Growth science · Bone health

Vitamin D, sunlight, and your child's bones

GrowSense Growth Science · Educational, not medical advice

A child can live under a bright, sunny sky — in Bangkok, São Paulo, Lagos, Los Angeles — and still be short on vitamin D. It sounds impossible: if sunlight makes vitamin D, shouldn't children in sunny places have more than enough? The missing detail is exposure — how much ultraviolet-B light actually reaches a child's uncovered skin during an ordinary day.

Between classrooms, cars, malls, glass, covering clothes, heat, sunscreen and haze days, a child can be surrounded by sun and still receive very little of the light that makes vitamin D. This guide follows the real chain — sun to skin to bone — and why a sunny forecast doesn’t guarantee a sufficient child anywhere in the world.

1. What vitamin D does for growing bone

Children grow in length at the growth plates, where cartilage is made, enlarged, and replaced by bone. But building a cartilage scaffold is only half the job — the new tissue must be mineralised with calcium and phosphate. Vitamin D’s role is to raise intestinal absorption of those minerals and help the hormonal system keep their blood levels steady.^[2]

When deficiency becomes severe, calcium and phosphate can fall, parathyroid hormone rises, and the growth plate fails to mineralise — nutritional rickets: widened wrists or ankles, bowed legs or knock-knees, bone pain, muscle weakness, delayed teeth, and in prolonged cases poor growth, with a raised alkaline phosphatase and abnormal calcium/phosphate/PTH.

2. How sunlight actually becomes vitamin D

The chain starts when UVB (mainly ~290–315 nm) hits uncovered skin and converts a cholesterol precursor into previtamin D3, which rearranges into vitamin D3. That isn’t the active form yet — it’s processed in two steps: the liver turns it into 25-hydroxyvitamin D (25(OH)D), and the kidney converts part of that into the active hormone, calcitriol.^[1]

The one number that matters for status is 25(OH)D — the blood test that reflects vitamin D arriving from skin, food and supplements over time. (The active hormone is the wrong test for ordinary deficiency.)

3. Why a food log looks low — and can’t be trusted alone

Few foods naturally carry much vitamin D: fatty fish and fish-liver oils, egg yolk, a little in liver, and UV-exposed mushrooms. Fortified products — milk, formula, some plant drinks, cereals, yoghurt, margarine — add more, but fortification varies wildly by country, brand and serving. A milk that’s fortified in one country may contain none in another.

For ages 1–18 the reference intake is around 600 IU (15 µg) a day assuming little sun (the AAP has long urged at least 400 IU, especially in infancy).^[13] A child who eats eggs, plain milk and yoghurt may log only a fraction of that unless the products are fortified or fish is regular — which looks alarming but often just means the sun is meant to supply the rest. That’s exactly why a food-only dashboard reads low, and why it can’t diagnose deficiency. The supplement side is covered in Growth supplements that matter.

4. Why sunny cities still fall short — the same biology, different photons

The pathway is identical in Thailand, the Netherlands, Korea or Australia — the same skin precursor, liver step and kidney activation. What differs is how many UVB photons reach the skin, and how many get absorbed.

• Latitude and season. At higher latitudes the winter sun sits low; UVB travels through more atmosphere and can be too weak for efficient production for months.^[7] Near the equator UVB is steadier year-round — but that edge is easily lost to indoor living, clothing, shade and haze.
• Skin pigmentation. Melanin absorbs UV (a protective adaptation), so more deeply pigmented skin generally needs more exposure for a similar vitamin D response.^[5][6] When deeply pigmented children live in low-UV or mostly-indoor settings, deficiency risk rises.
• Clothing and indoor life. Two children on the same street can get wildly different effective doses if one plays outside in shorts and the other moves between air-conditioned rooms fully covered.
• Air pollution and urban form. Tall buildings, aerosols and particulates cut ground-level UVB, and city life cuts outdoor time on top of that.^[11]

The same story repeats worldwide through different mechanisms:

These aren’t a ranking — they’re different routes to the same outcome: a sunny sky doesn’t guarantee a vitamin-D-sufficient child anywhere.

5. Why you can’t turn “minutes of sun” into a dose

It’s tempting to want “10 minutes outside = X IU.” It doesn’t work that way. Researchers can estimate the vitamin D equivalence of controlled UV exposure, but real sunlight isn’t a fixed dose — production can vary several-fold between two children outside for the same time.^[4] It depends on UV intensity, time of day, latitude, season, cloud, pollution, skin tone, clothing, exposed skin area, sunscreen, age — and whether they were behind glass.

And no — a full day at the beach isn’t a vitamin D strategy. Skin self-limits (excess previtamin D breaks down), so sun rarely causes vitamin D toxicity — but the UV damage keeps accruing after production plateaus. WHO advises sun protection once the UV index reaches 3+: shade around solar noon, hats, clothing, sunglasses, broad-spectrum sunscreen, no intentional tanning, and no sunburn — childhood UV damage adds to lifetime skin-cancer risk.^[12] A child can be outdoors and active and protected.

6. PM2.5 haze: protect the lungs first

Pollution can lower vitamin D two ways. Atmospherically, particulates scatter and absorb UVB; observational reviews link higher pollution with lower 25(OH)D, though it can’t be turned into an exact per-child reduction.^[11] Behaviourally — and this is the bigger everyday effect — haze keeps children in: cancelled sport, closed windows, indoor weekends.

7. The blood test, briefly

Status is measured as 25(OH)D, in ng/mL or nmol/L (1 ng/mL = 2.5 nmol/L). There’s genuine debate about ideal cut-offs, but broadly: below ~12 ng/mL (30 nmol/L) is deficient; 12–20 (30–50) is low/insufficient in many systems; ≥20 ng/mL (50 nmol/L) is adequate for most healthy children. Targets differ for rickets, malabsorption, kidney disease, obesity or certain medications, and interpretation depends on the lab and clinical picture. How this number is read alongside calcium, phosphate and PTH is covered in Your child’s growth blood tests, explained.

8. Does more make a child taller? And can supplements be excessive?

No, and yes. Once a child is sufficient, extra vitamin D doesn’t keep driving the growth plate — supplementation trials in otherwise healthy children generally show no reliable height gain from raising levels above deficiency. Correcting a real deficiency can restore mineralisation and growth if the deficiency was severe; that’s the honest claim — it protects normal growth, it doesn’t enlarge genetic potential.

And supplements can overshoot. Sun rarely causes toxicity, but high-dose products can, through excess calcium (hypercalcaemia): nausea, vomiting, constipation, thirst, frequent urination, kidney stones or injury. NIH tolerable upper limits for routine intake run from 1,000 IU (infants) up to 4,000 IU (ages 9–18).^[13] The sneaky risk is stacking — vitamin D hiding in a multivitamin and a calcium supplement and cod-liver oil and fortified milk and a growth supplement at once. Check the labels. (More on this in Growth supplements that matter.)

9. A practical parent framework

• Food: check labels for fortification — don’t assume all milk or yoghurt contains vitamin D.
• Outdoor time: record genuine outdoor exposure, not time behind glass.
• UV safety: don’t chase a tan or allow sunburn; use the UV index and protect during strong sun.
• Air quality: on high-PM2.5 days, respiratory safety wins — don’t send a child out for vitamin D into hazardous air.
• Supplements: an age-appropriate dose when recommended; avoid high-dose treatment without clinical advice; watch for stacking.
• Testing: consider discussing a 25(OH)D test if there are real risk factors — symptoms, bone signs, malabsorption, chronic disease, deep pigmentation with low exposure, or prolonged indoor life.

10. The questions parents actually ask

How this connects to the whole system

Vitamin D never works alone — it sits inside a bone-health system with calcium, phosphate, protein, weight-bearing activity, sleep, and (behind all of it) how much safe UVB a child’s life actually allows. That’s why no single metric — a food log, a sunny forecast, one 25(OH)D value — tells the story on its own; the signal is in how they combine, read against real exposure and, when it matters, a blood test.

The parent takeaway

Sunlight makes most of a child’s vitamin D, but a sunny sky guarantees nothing — from Bangkok to London to Los Angeles, indoor life, glass, clothing, skin tone, latitude and haze all cut the UVB that actually reaches skin. Vitamin D hardens the bone the growth plate builds; correcting a real deficiency protects growth, but extra doesn’t add height. You can’t convert minutes of sun into a dose, a window doesn’t count, and on polluted days lungs come before sunlight.

So the useful question is never “is it sunny where we live?”

It’s “how much safe UVB actually reaches my child’s skin — and if that’s little, are food, fortification or a clinician’s advice filling the gap?”

This article is educational and does not diagnose vitamin D deficiency or interpret your child's results. Only a blood test read by a clinician can assess vitamin D status, and thresholds and treatment depend on the child's full clinical picture. Do not start high-dose vitamin D without medical advice, and never increase a child's sun exposure on days when the air is unhealthy. If you're concerned about your child's bones or growth, speak with a paediatrician.