The Science of Vitamin D Vitamins: Managing Vitamin D Deficiency for Wellness

The most reliable way to fix or prevent a vitamin D deficiency is a combination of some regular sunlight, vitamin D-rich foods such as fatty fish and fortified dairy, and a supplement, ideally vitamin D3 (cholecalciferol). A supplement matters most if you live in a northern latitude or spend most of your time indoors. A blood level of about 30 to 60 ng/mL is often considered the optimal range.

Vitamin D is fat-soluble and acts more like a hormone than a typical vitamin, influencing the activity of over 200 genes. It affects your bones, your immune system and your metabolism, which is why low levels are worth taking seriously.

What vitamin D is

Vitamin D is a group of fat-soluble secosteroids. The two main forms are vitamin D2 (ergocalciferol), which comes mostly from plants and fungi, and vitamin D3 (cholecalciferol), which your skin makes when exposed to UVB light and which is also found in animal foods. Research suggests D3 is significantly better at raising and keeping up your blood levels over time.

Your body has to convert vitamin D twice before it can use it. The liver turns it into 25-hydroxyvitamin D [25(OH)D], the form measured in blood tests. The kidneys then turn that into 1,25-dihydroxyvitamin D [1,25(OH)2D], the active form. This binds to the vitamin D receptor (VDR), found in cells throughout the body, including the brain, heart and immune system.

Why deficiency is so common

Low vitamin D is widespread. In a 2011 study in Nutrition Research, Forrest and Stuhldreher found that about 41.6% of US adults were deficient (below 20 ng/mL), making it one of the most common nutritional deficiencies. The rate is higher in people with darker skin, because melanin reduces how much vitamin D the skin makes from sunlight.

Indoor work, high-SPF sunscreen (which blocks UVB) and living above 37 degrees latitude all make it harder to keep levels up. Your skin also makes less vitamin D as you age, so supplements matter more for older adults. Our guide to vitamin D supplements explains how to choose one.

Vitamin D and your immune system

Vitamin D receptors are found on nearly all immune cells. In the Journal of Investigative Medicine (2011), Aranow et al. described how deficiency is associated with more infections and autoimmune conditions. Vitamin D helps regulate both the fast, general immune response and the slower, targeted one. It strengthens the germ-fighting action of monocytes and macrophages while reducing inflammation.

With enough vitamin D, your body is better able to make antimicrobial peptides such as cathelicidin, an early defense against respiratory infections. It also works alongside other nutrients, as our article on how multivitamins support immune function explains, helping keep the immune response active without tipping into excess inflammation.

Signs of vitamin D deficiency

The symptoms are often vague, and many people put them down to age or stress. The most common signs of vitamin D deficiency are ongoing fatigue, getting sick often, bone pain and low mood, especially in winter.

Over the long term, severe deficiency can lead to secondary hyperparathyroidism and loss of bone mineral, raising the risk of fractures and osteoporosis. Vitamin D may also play a part in brain health, including serotonin regulation, and low levels have been linked to brain fog and low mood, including the winter pattern known as seasonal affective disorder (SAD).

Why it matters for women

Women's needs change through menstruation, pregnancy and menopause, and addressing nutrient deficiencies in women is a key part of prevention. Vitamin D is especially important for bone health because falling estrogen during menopause can speed up bone loss. Getting enough earlier in life helps build bone reserves for later.

Low vitamin D often sits alongside other gaps. B12 deficiency, for example, affects about 6% of adults under 60 and nearly 20% of those over 60, with vegans at far higher risk, according to Allen et al. in the American Journal of Clinical Nutrition (2009). Several shortfalls together can weigh heavily on energy and concentration. Our women's multivitamin guide covers how vitamin D fits with B vitamins, iron and folate.

How to absorb vitamin D well

Because vitamin D is fat-soluble, take it with a meal that contains fat, such as avocado, nuts or olive oil. Many supplements come in oil-based softgels for this reason. Magnesium also matters: the enzymes that process vitamin D in the liver and kidneys depend on it.

Look at the whole formula, too. Other nutrients, such as vitamin C for collagen, support your overall health. And choose a supplement with independent testing, so you know you're getting the dose on the label without contaminants or unnecessary fillers.

Testing and getting the dose right

A blood test for 25-hydroxyvitamin D is the standard way to check your status. Deficiency is usually defined as below 20 ng/mL, although some practitioners aim for 40 to 60 ng/mL.

How much you need depends on your body mass index (BMI), variations in your VDR gene and your starting level. Someone with a severe deficiency may need a higher starting dose than someone keeping their level steady through winter. Ask your healthcare provider for the right dose, because too much for too long can cause toxicity (hypercalcemia).

Where Go Wise Multivitamin fits

Go Wise Multivitamin is a powder stick you pour onto your tongue, with no water needed and a natural berry flavor. Each stick has vitamins A, C, D3, E, B6 and B12, niacin, pantothenic acid, biotin, folic acid, zinc and chromium, plus elderberry extract and kelp as a natural source of iodine. It's vegan, gluten-free and has no artificial colors or flavors. Every batch is third-party tested by Eurofins for heavy metals and microbial contamination, and it is made in a GMP-certified facility.

Your first step is a 25(OH)D blood test, so you and your healthcare provider can decide how much vitamin D you need.

Frequently asked questions

How long does it take to correct a vitamin D deficiency?

It usually takes 2 to 3 months of regular supplementation to raise blood levels significantly. The exact time depends on how low you started, the dose and how well you absorb it.

Can I get enough vitamin D from food alone?

It's very hard, because few foods contain much vitamin D. Fatty fish (such as salmon and mackerel), cod liver oil and egg yolks are the best sources, but most people don't eat enough of them to correct a deficiency.

Is vitamin D3 better than vitamin D2?

Yes. Research generally shows that vitamin D3 (cholecalciferol) is better than vitamin D2 (ergocalciferol) at raising and maintaining your body's vitamin D stores. Most experts recommend D3.

What is the best time of day to take vitamin D?

Take it with your largest meal of the day. It's fat-soluble, so food with some fat improves absorption significantly. Some people prefer the morning, since it may interfere with melatonin if taken late at night.

How do I know if my vitamin D levels are too high?

Vitamin D toxicity is rare but can happen with very high doses over long periods. Symptoms include nausea, vomiting, frequent urination and weakness. The only way to know for sure is a blood test, with toxic levels above 100 to 150 ng/mL.

Does sunblock prevent vitamin D synthesis?

Yes. Sunscreen with a high SPF (30 or higher) can block up to 95 to 98 percent of UVB, which your skin needs to make vitamin D. Sun protection matters for your skin, but 10 to 15 minutes in the sun without sunscreen a few times a week is often recommended for natural vitamin D production.

Related reading: nutrient deficiencies in women.

References

• Forrest and Stuhldreher. Nutrition Research. 2011.
• Aranow et al. Journal of Investigative Medicine. 2011.
• Allen et al. American Journal of Clinical Nutrition. 2009.

This article is for general information and isn't medical advice. Talk to your healthcare provider before starting a supplement, especially if you are pregnant, nursing or taking other medication.