Essential Nutrient #41, 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 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.

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.

Summary of Food Sources

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.