Vitamin D: Hormone, Not Just a Vitamin — And Why You Probably Don’t Need Supplements

Vitamin D is essential for human health — but despite its name, it is not truly a vitamin.

By definition, vitamins are substances that must be obtained from the diet because the body cannot produce them.

In contrast, humans synthesize vitamin D naturally in the skin when exposed to sunlight.

For this reason, vitamin D is technically a hormone, not a vitamin.

Why We Need Vitamin D (Hormone D)

Vitamin D plays powerful roles in the body:

• Strengthens bones and teeth by regulating calcium absorption

• Supports immune system function

• Helps maintain muscle strength and balance

• Regulates inflammation and promotes tissue repair

• Supports mood and brain health

Signs and Symptoms of Deficiency

X-ray of a 2-year-old with rickets

In children, severe vitamin D deficiency causes rickets, leading to soft, weak, and deformed bones. If untreated, it can cause growth problems and lifelong disability.

In adults, deficiency causes osteomalacia, resulting in bone pain, muscle weakness, and an increased risk of fractures.

Other possible signs include fatigue, frequent infections, low mood, and delayed wound healing — though these symptoms can occur with many other health conditions as well.

Can You Get Too Much?

Vitamin D Toxicity (StatPearls, 2023)

While vitamin D is vital, too much can be dangerous. Toxicity is almost always caused by excessive supplements, not sunlight.

Signs of vitamin D toxicity include:

• Nausea, vomiting, poor appetite, and constipation

• Muscle weakness and weight loss

• Elevated blood calcium (hypercalcemia), which can lead to confusion, kidney damage, and irregular heartbeat

☀️ Important: Your skin naturally stops making vitamin D after healthy sun exposure, preventing overdose.

Is Sunlight Safe? (YES — and it's free!)

Studies show that brief midday sun exposure — about 5–30 minutes on arms, legs, and face, between 10 a.m. and 4 p.m., at least twice a week — usually produces enough vitamin D (NIH Office of Dietary Supplements, 2022; Webb et al., 2018).

Factors like latitude, skin color, age, and season affect how much vitamin D you make.

Good news: Prolonged sun exposure doesn’t cause toxicity! Extra vitamin D simply breaks down into inactive compounds.

Is Low Vitamin D a Cause or a Marker?

Low vitamin D levels are linked to many diseases — but supplementation does not prevent them. A major review found that low vitamin D is more likely a marker of poor health, not a direct cause (Autier et al., 2014).

Vitamin D supplements have not been proven to prevent:

• Heart disease

• High cholesterol

• Inflammation

• Diabetes

• Weight gain

• Infections

• Multiple sclerosis

• Depression

• Cognitive decline

• Early death

✅ Possible exception: a slight reduction in colorectal cancer risk.

Do You Need Supplements?

Probably not! Humans thrived for millions of years relying on natural sunlight, not supplements.

Routine vitamin D testing is not recommended for healthy adults. The U.S. Preventive Services Task Force gave it a “D” grade, meaning it is ineffective and could cause more harm than good (USPSTF, 2021).

The only people who might benefit from supplements are:

• Frail elderly women at risk of falls and fractures

• People who are housebound or rarely get outdoors

Smart Sun Exposure Tips

🌞 Start sun exposure gradually in spring to avoid burns.

🌞 Aim for midday (10 a.m.–2 p.m.) for best vitamin D production.

🌞 Expose large skin areas (arms, legs, torso) briefly without sunscreen.

🌞 After healthy exposure, protect your skin:

• Wear hats and protective clothing 🤠👚👖

• Use safe sunscreens (non-nano zinc oxide or titanium dioxide — the kind that leaves a white residue)

🚫 Never burn — burning increases skin cancer risk.

🧠 Fun fact: Sunlight also boosts mood, sleep cycles, and blood vessel health!

Key Takeaways

☀️Vitamin D is a hormone, not a traditional vitamin.

☀️ Your body makes it naturally from safe, moderate sun exposure.

☀️ Low levels often reflect existing health issues rather than cause them.

☀️ Supplements and routine testing are unnecessary for healthy, sun-exposed people.

☀️ Smart sun habits protect both your vitamin D levels and your skin.