2. In just the same way, when ultraviolet light hits the cells of our skin, one form of cholesterol found in our skin cells—called 7-dehydrocholesterol—can be converted into cholecalciferol, a form of vitamin D3. (The revolution in our understanding of vitamin D has led to extensive research on both D2 and D3, and it is the overwhelming consensus of researchers that D3 is our best bet when supplementing with vitamin D. In fact, in 2006, the American Journal of Clinical Nutrition argued that D2 should no longer be considered as a nutrient “suitable for fortification or supplementation,” given the strong hormonal advantages of D3. You’ll find more about the issue of delivery form and supplementation in our section entitled “Form in Dietary Supplements.”)

In the life of a plant, the ergocalciferol form of vitamin D2 serves to accomplish most of the desired purposes that were intended for this substance. In the life of a human, however, cholecalciferol is not the final intended form for this vitamin. In order for our bodies to come up with the fully active form of vitamin D3, further metabolism is required. A first step involves conversion of cholecalciferol into hydroxyvitamin D, also called 25-hydroxyvitamin D or 25(OH)D. Hydroxyvitamin D can be formed in the liver, kidney, lung, skin, prostate, brain, blood vessel linings, and macrophage cells of the immune system. An enzyme called CYP27A1 is required for formation of hydroxyvitamin D. A second step involves conversion of hydroxyvitamin D into dihydroxyvitamin D (also called 1,25-dihydroxyvitamin D or 25(OH)2D). This second step can take place in the lung, brain, liver, stomach, spleen, kidney, colon, thymus, lymph nodes, skin, placenta, and in the monocyte and dendritic cells of the immune system. An enzyme called CYP27B1 is required for formation of dihydroxyvitamin D. The different forms of vitamin D and their relationships are summarized in the chart below:

Form of Vitamin D

Where Found

What’s Needed to Activate This Form of Vitamin D

New Form of Vitamin D That Get’s Created

7-dehydrocholesterol

Skin

UVB sunlight

Cholecalciferol

cholecalciferol

Many cell types

CYP27A1

Hydroxyvitamin D 25(OH)D

25(OH)D Hydroxyvitamin D

Many cell types

CYP27B1

Dihydroxyvitamin D 25(OH)2D

25(OH)2D dihydroxyvitamin D

Many cell types

Already most active form

No new form needed

Dihydroxyvitamin D (the only fully active hormonal form of vitamin D) does not last for very long in our body. The half-life of this hormone is between 2-3 weeks. For this reason, our vitamin D needs must be met on a fairly regular basis.

Before leaving this introductory description of vitamin D, it is important to note that our revolutionized understanding of vitamin D as a hormone found in a wide variety of cell types and responsible for the regulation of many different physiologic process has brought along with it a new understanding of how much we need. (You’ll find more information on this topic in our section entitled “Public Health Recommendations.”) There is a definite bottom line here: we need much more than we thought! When researchers had been limiting their research on vitamin D to bone health and rickets, they had been arriving at a clinical determination of about 15-20 ng/mL of vitamin D in our blood to keep us healthy. Once the research on vitamin D was expanded to include muscle health, immune health and other aspects of vitamin D function, this blood level of 15-20 ng/mL was clearly determined to be insufficient. Researchers quickly showed that blood levels of 30-50 ng/mL were needed to support these other health functions. In other words, our understanding of “normal vitamin D” has changed completely! Our blood levels need to be about twice as high as we previously thought, and it takes far more vitamin D intake than we previously thought to achieve those higher blood levels. (Once again, you will find more information on this topic in our sections entitled “Form in Dietary Supplements” and “Public Health Recommendations.”)

How it functions

What is the function of vitamin d?

The hormonal functions of vitamin D include regulation of bone health, regulation of muscle health (including both skeletal and heart muscle), regulation of immune response, regulation of insulin and blood sugar, and regulation of calcium and phosphorus metabolism. Further details about these functions are presented in the paragraphs below.

Regulation of bone health, calcium, and phosphorus

Bone composition involves many different substances, including collagen proteins, keratin proteins, and a variety of minerals including silicon, boron, and magnesium. But two especially critical bone components are the minerals calcium and phosphorus. These minerals form the bulk of a substance called hydroxyapatite, which accounts for over half of all bone composition.

The importance of hydroxyapatite in bone places a premium on calcium and phosphorus metabolism and their passage in and out of the body. While bone health is regulated by many different substances in the body—including growth hormone, testosterone, and estrogens—the importance of calcium and phosphorus in bone health also points to the special importance of two bone health regulators, namely parathyroid hormone (PTH) and vitamin D.

It’s the job of our parathyroid glands to put out PTH whenever our blood calcium level gets too low. When that happens, PTH triggers release of calcium from our bones in order to boost our blood level back up to normal. PTH also triggers our kidneys to retain more calcium (keeping it available for our bloodstream) and excrete more phosphorus (thereby helping to create a more favorable ratio of calcium to phosphorus in our blood). If there is too much PTH released from our parathyroid glands, however, we may end up removing too much calcium from our bones and leaving too much in our blood, compromising both our cardiovascular and our bone health. Research has shown vitamin D deficiency to be a key risk factor for overproduction of PTH and optimal levels of vitamin D to be associated with healthy parathyroid function (and desirable PTH levels). Like PTH, vitamin D helps the intestines absorb more calcium from our food, and it also helps our kidneys hang on to calcium. But unlike PTH, vitamin D also helps our kidneys retain phosphorus. The two hormones work together in order to assure proper balances of calcium and phosphorus in our bloodstream and in our bones. Interestingly, PTH “knows” that it must act in partnership with vitamin D because it triggers conversion of hydroxyvitamin D into dihydroxyvitamin D (the hormonally active form).

Regulation of immune function

It would be very difficult to overestimate the importance of recent discoveries about vitamin D and the immune system. Vitamin D’s role in immune regulation has revolutionized research in this area to such a degree that it is virtually impossible to investigate an autoimmune disease without considering the possible role of vitamin D. This statement holds true for health conditions like rheumatoid arthritis, multiple sclerosis, Crohn’s disease, systemic lupus erythematosus, and numerous other autoimmune conditions.

Discovery of vitamin D’s critical role in immune function was aided by the discovery of vitamin D receptors (VDRs) on the immune system’s macrophage and dendritic cells in the past 15 years. Once triggered by vitamin D, macrophage cells are capable of releasing antibacterial peptides (parts of protein) like cathelicidin, and these antibacterial proteins play a critical role in the immune system’s prevention of infection. Of special interest in this area has been infection by Mycobacterium tuberculosis (responsible for tuberculosis) and Mycobacterium leprae (responsible for leprosy). Vitamin D deficiency has emerged as a clear risk factor for these diseases.

Autoimmune conditions remain an extremely active area of vitamin D research. In current research on multiple sclerosis, for example, clinicians are experimenting with vitamin D doses up to 40,000 IU, and in research on rheumatoid arthritis, doses up to 100,000 IU are being used in some clinical trials. (To get some perspective on these vitamin D supplementation levels, they can be compared to the current adult Dietary Reference Intake recommendations for vitamin D, which range from 200-600 IU.)

Regulation of blood pressure and cardiovascular health

Vitamin D plays a direct role in regulating our blood pressure by inhibiting the activity of a system called the renin-angiotensin system. It’s the job of the renin-angiotensin system to help increase our blood pressure whenever it gets too low. (The renin-angiotensin system accomplishes this task by helping our body retain sodium and water—thus providing more fluid in our blood vessels—and by causing our blood vessels to constrict and thereby increasing the pressure inside them.) We need optimal levels of vitamin D to hold this system in check, and to prevent it from raising our blood pressure under inappropriate circumstances.

Vitamin D deficiency has been shown to be a significant risk factor for high pressure in a variety of studies, and risk of high blood pressure during pregnancy (called pre-ecclampsia) has also been associated with maternal deficiency of vitamin D. In one interesting study on a group of individuals with high blood pressure, those whose UVB exposure was increased by using tanning beds (30 min/3 times a week) increased their vitamin D levels by 180% and decreased their blood pressure by about 5%. (Ultraviolet B light is the type of light required to convert 7-dehydrocholesterol in the skin into cholecaliferol, a preliminary form of vitamin D.)

The key role played by vitamin D in regulation of calcium metabolism has opened the door to research about broad cardiovascular benefits of vitamin D not limited to its role in regulation of blood pressure. Overloading of cells with calcium is a problem for heart tissue, and it is associated with increased risk of oxidative stress and tissue damage. By triggering unwanted release of PTH, vitamin D deficiency can result in precisely this situation of cellular calcium overload. In several studies, the ability of heart tissue to heal after an event like heart attack has been shown to suffer significantly in the absence of optimal vitamin D.

Regulation of insulin and blood sugar

While researchers are not entirely clear about the exact mechanisms for vitamin D regulation of insulin metabolism and blood sugar balance, there is no doubt that vitamin D plays an important role in this area of body function. Vitamin D deficiency is clearly a risk factor for development of type 2 diabetes, and vitamin D levels have been associated with insulin secretion by the beta cells of the pancreas as well as insulin activity once released into the bloodstream. Interestingly, when vitamin D is deficient in the body and parathyroid hormone (PTH) is released in inappropriately large amounts, too much calcium can accumulate in the cells. When too much calcium accumulates in fat cells, these cells can end up producing too much cortisol, a hormone that counteracts the effectiveness of insulin. Similarly, too much accumulation of calcium in our fat and muscle cells can inhibit the formation of a protein called GLUT-4. This protein helps carry sugar (glucose) out of our bloodstream and into our cells, and it is designed to perform this function whenever directed to do so by insulin. Without sufficient vitamin D, too little GLUT-4 is formed, and insulin lacks one of the proper tools to do its job.

Regulation of muscle composition and muscle function

Research in this area of vitamin D function has expanded enormously in the past ten years, and vitamin D deficiency has been shown to play a key role in prevention of muscle weakness and prevention of falls, especially in older persons. Interestingly, vitamin D deficiency has been associated with too much accumulation of fat throughout muscle tissue, in such a way that muscle strength is decreased and physical performance is compromised. In one study in California on healthy young women, decreased muscle strength was not only associated with vitamin D deficiency but also found to be independent of muscle mass. In other words, women with deficient vitamin D intake were found to have less muscle strength even when their muscles were the same size as the muscles of other women. Some of the research on vitamin D deficiency and risk of falling has also stepped outside of muscle function per se and looked at the broader issue of neuromuscular function and the relationship of muscle movement to nerve activity. Since vitamin D is a key regulator of calcium metabolism, and calcium is known to play a key role in nerve firing and nerve triggering of muscle contraction, this broader research focus may turn up important information about vitamin D and its role in reducing risk of falls.

Prevention of cancer

The role of vitamin D in cancer prevention is a lively area of current research, and the mechanisms linking vitamin D to cancer prevention are not completely evident. Nevertheless, research has shown a clear role for vitamin D in prevention of the following types of cancer: bladder cancer, breast cancer, colon cancer, ovarian cancer, prostate, and rectal cancer. In certain situations, vitamin D may also play a role in cancer treatment. Vitamin D analogs (vitamin-D like substances synthesized in the laboratory) are actively being tests as anticancer agents, especially with respect to breast and prostate cancers.

Other functions of vitamin D

The presence of vitamin D receptors (VDRs) in so many different tissue types—including the brain and skin—has left the door open to a wide variety of vitamin D functions. There is considerable research underway in the area of vitamin D deficiency and cognitive function, especially in aging persons. Senile dementia and Alzheimer’s disease are two areas where vitamin D deficiency is under active investigation. Mood disorders—especially depression in older persons—is also an area of active research with respect to vitamin D deficiency. Due to its role in immune regulation and the known presence of VDRs in skin, vitamin D is a particularly good candidate for investigation in a skin-related autoimmune disease like psoriasis. Research in this area is also well underway.

Deficiency symptoms

What are deficiency symptoms for vitamin d?

Bone pain, frequent bone fractures, and softening of the bones can all be symptoms of vitamin D deficiency. So can muscle aches and muscle weakness since vitamin D helps to regulate muscle composition and prevent too much fat accumulation alongside of muscle tissue. In this context, especially in older persons, frequent falls can point to deficiency of this vitamin. The key role of vitamin D in regulation of immune response means that lowered immunity can be a symptom of vitamin D deficiency, as can the presence of any autoimmune disorder. In older persons, cognitive problems (disturbances in thought processes) and depression can be symptomatic of vitamin D deficiency, and in children, stunted growth and severe asthma have also been shown to have vitamin D deficiency as potential causes.

Toxicity symptoms

What are toxicity symptoms for vitamin d?

Excessive intake of vitamin D can be toxic, and toxicity of vitamin D can come from either its plant-based (D2) or animal-based (D3) form. Symptoms of toxicity include loss of appetite, nausea, vomiting, high blood pressure, kidney malfunction, and failure to thrive. However, it is also important to note that vitamin D deficiency poses a far greater risk to the vast majority of individuals than vitamin D toxicity and that vitamin D toxicity from food intake is extremely unlikely. Less than one-third of all persons in the U.S. meet the Dietary Reference Intake level for vitamin D, and are far from consuming anything close to potentially toxic levels.

In 2010, the National Academy of Sciences set Tolerable Upper Intake Levels (ULs) for vitamin D as follows:

While these toxicity limits are based on credible data, they do not attempt to address the issue that is posed by higher vitamin D intake amounts potentially needed to offset chronic deficiency of this vitamin. Clinical research has clearly shown that vitamin D supplementation in the range of 1,000-2,000 IU per day is unable to restore optimal vitamin D levels in many individuals with chronic vitamin D deficiency. In many cases, individuals with chronic vitamin D deficiency will need to exceed the vitamin D Upper Limits established by the National Academy of Sciences in order to bring the levels of hydroxyvitamin D in their blood up to optimal levels. Remedy of chronic vitamin D deficiency will also typically require supplementation of vitamin D since everyday food intake is typically unable to provide deficiency-offsetting amounts of this vitamin. Steps to remedy chronic vitamin D deficiency should always be taken in consultation with a licensed healthcare provider who is able guide and monitor changes in blood levels of vitamin D.

Individuals with primary hyperparathyroidism (overactivity of the parathyroid gland not caused by vitamin D deficiency) are at increased risk for vitamin D toxicity and should not take supplemental vitamin D without consulting a physician.

Factors that affect function

What factors might contribute to a deficiency of vitamin d?

Insufficient sun exposure

By far the most important D-deficiency contributing factor faced worldwide is insufficient exposure to sunlight. More specifically, it is ultraviolet B sunlight in the range of 290-300nm that is needed to convert 7-dehydrocholesterol found in our skin cells into cholecalciferol (the preliminary form of vitamin D3).

Although the task of “getting enough sunlight” may seem like a fairly straightforward one, the relationship between our vitamin D status and our time in the sun is not nearly as simple as many people might think. First, there is the fluctuating nature of UVB light. UVB light—the kind needed for skin synthesis of vitamin D—is not always present with the same intensity just because there is visible sunlight. The intensity of UVB light varies dramatically with geographical location (latitude), time of year, time of day, degree of cloud cover, and other factors. In other words, there are times when your eyes might leave you thinking that you are getting good intensity UVB light and synthesizing optimal amounts of vitamin D in your skin cells, but you actually are not. In addition, there is the issue of latitude, angle of sunlight, and time of day. In higher-latitude countries across the globe, UVB light in the range of 290-300nm wavelengths may not be available except for a few hours in the middle of the day. In those countries, being outside on a “bright and sunny day” would not be enough to guarantee adequate skin synthesis of vitamin D. Skin pigmentation also plays an important role in skin synthesis of vitamin D. Darker skin pigmentation means less vitamin D synthesis per minute exposure to UVB light. (In the United States, for example, it is estimated that African American adults are 2-3 times more likely to have vitamin D insufficiency than Caucasian adults.) As all of these examples indicate, even when common sense tells you that you are getting good sun exposure and must be synthesizing plenty of vitamin D, you might not be, for a variety of reasons.

Some lifestyles and occupations provide unusual amounts of sun exposure. Individuals who work outdoors throughout the day in warmer climates have a great chance of getting good exposure to UVB light and synthesizing adequate amounts of vitamin D. Under optimal circumstances, our skin can synthesize between 10,000-20,000 IU of vitamin D (cholecalciferol) in 30 minutes. Yes, all of the practical factors still have to line up correctly (like angle of the sun, time of day, degree of cloud cover, etc.), but when people spend generous amounts of times outdoors in the sun on a regular basis, their chances of adequate vitamin D synthesis are greatly increased. At the other end of the spectrum, if an individual typically wears clothing that blocks a lot of the skin from exposure to sunlight, or typically uses sunblock, or works in a job that keeps him or her indoors throughout the day, chances of adequate vitamin D synthesis are greatly decreased.

Based on all of the factors listed above, we believe that the majority of U.S. adults are unlikely to be getting enough exposure to UVB sunlight in the 290-300nm wavelength required to allow for optimal skin synthesis of vitamin D. In addition, we believe that many people may assume that they get plenty of sunlight and have plenty of vitamin D synthesis in their skin cells when they actually do not. Except in cases where lifestyle and/or occupation makes exposure to sunlight an issue that is free from debate, we encourage individuals to avoid assumptions about the adequacy of their sunlight exposure and to treat their vitamin D needs as a matter for special focus and potential reason for follow-up healthcare consultation.

The importance of sunlight for vitamin D health has been the subject of longstanding research. The term “vitamin D winter” was coined several decades ago to summarize the impact of this lower-sunlight season on potential compromise in vitamin D status. More recently, the idea of a “vitamin D winter” has been revisited to include a look at diseases which are related to vitamin D deficiency. Researchers are discovering that many autoimmune conditions have greater severity in winter versus summer, and they are investigating possible links with vitamin D deficiency under these circumstances. Similarly, there has been a strong research trend for studying the occurrence of vitamin D-related diseases at different latitudes on the earth. Over the past 10 years, researchers have found “South to North” trends for increased occurrence of multiple sclerosis, Crohn’s disease, and type 1 diabetes. In other words, there is lesser incidence of these vitamin D deficiency-related diseases at lower latitudes (closer to the equator) where UVB light exposure is presumably higher, and greater incidence of these vitamin D deficiency-related diseases at higher latitudes (farther from the equator).

Breastfeeding and pregnancy

Since nursing mothers must help supply their infants with vitamin D, breastfeeding can pose a challenge both to mothers and infants in terms of vitamin D status. The American Academy of Pediatrics (AAP) and the Canadian Pediatric Society (CPS) have both recommended vitamin D supplementation for both mothers and infants for this reason. (See the section on “Public Health Recommendations” for more information.)

Insufficient dietary fat or inability to absorb dietary fat

Since vitamin D is a fat-soluble vitamin, a diet that is extremely low in fat and/or the presence of certain medical conditions that cause a reduction in the ability to absorb dietary fat may cause vitamin D deficiency. These medical conditions include pancreatic enzyme deficiency, Crohn’s disease, celiac sprue, cystic fibrosis, surgical removal of part or all of the stomach, gall bladder disease, and liver disease. Symptoms of fat malabsorption include diarrhea and greasy stools.

Health conditions that involve the parathyroid gland or kidney

Under certain circumstances, the conversion of inactive forms of vitamin D to calcitriol is impaired. For example, diseases that affect the parathyroid gland, liver, and/or kidney impair the synthesis of the active form of vitamin D.

Aging

The production of vitamin D precursors in the skin decreases with age. Additionally, with age the kidneys and many other organ systems and cell types are less able to convert vitamin D to its active hormone form.

Genetic susceptibility

Some individual’s genetic inheritance includes genetic polymorphisms that result in the production of vitamin D receptors (VDR) that don’t work very well. To help compensate for such VDR defects, these individuals need more vitamin D than would normally be necessary.

Nutrient interactions

How do other nutrients interact with vitamin d?

Vitamin D plays a role in maintaining normal blood levels of calcium. As a result, vitamin D impacts the absorption and storage of calcium. Vitamin D also stimulates the absorption of phosphorus.

Vitamin D helps to regulate the production of certain calcium-binding proteins that function in the bones and kidneys. Because these binding proteins are also dependent on vitamin K, interrelationships between vitamin D and vitamin K have become the subject of active research investigation.

Health conditions

What health conditions require special emphasis on vitamin d?

Vitamin D may play a role in the prevention and/or treatment of the following health conditions:

Food sources

What foods provide vitamin d?

Excellent sources of vitamin D include sardines while very good sources include vitamin-D fortified milk.

Good food sources of vitamin D include goat’s milk, shiitake mushrooms, and eggs.

Related Articles

References

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