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.
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 calcium profile to learn more about dietary calcium.
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.
The conversation about which foods contain vitamin D can be a bit challenging because the WHFoods 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.
|World's Healthiest Foods ranked as quality sources of|
|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|
|Mushrooms, Shiitake||0.50 cup||40.6||20.30||5||2.2||good|
Density>=7.6 AND DRI/DV>=10%
Density>=3.4 AND DRI/DV>=5%
Density>=1.5 AND DRI/DV>=2.5%
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 WHFoods.) 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 WHFoods.
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.
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.
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.
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.
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 WHFoods, including our food rating system.