Exactly How Long To Wait To Retest Vitamin D Levels After Starting A Supplement

October 2, 2021 — 11:28 AM

If you've recently started supplementing with vitamin D, kudos to you. Though it might seem like a simple routine tweak, that tiny softgel, capsule, or tablet has a lot to offer—and can make a notable impact on your health and well-being.* (Seriously, tell your friends to get on the bandwagon! After all, 41% of us are insufficient in the sunshine vitamin.)

Vitamin D has a hand in keeping your bones healthy, supporting your immune system, and promoting cardiovascular health—and given that nearly every cell in your body has binding sites for vitamin D, that's probably just the tip of the iceberg,* according to functional nutrition dietitian Whitney Crouch, RDN, CLT.

Once you make that much-needed vitamin D supplement a daily habit, though, you probably want to know how long it'll take to start feeling the difference—and seeing your vitamin D levels on the up-and-up. Here's a quick breakdown of what you need to know about how your vitamin D status is actually measured, how to supplement for the best results, and when to retest your levels after starting a supplement to see its effects in action.*

First things first here: some important background information about how health care providers and researchers measure vitamin D status in the body—and what level to aim for.

If your health care provider suspects you have a vitamin D deficiency or insufficiency, they will order a blood test that looks at your total serum 25-hydroxyvitamin D—or 25(OH)D—which is the form of vitamin D that circulates through your blood. The amount of that 25-hydroxyvitamin D you have in your blood (your status biomarker) is measured in "ng/ml."

You've probably heard that 30 ng/ml is considered the baseline cutoff for avoiding insufficiency, but scientific literature and many experts—including board-certified endocrinologist Brittany Henderson, M.D.—recommend achieving a level of 50 ng/ml to best support bone health, immunity, and more.* (In fact, Henderson typically likes to see patients between 50 and 100 ng/ml.)

Depending on how far below the mark your vitamin D status is, your provider will recommend a high-quality vitamin D supplement at a potency that matches your needs and takes your unique health factors into consideration.* They'll also recommend a supplement that provides vitamin D3, which is the bioactive form of the vitamin and is better at boosting your status than D2.* (Not sure where to start? We compiled a list of our favorite high-quality, high-potency vitamin D supplements.)

While your individual vitamin D needs may vary, know this: It takes 1,000 IU of vitamin D3 per day to increase 25(OH)D by about 10 ng/ml in the average adult.* For reference, that means it generally takes 5,000 IU per day to hit 50 ng/ml—though some people may need even more.* People with overweight or obesity, for example, need two to three times as much vitamin D (which means 10,000+ IU per day) to achieve the same result, working in partnership with your health care provider.*

So, say your health care provider confirmed with a 25(OH)D test that you need to get more of the sunshine vitamin into your system, stat—and that you promptly started (and have diligently stuck to) a daily vitamin D supplement routine. You might feel antsy to retest your vitamin D status and confirm that your habit change is paying off—but not so fast!

While we all want immediate gratification, you'll have to be a little patient here. The reason: "Vitamin D is fat-soluble, so vitamin D supplementation goes toward replenishing the vitamin D stored in fat, in addition to serum levels,"* Henderson explains. "As these storage levels are replenished, serum levels then start to rise."*

As a result, "waiting three months to retest your 25(OH)D status can help give you a more accurate reading of how well your supplement has done in replenishing your whole-body vitamin D levels,”* she adds. Research agrees, with studies similarly suggesting health care practitioners wait 12 weeks before rechecking a patient's vitamin D status after starting them on a supplement or adjusting the potency of that supplement. 

There's nothing you can do to speed up this process, but you can increase your chances of success by ensuring you take in some fat alongside your vitamin D supplement in order to promote optimal absorption.* (Remember, it's fat-soluble!) Better yet, opt for a supplement that contains vitamin D3 and a source of fat so you don't have to wait for snack- or mealtime to get your daily dose of the sunshine vitamin. (mbg formulated our vitamin D3 potency+ with a unique trio of organic, virgin oils so you can take your daily dose of D whenever is most convenient for you.)*