vitamin D

What can high-vitamin D foods do for you?

• Help optimize calcium metabolism
• Help optimize phosphorus metabolism
• Help prevent type 2 diabetes, insulin resistance, high blood pressure, heart attack, congestive heart failure, and stroke
• Help prevent falls and muscle weakness
• Help prevent osteoporosis while maintaining bone integrity
• Help regulate insulin activity and blood sugar balance
• Help regulate immune system responses
• Help regulate muscle composition and muscle function
• Help regulate blood pressure
• Lower risk of excessive inflammation
• Lower risk of some bacterial infections
• Support cognitive function, especially in older persons
• Support mood stability, especially in older persons
• Help prevent chronic fatigue
• Help prevent the following types of cancer: bladder, breast, colon, ovarian, prostate and rectal

What events can indicate a need for more foods rich in vitamin D?

• Muscle aches and muscle weakness
• Frequent falls, particularly among older persons
• Bone pain, frequent bone fractures, or soft bones
• Stunted growth in children
• Asthma in children (especially severe asthma)
• Impaired cognitive function, especially among older persons
• Lowered immunity
• Chronic low energy and fatigue
• Depression, particularly among older persons
• Presence of any autoimmune disorder
• Lack of exposure to sunlight for any reason, including geography, use of sunscreen, or wearing of protective clothing 

Concentrated food sources of vitamin D include salmon, sardines, shrimp, milk, cod, and eggs. Among salmon, wild-caught fish have been shown to average significantly more vitamin D than non-organically farmed fish.


For serving size for specific foods, see Nutrient Rating Chart below at the bottom of this page.

Description Function Deficiency Symptoms Toxicity Symptoms Cooking, storage and processing Factors that affect function

Drug-nutrient interaction Nutrient interaction Health conditions Supplements Food Sources Public Recommendations References

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.")