70% of Americans Aren't Getting Enough Vitamin D — Winter Makes the Gap Impossible to Ignore

December 26, 2025

Nearly 70% of Americans aren’t getting enough vitamin D, a nutrient that supports much more than just bone health. Our bodies primarily make vitamin D when sunlight hits our skin, and food sources like milk or fortified products usually aren’t enough.

Winter makes it even harder to get the vitamin D we need. Colder weather means less sun exposure, more layers, and a lower UV index, all of which limit your body’s natural production. Here’s why vitamin D matters and how to support healthy levels when sunlight is in short supply.

Natural Sources of Vitamin D

We reach our vitamin D needs through a variety of ways. The body’s preferred source of vitamin D is sunlight. When sunlight reaches your skin, it triggers a chemical reaction that allows your body to produce vitamin D naturally.

This process is efficient, as the body can meet all its daily vitamin D needs in as little as 10 minutes when the UV index is high. However, this only works when enough bare skin is exposed to UVB rays. Clothing, sunscreen, and limited time outdoors can all reduce the vitamin D you make—especially in winter. Winter also comes with shorter days and lower UV levels.

Apart from sunlight, the body gets a small amount of vitamin D from food. Very few food sources naturally contain high amounts of vitamin D. The best natural sources of vitamin D are:

Fatty fish/fish liver oils
Beef liver
Egg yolks

Since these foods are less common in the Western diet, manufacturers often add vitamin D to dairy products and breakfast cereals. This is why we think of milk when we hear vitamin D! Not because it naturally is high in vitamin D, but because it has been fortified with some vitamin D. Even then, one cup of milk only contains 15% of a person’s daily vitamin D needs.

What does Vitamin D do?

Vitamin D might do a lot more than you realize! What most people don’t know is that Vitamin D isn’t merely a vitamin; it’s a hormone. This means that Vitamin D plays a role beyond supporting your body. It actually helps control it.

Here are five of the primary functions of Vitamin D in the body:

Supports bone growth and remodeling
Helps regulate the immune system
Boosts your mood
Helps regulate your hormones
Supports quality sleep and daily energy

Bone Health

The best-known job of vitamin D is helping build and keep your bones strong. Without enough vitamin D, your body can’t absorb calcium well. Vitamin D works like a key, letting your gut take in calcium and phosphate, which are both needed for strong bones.

Having enough vitamin D helps your body replace old bone with new bone, which is always happening. This keeps your bones strong as you get older, supports your athletic performance, and helps prevent bone loss over time.

Boosts Immune System

Vitamin D is a powerhouse when it comes to immune defense. It supports the body’s ability to fight off viruses and bacteria by activating and regulating key cells of the immune system, such as T cells and macrophages. These cells act like your body’s security team, looking for invaders and responding quickly to infections.

This study shows that people with low vitamin D are more likely to get respiratory illnesses like colds, the flu, and even pneumonia. In winter, when there’s less sunlight and your vitamin D drops.

Vitamin D also helps regulate inflammation, allowing the immune system to work efficiently without overreacting. This balance supports overall health and makes vitamin D essential for a resilient immune system.

Impacts Mood

Vitamin D influences mental well-being as well as physical health. Scientists have found vitamin D receptors throughout the brain, especially in areas that control mood. This means vitamin D can help make neurotransmitters such as serotonin, which is often called the “feel-good” chemical.

One meta-analysis examined 31 studies investigating the relationship between vitamin D supplementation and depressive symptoms. Analyzing data from over 24,000 participants, they found that for every extra 1000 IU of vitamin D taken daily, depression scores continued to drop.

Keeping your vitamin D levels in a healthy range may help support a more positive outlook, especially during the darker, colder months when sunlight is scarce and mood dips are common.

Assists Hormones

Vitamin D quietly plays a big role in keeping your hormones balanced. Nearly every tissue and organ in the body has vitamin D receptors, highlighting its wide-ranging effects. In addition to functioning as a vitamin, it acts as a hormone, guiding the body’s hormone production and signaling.

Vitamin D is important for how your body makes and uses hormones. It attaches to cell receptors that control gene expression, turning certain processes on or off. This helps guide your body’s hormone production and signals.

The most notable hormone pathways that vitamin D impacts might be testosterone. In one study, it was observed that Vitamin D supplementation significantly increased total testosterone levels in men.

Vitamin D also helps your body make other hormones. When your vitamin D levels are good, it helps keep your hormones balanced, which is especially helpful when you’re under extra stress in the winter.

Supports Sleep and Energy

One of the most common symptoms of vitamin D deficiency is fatigue. Low levels of this key nutrient were studied in a 2018 meta-analysis, which included over 9,000 participants. The scientists consistently saw shorter and lower-quality sleep, brain fog during the day, and a higher risk of developing sleep disorders in people who had low vitamin D.

So why is vitamin D important for sleep and energy? Researchers believe that vitamin D plays a role in regulating circadian rhythm, the internal clock that determines sleep and wake cycles. It also supports the body’s production of melatonin, the hormone that signals your brain to wind down at night.

When your vitamin D is in a healthy range, you’re more likely to fall asleep easily, sleep through the night, and wake up refreshed. You might also think more clearly and feel more focused during the day, with less brain fog or tiredness.

Making Vitamin D a Priority

As you can see, vitamin D is important for much more than strong bones. It affects your immune system, hormones, mood, energy, and sleep. Since many people don’t get enough, especially in winter, it’s a good idea to check your vitamin D levels.

Paying attention to sun exposure and recognizing the signs of deficiency can help you stay healthy throughout the season.