Vitamin D is unusual among nutrients: your body makes most of it not from food but from sunlight hitting your skin. That single fact is why deficiency is so widespread — modern life keeps us indoors, and half the planet spends part of the year somewhere the winter sun simply can't do the job. Estimates vary by population and by the threshold used, but a large share of people in higher-latitude countries run low at some point in the year.
The tricky part is that vitamin D deficiency rarely announces itself. Its symptoms are quiet and easily blamed on other things, so plenty of people carry a shortfall for years without knowing. Below is what the science actually says — the signs, the causes, who's most at risk, and how to work out your own likelihood without guessing.
What "deficiency" actually means
Vitamin D status is measured in the blood as 25-hydroxyvitamin D, or 25(OH)D. For years, clinical guidance leaned on tidy cut-offs — a level below 20 ng/mL (50 nmol/L) was widely treated as deficient, with 20–30 as "insufficient." But the ground has shifted. The Endocrine Society's 2024 guideline stepped back from those universal thresholds and stopped recommending routine testing for otherwise-healthy adults, focusing instead on ensuring adequate intake in the groups most likely to benefit. In other words, the field is moving from "everyone hit a number" toward "protect the people actually at risk."
Why this matters for you: if your doctor is no longer routinely testing healthy adults, a blood number may not be something you'll ever see. That makes understanding your risk — from sun, skin, latitude, and lifestyle — more useful than ever, because it's the signal you can actually act on.
The signs worth knowing
Because vitamin D is involved in bone metabolism, muscle function, immune regulation, and mood, a shortfall can surface in several loosely-connected ways. None of these is proof of deficiency on its own — that's the whole difficulty — but the more that stack up, especially alongside a high-risk profile, the more worth investigating:
- Persistent fatigue — one of the most commonly reported symptoms, and one of the least specific.
- Bone pain and muscle aches — vitamin D governs calcium absorption; a long shortfall can leave bones aching and, in severe cases, soft (osteomalacia in adults, rickets in children).
- Muscle weakness — particularly in the legs, and a contributor to falls in older adults.
- Low mood — deficiency is associated with depressed mood, though cause and effect are genuinely hard to untangle.
- Frequent infections — vitamin D helps regulate immune defence, and low levels are linked to more respiratory infections.
- Slow wound healing and hair thinning — reported in more significant deficiency.
The honest summary: these symptoms should raise a question, not settle it. Only a blood test confirms deficiency — but knowing your risk tells you whether that question is worth asking.
Why it happens: the sunlight problem
Your skin makes vitamin D when UVB radiation reaches it, and UVB is the part of sunlight most easily blocked — by a low sun, cloud, clothing, glass, and melanin. That's why the same lifestyle produces wildly different vitamin D depending on who and where you are:
The biggest single driver is skin type: melanin is a natural sunscreen, so darker skin needs substantially longer sun to make the same vitamin D as lighter skin. Add a high latitude, and there's a stretch of winter — often called "vitamin D winter" — when the midday sun never climbs high enough for any meaningful UVB to reach the ground. Layer on age (skin's capacity to synthesise vitamin D declines over the years), time indoors, sunscreen, and covering clothing, and you can see why deficiency clusters in specific groups.
Who's most at risk
| Group | Why the risk is higher |
|---|---|
| Darker skin (higher Fitzpatrick types) | Melanin slows synthesis — needs far more sun for the same yield |
| Older adults, especially 75+ | Skin makes less; often less time outdoors — a 2024-guideline priority group |
| Higher latitudes in winter | Months of "vitamin D winter" with no usable UVB |
| Limited sun exposure | Indoor lifestyles, covering clothing, consistent sunscreen |
| Pregnant people & infants | Higher needs; guideline priority groups |
| Obesity or fat malabsorption | Vitamin D is fat-soluble and can be sequestered or poorly absorbed |
What to do about it
The reassuring news is that vitamin D deficiency is one of the more fixable nutritional problems. In the right season and latitude, sensible sun exposure maintains healthy levels. Dietary sources — fatty fish, egg yolks, fortified foods — contribute, and supplements are a reliable route when sun and diet fall short, particularly through winter or for anyone in a high-risk group. What you shouldn't do is guess at a confirmed deficiency: if you have persistent symptoms and a high-risk profile, that's a conversation to have with a healthcare professional, who can test and advise on a dose.
How to gauge your own risk
This is exactly what we built SolGlow to help with. Rather than a generic "get some sun" nudge, it estimates the vitamin D you actually produced today by combining the factors that drive deficiency: it reads your Time in Daylight from Apple Watch, computes the sun's angle from your location and the date, factors in your Fitzpatrick skin type and how much skin your typical outfit exposes, checks the day's UV, and adds any vitamin D you log from food and supplements. From that it surfaces a running Deficiency Risk read — a plain-language sense of whether you're topping up or falling behind — plus a burn-risk warning so "more sun" never tips into skin damage.
It won't replace a blood test, and it doesn't pretend to: SolGlow gives an estimate and a risk signal, computed entirely on your device, with the only network call being a UV forecast for your area. But for the very common situation where no one is going to test you, an honest estimate of where you stand is a far better guide than the "15 minutes a day" folklore.
Know where you stand
SolGlow turns your skin type, location, the day's UV, and Apple Watch Time in Daylight into a real estimate of the vitamin D you made today — with a Deficiency Risk read and a burn-risk warning built in. Free on the App Store.
Check your risk with SolGlowFrequently asked questions
What are the symptoms of vitamin D deficiency?
Most often fatigue, bone or muscle aches, muscle weakness, low mood, frequent infections, and slow healing — but all are non-specific, and mild deficiency is frequently symptomless.
How do you know if you're deficient without a blood test?
You can't confirm it, but you can estimate your risk from sun exposure, skin type, latitude, season, age, and diet. A high-risk profile plus vague symptoms is a good reason to ask about testing.
Can sunlight alone fix a deficiency?
Sometimes in summer at lower latitudes — but not through a higher-latitude winter, when there's no usable UVB. Confirmed deficiency is usually corrected with diet and supplements under professional guidance.
Who is most at risk?
People with darker skin, older adults, those who get little sun, higher-latitude populations in winter, pregnant people and infants, and those with obesity or malabsorption.
Related reading: How much sun do you actually need for vitamin D? and how SolGlow estimates vitamin D from sunlight.
This article is for general education and is not medical advice. Vitamin D deficiency can only be confirmed by a blood test, and symptoms have many possible causes. SolGlow provides vitamin D estimates and a risk signal — it is not a medical device and does not diagnose deficiency. Talk to a healthcare professional about testing, symptoms, or supplementation, especially if you're pregnant, older, or managing a health condition.