What Vitamin D Really Does in the Body

Vitamin D is often called a vitamin, but technically, it acts more like a hormone. Once it’s activated in your body, it gets to work almost everywhere, helping you absorb calcium, deal with inflammation, and even influence the way hundreds of your genes function. Think of it as a behind-the-scenes coordinator that quietly keeps things running smoothly.

One of its most important jobs is supporting strong bones. Vitamin D helps your body absorb calcium and phosphorus, the minerals that build and maintain bone strength. Without enough of it, your bones can become weak or brittle over time, which increases the risk of fractures or osteoporosis.

But its influence doesn’t stop there. Vitamin D also helps your immune system stay alert and balanced, making sure that it’s strong enough to defend against viruses, but calm enough to avoid overreacting. Studies have shown that people with adequate vitamin D levels are less likely to experience frequent respiratory infections.

Your brain depends on it, too. Vitamin D helps regulate serotonin, one of the key neurotransmitters that influences mood, focus, and mental clarity. Low levels have been linked to fatigue, seasonal mood changes, and even cognitive fog.

And finally, vitamin D plays a quiet but very important role in hormone balance. It supports insulin sensitivity, reproductive health, and overall metabolic stability, which helps your body use energy more efficiently.

“Clients are often surprised that fatigue, low mood, or even hair loss can sometimes trace back to low vitamin D levels,” says Bridget Isaacs, MS, RD.

So while it’s easy to overlook, this “sunshine vitamin” truly touches nearly every part of your health…from your bones to your brain.

How Much Vitamin D Do You Need?

The Recommended Dietary Allowance (RDA) for vitamin D varies by age and individual factors:

However, these recommendations assume sufficient sunlight exposure. People with darker skin, those living in northern climates, or anyone that spends most of their time indoors may need more.

“Blood testing is the only reliable way to know your vitamin D status,” notes Jessica Kelly, MS, RDN, LDN. “Ideally, aim for levels between 30–50 ng/mL for optimal health.”

The Sunlight Equation: Can You Get Enough Naturally?

Your skin synthesizes vitamin D when it is exposed to UVB rays. But latitude, season, sunscreen, skin tone, and the amount of time spent indoors can all limit its production. During late fall and winter, UVB intensity often isn’t strong enough in northern regions to trigger vitamin D synthesis.

Practical Tip: Get short, regular sun exposure (about 10–20 minutes, arms and legs exposed, 2–3 times per week) when possible,but be sure to avoid getting a sunburn. In colder or cloudier months, dietary sources and supplements become essential.

The Top Food Sources of Vitamin D

“Salmon, eggs, and fortified dairy are easy ways to boost vitamin D without supplements,” says Jessica Kelly, MS, RDN, LDN. “But if your levels are low, food alone often isn’t enough.”

Remember

Supplementing Smartly

Sometimes, sunshine and food just aren’t enough, especially if you live in a place with long winters, spend most of your day indoors, or already have low vitamin D levels. That’s where supplements can really help. But before you grab a bottle off the shelf at your local pharmacy, there are a few things to know.

There are two main types of vitamin D supplements out there. Vitamin D2 (ergocalciferol) is plant-based, but not as effective at raising your levels. Vitamin D3 (cholecalciferol) is what your body makes from sunlight, and it’s usually better absorbed, plus, there are vegan-friendly versions available if you prefer a plant-based option.

To get the most from your supplement, take it with a meal that contains some fat, because this helps your body absorb it more easily. And while it might be tempting to go for a high dose, more isn’t always better. Unless your doctor or dietitian recommends it, steer clear of megadoses.

Once you’ve been supplementing consistently for a few months, it’s smart to get your blood levels checked again. That way, you’ll know if your routine is working or needs a tweak.

“Supplements can be powerful, but they’re not one-size-fits-all,” says Bridget Isaacs, MS, RD. “Your needs depend on your blood levels, medications, and even genetics.”

Vitamin D and Mood: The Sunshine Connection

Several studies link low vitamin D levels with increased risk of depression, especially in winter months. Vitamin D receptors are found in areas of the brain that regulate mood and serotonin.

“Low vitamin D doesn’t automatically mean depression, but optimizing levels can make a noticeable difference in mood and motivation,” says Bridget Isaacs, MS, RD.

Other studies suggest that combining vitamin D with exercise and adequate sleep provides additional benefits for mood stability and mental health.

Vitamin D, Hormones, and Women’s Health

Vitamin D plays a surprisingly important role in hormone balance. It helps regulate the production of important hormones like estrogen, testosterone, and insulin, all of which influence everything from mood to metabolism to menstrual health. When vitamin D levels are too low, the ripple effects can show up as PMS symptoms, irregular cycles, PCOS, or even fertility challenges.

“For women with PCOS or hormonal imbalances, vitamin D can be a key piece of the puzzle—especially for insulin sensitivity and menstrual regularity,” notes Bridget Isaacs, MS, RD.

There’s growing research connecting healthy vitamin D levels with smoother cycles, better blood sugar control, and even improved egg quality. While it’s not a cure-all, vitamin D often works behind the scenes to boost the systems that keep your hormones in sync. If you’re dealing with hormonal symptoms that feel out of the ordinary, checking your vitamin D levels might be a helpful place to start.

Let’s Get Your Vitamin D Levels Where They Should Be

If you’re feeling tired, run-down, or noticing mood changes, it might be time to check your vitamin D levels. The Nutrition Experts at Berry Street can help you interpret lab results, adjust your diet, and find the right supplement strategy. Book your 1:1 session today!

Vitamin D is essential year-round, not just in sunny weather. Whether you get it from sunlight, food, or supplements, maintaining optimal levels supports immunity, mood, and long-term health.

“Checking your vitamin D status is one of the simplest ways to invest in your future health,” says Jessica Kelly, MS, RDN, LDN. “When your levels are right, you feel the difference—in energy, focus, and mood.”

At Berry Street, we help clients create realistic, evidence-based plans to maintain vitamin D levels all year long. Contact us today to find out how we can help.

Frequently Asked Questions

Can I get enough vitamin D from sunlight alone?

In the summer months, it’s possible, especially if you spend time outside with your arms and legs exposed a few times a week. But for most people, especially in fall and winter or in northern climates, sunlight alone usually isn’t enough. That’s when food sources and supplements become more important to help maintain healthy levels year-round.

What’s the best time to take vitamin D?

The best time to take vitamin D is with your largest meal of the day, ideally one that includes some healthy fat, like olive oil, eggs, or avocado. Because vitamin D is fat-soluble, your body absorbs it better when it’s taken with food.

Can too much vitamin D be harmful?

Yes, taking too much vitamin D over time can cause problems. It may raise calcium levels in your blood, which can lead to symptoms like nausea, confusion, or even kidney issues. That’s why it’s always a good idea to check your blood levels and talk to your healthcare provider before starting high-dose supplements.

Are vitamin D drops better than pills?

Both drops and pills can be effective. It really comes down to personal preference. Drops can be a great option for anyone who has trouble swallowing pills or prefers to mix their supplement into a drink or smoothie. The most important thing is choosing a high-quality product and taking it consistently.

Does sunscreen prevent vitamin D production?

Sunscreen can slightly reduce your skin’s ability to make vitamin D, but that doesn’t mean you should skip it. Protecting your skin from UV damage is still very important. A little sun exposure (like 10 to 15 minutes a few times a week) can help, but it’s all about balance. When in doubt, lean on food and supplements to stay covered.