Fatigue that doesn't go away, getting sick more often than usual, bone or joint pain you can't quite explain: these are some of the most common signs of a vitamin D deficiency, and they're easy to mistake for something else entirely, stress, poor sleep, just getting older.
Part of what makes it so easy to miss is that vitamin D breaks the usual rule that food is where your nutrients come from. Your skin makes it on its own, using nothing but sunlight, which means the "right" amount to get from food isn't a fixed number the way it is for most other nutrients. It depends on where you live, what season it is, and how much of your skin actually sees the sun.
That's why vitamin D is one of the most commonly low nutrients around, even in places that get plenty of sunshine. It's one of the 8 micronutrients Fobalanz tracks alongside your macros and calorie target; for the full picture of all 14, see the 14 nutrients that actually matter.
Signs you might be running low
Mild vitamin D deficiency often doesn't announce itself clearly. The more established effects show up over time: bone pain and muscle weakness in adults (a condition called osteomalacia), and in children, softened, weakened bones known as rickets. Along the way, people often report general fatigue, getting sick more frequently, or bone and joint discomfort they can't quite explain.
Certain groups are especially likely to run low: older adults, people who cover most of their skin or spend most of their time indoors, people with darker skin, and anyone with a condition that affects fat absorption, like celiac or Crohn's disease, since vitamin D needs dietary fat to be absorbed properly.
Fatigue in particular is a symptom that overlaps with several other nutrient gaps, iron included. If you're tired more often than seems normal, it's worth reading up on the signs you might be low on iron, or the most common silent deficiencies more broadly, since these get mixed up more often than you'd think.
What vitamin D actually does
Vitamin D's best known job is helping your body absorb calcium, which is why it's so closely tied to bone strength. Without enough of it, your bones don't get the calcium they need no matter how much calcium you're eating, which is part of why the two nutrients are almost always discussed together.
That's not all it does, though. Vitamin D supports your immune system's ability to fight off bacteria and viruses, helps your muscles move properly, and plays a role in how your nerves carry signals between your brain and the rest of your body. Some research has found a link between low vitamin D levels and mood, though the evidence on whether taking a supplement actually improves mood is mixed, so it's worth being cautious about that particular claim. The bone and immune effects are much better established.
Why it varies so much from person to person
Two people can match on paper, same age, same diet, same weight, and still need completely different amounts of vitamin D from food. Here's what actually moves the number:
- Where you live. The closer you are to the equator, the more direct UVB exposure you get year-round, which is what your skin needs to produce vitamin D. Higher latitudes get far less usable sunlight, especially outside of summer.
- The season. In many northern regions, the sun sits too low in the sky for several months to trigger meaningful vitamin D production at all, regardless of how much time you spend outside.
- Skin tone. More melanin means more natural protection from UV radiation, which also means it takes longer for the skin to produce the same amount of vitamin D. People with darker skin generally need more sun exposure to make an equivalent amount.
- Time spent outdoors, and how covered up you are. Someone who works outside gets a very different amount of exposure than someone who commutes by car and spends the day indoors. Clothing, sunscreen, and cultural or personal coverage choices all reduce how much skin is actually exposed.
- Age. Skin becomes less efficient at producing vitamin D as you get older, which is part of why recommended intake goes up after 70.
- Glass and clouds. Vitamin D production doesn't happen through a window, since window glass blocks UVB rays. Heavy cloud cover and pollution cut it down too.
Hand two people who differ on any of these a single generic vitamin D target, and you've almost certainly gotten it wrong for at least one of them. This is exactly why Fobalanz asks about sun exposure when building your profile instead of assuming everyone's skin is doing the same amount of work.
How much vitamin D do you actually need
Recommended amounts are given in micrograms (mcg) and International Units (IU), and they rise with age:
These are the official intake guidelines, and they assume little to no sun exposure. Get regular sunlight and your actual dietary need drops. Get almost none and it's worth paying closer attention to food and, if needed, talking to a doctor about supplementing.
The upper limit for adults is 100 mcg (4,000 IU) a day from all sources combined: food, supplements, and fortified products. You cannot get too much vitamin D from sunlight alone. Your skin caps its own production, which is one more reason sunlight and food behave so differently as sources.
Where to actually get it from food
Very few foods contain vitamin D naturally, which is part of why deficiency is so common. The short list that actually matters:
Fortified milk and plant-based alternatives (soy, oat, almond) are where most people in the US actually get the bulk of their dietary vitamin D. Cheese and ice cream, despite being dairy, usually aren't fortified, so don't count on them the way you would fortified milk.
How to actually stay on top of it
Get some safe sun exposure when you can. A modest amount of midday sun on bare arms and legs a few times a week helps, without needing to skip sunscreen for extended periods or risk sunburn to get there. Skin cancer risk is real, so this is about balance, not maximizing exposure.
Lean on the food sources that actually count. Fatty fish a couple of times a week, fortified milk or a fortified plant alternative, and eggs all add up meaningfully over a week even if no single meal looks impressive on its own.
Adjust your target to your actual life, not a generic average. Someone who works outdoors in summer and someone stuck at a desk through a gray winter are not the same case, even if every other stat about them matches. A number built for the average doesn't reflect either of them accurately.
The bottom line
Vitamin D breaks the usual rule that food is where your nutrients come from. Your skin does part of the job for you, which means the right target isn't one number for everyone, it's a number that depends on your actual life: where you live, what season it is, and how much sun you really get. Get that context wrong and even a carefully tracked diet can leave you running low.
Try Fobalanz free and get a vitamin D target that's actually built around your life, not a generic average.
This article is general information, not medical advice. If you're considering a vitamin D supplement, or think you might be deficient, check with a healthcare professional before making changes.
Recommended intakes and sources are based on the NIH Office of Dietary Supplements Vitamin D Fact Sheet and U.S. Dietary Guidelines for Americans; Fobalanz's own baseline is personalized from there.
