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 WHFoods, 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 World's Healthiest Foods, 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.
Which foods contain vitamin D?
Very few foods have vitamin D in them. Foods with a higher amount of vitamin D include fish, liver, and egg yolk.
Excellent sources of vitamin D are foods and beverages that have vitamin D added to them. Cow milk always has added vitamin D. Orange juice, margarine, and soy beverage usually have it added. For other foods, check the label. You know vitamin D has been added if you see 'fortified' or 'enriched' on the label.
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, we only consider vitamin D to be a helpful 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 is one of many hormones involved in the maturation of white blood cells, our first line against most types of infection. In particular, researchers have uncovered a relatively consistent link between low vitamin D level and increased risk of respiratory infection. This research is still relatively new, and we do not know at this point whether increasing dietary intake will reverse this correlation.
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.
Even worse, vitamin D deficiency appears to be on the rise, 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 World's Healthiest Foods, 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 WHFoods, 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.
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.
Public Health Recommendations
In 2013, 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:
0-12 months: 400 IU (10 micrograms of cholecalciferol)
1-70 years: 600 IU (15 micrograms of cholecalciferol)
70+ years: 800 IU (20 micrograms of cholecalciferol)
Pregnant women: 600 IU (15 micrograms of cholecalciferol)
Lactating women: 600 IU (15 micrograms of cholecalciferol)
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.
Food Sources of Vitamin D
Food Serving Size Amount of Vitamin D
Milk 250 mL 104 IU
Fortified rice 250 mL 87 IU
Fortified orange juice 125 mL 50 IU
Fortified margarine 10 mL 51 IU
Egg yolk 1egg 32 IU
Herring, cooked 75 g 161 IU
Trout, cooked 75 g 150 IU
Mackerel, cooked 75 g 81 IU
Salmon, Atlantic, cooked 75 g 246 IU
Salmon, chum, canned 75 g 202 IU
Salmon, pink, canned 75 g 435 IU
Salmon, sockeye, canned 75 g 557 IU
Sardines, Atlantic, canned 75 g 70 IU
Sardines, Pacific, canned 75 g 204 IU
Tuna, canned, light 75 g 36 IU
Tuna, yellowfin cooked 75 g 105 IU
Tuna, bluefin, cooked 75 g 219 IU
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