Dietitians Explain Just How Long It Takes Vitamin D to Work in Your Body

Updated: Mar. 20, 2022

Research suggests low levels of vitamin D increase your risk of important health issues, possibly including premature death. Here are smart ways to tell if your vitamin D is working, and what to do if it's not.

Am I getting enough vitamin D?

Vitamin D helps keep your bones strong, your heart healthy, your immune system tough, your mood light, and so much more—in fact, many experts agree that Vitamin D is important for most of the things your body does. In fact, a 2016 peer-reviewed analysis of studies suggested that having just moderately low levels of this supernutrient can increase your risk of premature death from any cause. One 2020 study in the Journal of the American Medical Association even found that more than 80 percent of people hospitalized for COVID-19 had low vitamin D levels.

Yet, estimates show that about one in three U.S. adults has insufficient vitamin D levels, according to 2020 research published in the European Journal of Clinical Nutrition. (The study noted that in Europe, that figure represents upwards of 50 percent of adults.) What’s extra tricky is that you might not even know you’re lacking—while there are signs of low vitamin D levels, symptoms aren’t obvious for everyone.

Only a blood test can confirm where you stand and whether you need to take action to increase your vitamin D. One tip, says Paula Doebrich, RDN, MPH, is to get checked in both the spring and fall to help you understand how your levels trend. This is because human skin naturally manufactures vitamin D in response to sunlight, so the repeat testing shows how well your body responds to sun exposure, and how well it naturally stores the vitamin.

How can I raise my vitamin D level quickly?

While it may be tempting to recharge your vitamin D with a bit of sunbathing, Doebrich says this is not the most efficient method to maintain appropriate levels year-round. The sunshine approach is typically recommended only when someone has a fat malabsorption issue, meaning your body isn’t taking in vitamin D very efficiently, explains Emily Clairmont, RD, a registered dietitian with the University of Vermont Medical Center.

Even then, it only takes about 20 minutes in the sun to get the vitamin D you need for the day. Beyond that, sunlight can actually start degrading the nutrient in your skin, according to research published in BMC Public Health.

And, you may suspect what’s coming next: “The increased risk of skin cancer is not worth it when there are other methods of replenishing [your vitamin D],” Doebrich says.

Eat more vitamin D

In general, getting your nutrients from dietary sources is superior to taking supplements, according to 2019 research published in Annals of Internal Medicine. Oily fish, like rainbow trout and salmon, are especially vitamin D-rich, netting you up to 80 percent of your daily requirement with a three-ounce portion. Check out five more ways to incorporate enough vitamin D into your diet—including what to do if you’re eating plant-based.

Take a vitamin D supplement

If you’re not meeting your vitamin D needs through dietary means alone, the experts say you should speak with your doctor about taking a supplement. There are two types of vitamin D supplements available: Vitamin D2, derived from plants, and vitamin D3, derived from animal sources.

The experts suggest you opt for D3 when possible—and a review of studies published in Nutrients points to why: D3 routinely works better and faster than a D2 supplement at both raising and sustaining the body’s vitamin D levels.

While animal-based D3 supplements may not be suitable for vegans and vegetarians, Clairmont says D3 can now be sourced from sheep lanolin. “Though this is still from an animal, no animal is harmed in the process,” she explains. That said, if you follow a strict vegan lifestyle and want to stick with vitamin D2, this dietitian recommends you speak with your doctor about whether you need a higher dose. For more wisdom into all this, read The 4 Best Vitamin D Supplements Depending on Your Specific Needs, from Registered Dietitians.

How long does it take for vitamin D to work?

Black woman leaning out car window

Many factors can influence how long it will take for your vitamin D levels to hit a healthy range, such as how well you absorb vitamin D, any underlying health conditions you have, and your initial blood serum levels. For instance, the National Institutes of Health says that the greater your starting deficiency, the longer it will take for levels to improve.

In general, most protocols aim to reach ideal levels within about three months, Clairmont says. But it’s important to stick to your healthcare provider’s plan and get your levels checked until you reach your goal. “Some people see an increase after six weeks,” Doebrich says. “Others need to take supplements for four months before they feel better.”

How do I know vitamin D is working?

“If you had physical symptoms of a deficiency, such as fatigue or bone pain, you will likely start feeling better as your serum levels increase,” Doebrich says.

While research is ongoing, you might also start snoozing more soundly, too, according to Nicole Avena, PhD, Assistant Professor of Neuroscience at Mount Sinai School of Medicine in New York City and author of Why Diets Fail. “The higher the levels of vitamin D in the body, the fewer sleep disturbances and disorders [that are] recorded,” she explains. “This may be because so many vitamin D receptors are found in the brain, which is the control center of the body’s sleep function.”

Still, some people may not notice feeling differently at all, as symptoms of low vitamin D levels can be subtle or insubstantial. The experts emphasize that it all depends on the individual person, health status, and vitamin D levels before supplementation. (That’s why it’s so important to get blood tests to confirm you’re in a healthy range.) But even if you don’t feel the effects of maintaining great vitamin D levels, it doesn’t mean the supernutrient isn’t hard at work. Here are 23 ways the sunshine vitamin quietly keeps your body in great shape.

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