Vitamin D Deficiency: Signs, Causes, and How to Get Enough from Sunlight

Vitamin D deficiency is the most common nutritional deficiency in the world. An estimated 1 billion people globally have insufficient levels, and in some populations — office workers, northern latitudes, darker skin tones — the rate exceeds 70%.

The strange part: your body is designed to make all the vitamin D it needs. The raw material is free, and it falls from the sky. So why are so many people deficient?

What Vitamin D Does

Vitamin D isn’t really a vitamin — it’s a hormone precursor. Once your body converts it to its active form (calcitriol), it regulates:

Calcium absorption — without adequate vitamin D, your intestines absorb only 10–15% of dietary calcium instead of the normal 30–40%
Bone metabolism — vitamin D deficiency directly causes osteomalacia (soft bones) in adults and rickets in children
Immune function — vitamin D activates T cells and modulates innate immunity. Low levels are associated with increased susceptibility to respiratory infections
Muscle function — deficiency is linked to muscle weakness and increased fall risk, particularly in older adults
Mood regulation — receptors for vitamin D are found throughout the brain. Seasonal affective disorder (SAD) correlates with winter vitamin D lows

The recommended daily intake is 600–800 IU for most adults, though many researchers argue this is too low. The Endocrine Society suggests 1,500–2,000 IU daily for adults at risk of deficiency.

Signs You Might Be Deficient

Vitamin D deficiency is often subtle. Many people are deficient for years without obvious symptoms. But common signs include:

Fatigue and tiredness — even with adequate sleep
Bone and joint pain — especially lower back pain
Frequent illness — catching colds and respiratory infections often
Slow wound healing — vitamin D plays a role in controlling inflammation and fighting infection
Muscle weakness — difficulty with physical tasks that were previously easy
Mood changes — particularly depression during fall and winter months
Hair loss — severe deficiency is linked to alopecia

The only way to confirm deficiency is a blood test measuring 25-hydroxyvitamin D (25(OH)D). Levels below 20 ng/mL are considered deficient; 20–29 ng/mL is insufficient; 30+ ng/mL is generally adequate.

Why So Many People Are Deficient

Indoor lifestyles

The average American spends 93% of their time indoors. Even people who feel like they “get outside” often don’t get meaningful UVB exposure — window glass blocks virtually all UVB radiation.

Latitude

Above ~37°N latitude (roughly the line from San Francisco to Richmond, Virginia), the sun angle is too low for vitamin D production during winter months. Boston has zero D-Window days from November through February. London loses it from October through March.

Sunscreen and clothing

SPF 15 blocks about 93% of UVB. SPF 30 blocks 97%. Modern sun-safety messaging — while important for skin cancer prevention — has the side effect of dramatically reducing vitamin D synthesis when people apply sunscreen before any unprotected exposure.

Skin pigmentation

Melanin absorbs UVB, which is protective against sunburn but also reduces vitamin D production. A person with Fitzpatrick Type VI skin may need 5–10x longer sun exposure than a person with Type I skin to produce the same amount of vitamin D.

This creates a particularly difficult situation for dark-skinned individuals living at high latitudes — they face both reduced UVB availability and higher melanin filtering.

Age

Vitamin D synthesis efficiency decreases with age. A 70-year-old produces roughly 25% of the vitamin D that a 20-year-old produces from the same sun exposure, because the concentration of 7-dehydrocholesterol in the skin declines.

Body composition

Vitamin D is fat-soluble. It gets sequestered in adipose tissue, making it less bioavailable. People with higher body fat percentages tend to have lower circulating vitamin D levels.

How to Get Enough Vitamin D from Sunlight

Diet alone is a poor source of vitamin D. Very few foods contain significant amounts — fatty fish, egg yolks, and fortified milk provide some, but you’d need to eat salmon daily to approach 1,000 IU.

Sunlight is by far the most efficient source. A single session of moderate sun exposure can produce 10,000–25,000 IU — more than any supplement or diet.

The challenge is doing it correctly:

1. Expose skin during the D-Window

Your “D-Window” is the daily time range when the sun is above 35° altitude — the minimum angle for UVB to penetrate the atmosphere. Outside this window, you can’t produce vitamin D no matter how long you stay out.

The D-Window varies by:

Latitude — closer to the equator = longer window
Season — summer has a much longer window than spring/fall
Date — it shifts by minutes every day

2. Expose enough skin area

Face and hands (~10% body area) produce minimal vitamin D. Arms and legs (~25% body area) are the practical minimum for meaningful production. The more skin exposed, the faster you produce — and the less time you need to spend outside.

3. Start short, build up

For fair-skinned individuals, 10–15 minutes of midday sun with arms and legs exposed can produce ~1,000 IU. Darker-skinned individuals may need 30–60 minutes for the same production.

The key: stay well below your burn time. You don’t need a sunburn to get vitamin D. Holick’s Rule shows that just 1/4 of a Minimal Erythemal Dose on 25% of your skin produces ~1,000 IU.

4. Skip sunscreen for your D-session

This is controversial but backed by the science: sunscreen blocks the UVB that triggers vitamin D production. Consider brief unprotected exposure during your D-Window, then apply sunscreen for any additional time in the sun.

Always protect your face and any areas prone to sun damage.

5. Check the conditions

Cloud cover, altitude, and surface reflection all affect your production rate. A cloudy day at sea level on grass is very different from a clear day at 2,000m on snow — even at the same latitude and time.

Tracking Your Exposure

The number of variables — sun angle, skin type, cloud cover, clothing, SPF, altitude, surface type — makes it genuinely hard to estimate vitamin D production by feel.

RayDay calculates all of this automatically. It shows your D-Window for today and the week ahead, tracks IU production in real-time during sun sessions, and includes a burn countdown so you know exactly when to stop. It accounts for your Fitzpatrick skin type, what you’re wearing, SPF level, weather conditions, altitude, and surface albedo.

The goal is straightforward: get your vitamin D, don’t get burned, and don’t waste time in the sun when UVB isn’t even reaching you.