What’s the most hyped up vitamin during the cooler seasons? Vitamin D! Here’s the breakdown of why you need it, how to know if you’re deficient, and how to get some more.

So what does vitamin D do?

Vitamin D has many roles in the body. You’ve probably heard you need it for strong healthy bones, and you've heard right. Vitamin D helps bring calcium into your bones and teeth and helps regulate calcium levels in your blood. It also supports the immune system, regulates insulin levels, influences gene expression, and supports cardiovascular health. Vitamin D receptors have also been found in the brain, and studies have found that people with lower levels of vitamin D have higher rates of depression. Having trouble sleeping? One recent study found that people who had low levels of vitamin D were more likely to wake up more frequently and supplementation gave them more restful Zzzs.

Are you at risk for vitamin D deficiency? 

Vitamin D is dubbed the “sunshine vitamin” for a reason - we need a little time in the sun each day to make the vitamin and it’s hard to come by in food. In modern times we’ve spent more and more time indoors and started slathering on sunscreen, causing vitamin D deficiency to become more common. The Centers for Disease Control and Prevention estimates that on average about 40% of the population has a vitamin D deficiency, with some populations at an even higher risk.

Melanin, a pigment found in skin, acts like sunscreen to protect the skin from UV rays. By blocking the sun’s rays, melanin affects the skin’s ability to activate vitamin D’s precursor. People with darker skin tones have more melanin in their skin than people with lighter skin tones, and so are at a much higher risk of deficiency. The CDC found that 82% of African Americans and 69% of Hispanics were found to have a vitamin D deficiency.

You are more likely to have a vitamin D deficiency if you have dark skin, are elderly, are overweight, do not eat fish, milk or fortified products, live far from the equator, always wear sunscreen, work at night, or do not frequently go outdoors. People with certain medical conditions such as liver disease, cystic fibrosis, celiac disease and Crohn's disease and people who have had gastric bypass surgery may need extra vitamin D.

How do you know you have a vitamin D deficiency? 

Symptoms of vitamin D deficiency include fatigue, depression, more frequent colds, bone and back pain, muscle weakness and bone loss.

Many doctors are now offering vitamin D tests during annual check ups. If you’re concerned, ask your doctor for a 25-hydroxy vitamin D blood test. Between 20 - 50 ng/mL is considered normal, and levels under 12 ng/mL are considered deficient. In recent years, some doctors have been calling for supplementation even when people are at the low range of normal.

Remember there can be too much of a good thing. Unlike water soluble vitamins where excess levels are flushed from the body, fat soluble vitamins, like vitamin D, are stored in fat throughout the body. Overdose only occurs with supplementation (our bodies only make as much as they need). Signs of overdose include nausea, vomiting, poor appetite, constipation, weakness, weight loss, confusion, and irregular heart beats.

How can I get enough vitamin D in my diet?

Very few foods are naturally high in vitamin D. Most of the vitamin D in the American diet comes from fortified foods, such as milk, cereal, and orange juice. The best sources of vitamin D are found in fatty fish, such as salmon, tuna and mackerel. Small amounts are found in cheese, butter, liver and egg yolks.

The amount of vitamin D you need each day depends on your age. The Food and Nutrition Board recommends 400 IU for infants up to 12 months, 600 IU from age 1 to 70 (including pregnant and breastfeeding women), and 800 IU for adults over age 70.

How much sun do I need to get enough vitamin D?

The amount of vitamin D you make in the sun includes a lot of factors, and can make the answer rather complicated. The short answer? Aim for 10 - 20 minutes out in the sun, sans sunscreen. (Any time after that and you should slather up to reduce your risk of skin cancer and premature aging!)

The long answer? There are less UV rays on cloudy days and the further you are located from the equator. The sun’s strength peaks in the summer and is weakest during the winter. Generally the sun is strongest between the hours of 10 am to 2 pm. One study found that in winter, when only about 10% of our skin is left exposed, it can take up to two hours to get adequate levels. Unfortunately sitting inside by a sunny window doesn’t count towards your daily sunshine time as UVB radiation does not penetrate glass.

And remember, skin tone makes a huge difference in how quickly we produce vitamin D. It may take 5 times longer for people with darker skin tones to make the same amounts of vitamin D as as, say, a redhead. Dr. Holick, who wrote the book The Vitamin D Solution, suggests you take the time it takes for your skin to begin to turn pink, then only spend a quarter to half of that time in the sun without sunscreen.

Remember how vitamin D is stored in fat? The metabolism of vitamin D is complex (this vitamin is rather complicated, isn’t it?) but your body does store some vitamin D. One estimate is that it lasts for approximately 2 months in the body, so doses of sun do add up and hiding out all day inside won’t suddenly drop your vitamin D levels and send you spiraling into depression.

How much vitamin D should you take in a supplement?

Again, tricky. The Food and Nutrition Board recommends 400 IU for infants up to 12 months, 600 IU from age 1 to 70 (including pregnant and breastfeeding women), and 800 IU for adults over age 70. If you’re not eating foods high in vitamin D, fortified foods, or getting much sun you may consider getting those amounts through supplements. Experts at Harvard suggest taking 2,000 IU a day. There are two types of vitamin D - D3 (cholecalciferol) and D2 (ergocalciferol). D3 raises blood levels more quickly than D2, and is considered the “natural” form, since it is the form that humans make when thes un strikes our skin.

In recent years, some experts have called for much higher doses. Your skin can make between 10,000 and 25,000 IU in the summer sun before it begins to burn. Before you dabble in extremely high doses, remember that your skin is smarter than a pill and will only make as much as it needs.