Did you know DNA influences your risk of vitamin D deficiency?

Genes, vitamin D and why you need a bit of sun

Ahh the winter blues…. Anyone who has spent a winter in Vancouver or Seattle knows the true meaning of this phrase. These are breathtakingly beautiful cities in the summer with their ocean, mountains and lush, green forests. But come winter, they turn dark and gloomy, because it seems to rain non-stop! If you’ve blamed the damp air and the lack of sunlight for your tiredness, aching bones, or for just feeling down in the dumps, you are on the right track to uncovering one of the reasons behind this phenomenon. For these are symptoms of vitamin D deficiency. Two genes, CYP2R1 and GC, that control the levels of this vitamin hold the key to understanding why some of us are hit much harder by the winter blues than others.

Vitamin D is often called the ‘sunshine’ vitamin, because sunlight or ultraviolet (UV) light is essential for making this vitamin in the skin. A few natural foods also contain this vitamin (e.g. fatty fish and fish liver oils), and there are many sources of fortified foods, including cereals and orange juice. A list of food sources and recommended dietary allowances can be found at the end of the article. 

Regardless of whether it is synthesized in the skin or acquired from food, vitamin D needs to be activated before it can be used. The CYP2R1 (cytochrome P450 2R1) enzyme is responsible for the activation and it is then distributed around the body by a binding protein (encoded by the GC gene). Given the part they play in vitamin D metabolism, it’s not surprising that genetic variants of CYP2R1 and GC can influence levels of this vitamin in the blood. Three separate variants of GC and one variant of CYP2R1 all affect vitamin D levels. People with these risk variants have lower than average levels and increased risk of deficiency. Additionally, GC and CYP2R1 variants increase the risk of autoimmune disease, cancer and infections.

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Vitamin D plays a role in cell growth, immune function and inflammation. All of these functions are compromised when there are lower levels of active vitamin D in the blood. The most important function of this vitamin is its involvement in the absorption of calcium and phosphate. Calcium is essential for proper bone formation, bone growth and bone remodelling. Vitamin D deficiency can lead to calcium deficiency, and thin, soft, brittle and misshapen bones. Muscle weakness, unexplained fatigue, difficulty thinking clearly and bone pain are other symptoms of deficiency. In the USA, more than 40% of adults over 50 years of age are vitamin D deficient. It is the most common nutritional deficiency worldwide, affecting over a billion people.

To make enough vitamin D in our bodies, we need to be outside in the sun between 10 am and 3 pm for 5 to 30 minutes at least twice a week. However, many other factors like cloud cover, sun screen use and skin pigmentation also factor into how long we should stay in the sun. And, of course, prolonged exposure to UV light is also detrimental, increasing the risk of skin cancer. Making vitamin D food sources part of your diet, in addition to sun exposure, ensures that you meet the daily requirement.

While you can’t get too much vitamin D from your diet, or from being in the sun too long, vitamin D toxicity is a real concern when taking supplements. Symptoms of toxicity can be non-specific like anorexia, weight-loss, polyuria (excessive urine production) and heart arrhythmia. The most dangerous consequence of toxicity involves calcium. As vitamin D levels go up, more calcium is absorbed. Too much calcium can cause calcification or build up of calcium deposits in soft tissue like the heart and kidneys, causing permanent damage. Vitamin D can reduce the risk of depression, asthma, multiple sclerosis, dental cavities, cardiovascular disease, type 2 diabetes, osteoporosis and cancer, but only when it’s taken at the correct doses. See the table at the end of this article for tolerable upper limits of vitamin D for each age group.

Until quite recently we haven’t been paying much attention as to how our genetic makeup might influence our nutritional needs. Vitamin D is one prime example, where genetics can account for ~50% of the variation between people. Some have naturally high levels of this vitamin, because they either spend lots of time outdoors or eat a lot of fish and are blessed with the right genes. Then there are those of us that succumb easily to the winter blues, because our genes necessitate higher vitamin D intake. With a simple swab from your cheek, you can determine if the culprit is genetics. Then all you will need to make it through winter could be eating extra fish and investing in a UV lamp.

Recommended dietary allowances for vitamin D.

Recommended dietary allowances are listed as both micrograms (mcg) and international units (IU). These allowances are set on the basis of minimal sun exposure. For infants from birth to 12 months, an adequate intake is shown, which is equivalent to the mean intake of vitamin D in healthy, breastfed infants. 

Age Male Female Pregnancy Lactation
0–12 months 10 mcg (400 IU) 10 mcg (400 IU)
1–13 years 15 mcg (600 IU) 15 mcg (600 IU)
14–18 years 15 mcg (600 IU) 15 mcg (600 IU) 15 mcg (600 IU) 15 mcg (600 IU)
19–50 years 15 mcg (600 IU) 15 mcg (600 IU) 15 mcg (600 IU) 15 mcg (600 IU)
51–70 years 15 mcg (600 IU) 15 mcg (600 IU)
51+ years 20 mcg (800 IU) 20 mcg (800 IU)

Selected foods sources of vitamin D.

The “IU per serving” refers to the international units measurement, which is commonly listed on food and supplement labels. The Daily Value (DV) for vitamin D is 400 IU for adults and children age 4 and older. This DV was developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet.

Food IU per serving Percent DV
Cod liver oil, 1 tablespoon 1,360 340
Swordfish, cooked, 3 ounces 566 142
Salmon (sockeye), cooked, 3 ounces 447 112
Tuna fish, canned in water, drained, 3 ounces 154 39
Orange juice fortified with vit D, 1 cup (check product labels, as amount of added vit D varies) 137 34
Milk, nonfat, reduced fat, and whole, vit D-fortified, 1 cup 115-124 29-31
Yogurt, fortified with 20% of the DV for vit D, 6 ounces (more heavily fortified yogurts provide more of the DV) 80 20
Margarine, fortified, 1 tablespoon 60 15
Sardines, canned in oil, drained, 2 sardines 46 12
Liver, beef, cooked, 3 ounces 42 11
Egg, 1 large (vit D is found in yolk) 41 10
Ready-to-eat cereal, fortified with 10% of the DV for vit D, 0.75-1 cup (more heavily fortified cereals might provide more of the DV) 40 10
Cheese, Swiss, 1 ounce 6 2

Tolerable upper intake levels for vitamin D.

Age Male Female
0–6 months 25 mcg (1,000 IU) 25 mcg (1,000 IU)
7–12 months 38 mcg (1,500 IU) 38 mcg (1,500 IU)
1–3 years 63 mcg (2,500 IU) 63 mcg (2,500 IU)
4–8 years 75 mcg (3,000 IU) 75 mcg (3,000 IU)
9–18 years 100 mcg (4,000 IU) 100 mcg (4,000 IU)
19+ years 100 mcg (4,000 IU) 100 mcg (4,000 IU)

Recommended dietary allowances, food sources and tolerable upper limits are obtained from the Vitamin D Fact Sheet for Health Professionals (National Institutes of Health).

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