IN BRIEF
Sunlight has to reach the skin.
Visible daylight is not enough. Vitamin D production depends on UVB reaching exposed skin directly.[1]
The opportunity changes through the day and year.
Useful UVB varies with the hour, season, latitude, weather and the amount of skin exposed, and across much of central and northern Europe it may become too limited for cutaneous Vitamin D production during the winter.[1][3]
Modern life can miss the useful window.
The hours when UVB is most effective often fall in the middle of the working day, while much of that time is spent indoors.[3]
Summer does not carry indefinitely into winter.
Vitamin D status tends to rise through the brighter months and fall again as exposure declines.[1][3]
Think across the whole year.
Sunlight makes a seasonal contribution. Oral vitamin D can remain part of a daily practice throughout the year.[5]
THE INDOOR DAY
Modern life is lived remarkably close to daylight, and often at some distance from the sun.
We wake indoors, commute to work and spend much of the day inside. A window can fill a room with light. The weather may be beautiful. Yet very little direct sunlight needs to reach the skin for the day to feel bright.
Vitamin D makes that distinction unusually clear.
Its production begins when ultraviolet B radiation, or UVB, reaches the skin and converts 7-dehydrocholesterol into previtamin D₃, the first step towards the Vitamin D₃ the body produces for itself. [1]
It is an old relationship between the body and its environment.
The relationship remains. The structure of the day around it has changed.
THE HOURS OF OPPORTUNITY
Sunlight is not biologically equivalent from morning to evening.
The amount of UVB reaching the earth changes with the height of the sun. As the sun rises, its rays travel through less atmosphere and more UVB reaches the surface. The opportunity for cutaneous Vitamin D production tends to be greatest around solar noon, then diminishes again as the sun falls. Latitude and season change the width of that opportunity considerably. [1][2]
The timing is significant because, for many people, it coincides with the working day.
A commute may give us time outside in the morning and again later in the afternoon. Between the two lies a long period indoors, which can include the part of the day when Vitamin-D-effective UVB is most available.
This does not mean that everyone who works indoors has low Vitamin D status. The biology is far more individual than that. Age, skin pigmentation, clothing, body composition, the amount of skin exposed, diet, latitude and season all matter. [2]
Working environment is one factor among them, but it is a measurable one.
Feldman and Pike reports that people who work outdoors generally have higher 25-hydroxyvitamin D, or 25(OH)D, concentrations than people who work indoors. In one Israeli study, outdoor workers received an average 4.4 hours of daily solar UVB exposure compared with 0.9 hours among indoor workers. A study in Delhi found the same progression: Vitamin D status was highest in outdoor workers, lower in those dividing their time between inside and outside, and lowest in indoor workers. [2]
The significance is straightforward. Sunlight can only contribute to Vitamin D status when the relevant UVB actually reaches the skin.
BEHIND GLASS
Glass gives modern indoor life one of its more convincing illusions.
Visible sunlight passes through it easily. UVB does not.
Feldman and Pike states that glass and Plexiglas absorb most, if not all, of the UVB photons required for cutaneous Vitamin D production. Skin exposed to sunlight through ordinary glass does not produce Vitamin D₃. [1]
A desk beside a window can therefore provide hours of daylight without providing the skin with the radiation required for this particular process.
The distinction is subtle because our eyes tell us we have been in the light.
Our skin has a different measure.
This is one reason a sunny season cannot be understood simply from the weather outside. What matters is how much of the available opportunity becomes exposure.
THE SUMMER ASSUMPTION
Summer is when the opportunity is greatest.
The sun sits higher, the useful UVB period broadens and clothing often leaves more skin uncovered. Across temperate regions, circulating 25(OH)D generally rises through the brighter part of the year and declines again as winter approaches. [1][2]
It is easy to turn this into a reassuring shorthand: summer takes care of Vitamin D.
Sometimes it may. But the season alone cannot tell us what happened at the skin.
Someone who works outdoors and someone who spends the same summer inside experience the same latitude and the same position of the sun. Their exposure can be quite different.
This is already visible in the occupational data. Feldman and Pike treats work environment, recreational time outdoors and clothing alongside season and latitude as determinants of Vitamin D status. [2]
The same point can be made more simply.
A sunny day is an opportunity, not a dose.
WHAT SUMMER LEAVES BEHIND
The Vitamin D produced during periods of greater sunlight does carry forward.
Vitamin D and its metabolites are distributed through the body, and 25(OH)D declines gradually rather than disappearing when the season changes. Pharmacokinetic studies reviewed in Feldman and Pike suggest a whole-body half-life for 25(OH)D of roughly two to three months, although the range between individuals and study conditions is considerable. [3]
Summer therefore matters to autumn.
What it does not provide is a fixed reserve that remains untouched until spring.
Vitamin D continues to be metabolised and cleared as the year progresses. If cutaneous production falls, the balance changes with it.
Feldman and Pike also draws an interesting distinction between older patterns of daily life and the modern pattern of concentrated sun exposure. Until relatively recently in human history, much more of everyday activity took place outdoors, allowing UV exposure to rise and fall gradually with the seasons. The book contrasts this with the “vacation model” common among indoor workers today: long periods inside interrupted by concentrated episodes of sun. [4]
The distinction is useful because our idea of having “had plenty of sun” is often built around a holiday.
Biologically, a holiday and a pattern of regular exposure are not the same thing.
WHEN WINTER ARRIVES
Winter adds a different problem.
During the brighter months, useful UVB may be available while we are inside. At higher northern latitudes in winter, the environmental opportunity itself can become very small or disappear for a period.
Feldman and Pike describes solar zenith angle as the most important environmental determinant of Vitamin D production from sunlight. As the sun sits lower, UVB travels through more atmosphere and less reaches the earth's surface. Modelling reviewed in Chapter 56 indicates that, around the end of the year, cutaneous Vitamin D production is not possible at latitudes above roughly 46°N under the conditions modelled, with the period of limitation extending farther into the year at higher latitudes. [2]
Circulating Vitamin D status follows this seasonal change. Studies across both hemispheres consistently show lower 25(OH)D concentrations during winter than summer. [2][5]
Winter is therefore the most obvious point in the cycle.
But it is not where the story begins.
The level from which we enter winter has already been shaped by the months before it: by season, latitude, time outdoors, skin exposure, diet and, where used, supplementation.
WHY IT MATTERS
The clearest consequences of severe Vitamin D deficiency remain skeletal.
Vitamin D has an established role in calcium and phosphate metabolism and normal bone mineralisation. Severe deficiency can cause rickets in children and osteomalacia in adults. [6]
The biology extends beyond bone, and Feldman and Pike documents an extensive field of research into immune, metabolic and other functions. The interpretation is less simple. Low Vitamin D status is associated with numerous diseases, but association does not establish that low Vitamin D caused them, or that increasing Vitamin D intake will prevent them.
The same care is needed around blood levels. Feldman and Pike devotes separate chapters to the debate over thresholds because there is no single 25(OH)D concentration established as optimal for every person and every health outcome. [6][7]
Certainty is strongest around avoiding deficiency and maintaining adequate Vitamin D status.
The question then becomes one of source.
CONTINUITY
Sunlight is a natural source of Vitamin D, but it is inherently variable.
The amount available changes with latitude, season and hour. The amount we receive changes with work, clothing, weather, time outdoors and the amount of skin exposed.
Diet contributes as well, although naturally rich sources are relatively limited. Fish, eggs and certain other animal foods contain Vitamin D, and fortified foods make an important contribution in populations where they are widely used. [8]
Supplementation introduces another kind of source.
It is independent of the height of the sun and of whether the working day allowed enough useful UVB to reach the skin.
Feldman and Pike's chapter on supplementation treats this distinction explicitly. For maintenance of Vitamin D status, it favours daily supplementation over intermittent bolus dosing. The chapter separates maintenance from the quite different clinical objective of correcting an established deficiency and notes that the parent Vitamin D molecule has a considerably shorter circulating lifetime than 25(OH)D. [9]
This gives daily intake a particular logic in the context of modern indoor life.
The natural source changes from day to day and season to season.
A daily oral source does not.
There is no need to make the two compete. Sunlight belongs to one part of Vitamin D physiology; diet and supplementation belong to another.
Together they make a more complete picture of how Vitamin D status is maintained across the year.
THE INDOOR DAY
Perhaps the most useful change is simply to look at Vitamin D through the structure of an ordinary day.
When are we outside?
When is useful UVB available?
How much skin does it reach?
Those questions are more revealing than whether the forecast says sunny.
Modern life has altered the answer without altering the underlying biology. We still make Vitamin D when the conditions are right. What has become less reliable is how often those conditions come together.
Winter makes the limitation visible because the light itself changes.
The rest of the year is quieter. The opportunity may be outside while the working day continues indoors.
Seen this way, Vitamin D is not principally a winter subject.
It is a year-round relationship between sunlight, skin and the way we now live.
And continuity, rather than season alone, becomes the more useful way to think about it.
DISCLAIMER
This article is for educational and informational purposes only. It does not constitute medical advice or promote any specific product. Individual Vitamin D requirements vary. Readers with questions about their Vitamin D intake, status or health should consult a qualified healthcare professional.
REFERENCES
[1] Holick MF, Slominski AT. Photobiology of Vitamin D. In: Hewison M, Bouillon R, Giovannucci E, Goltzman D, Meyer M, Welsh J, eds. Feldman and Pike's Vitamin D. 5th ed. Volume One. Academic Press; 2024. Chapter 3.
[2] Grant WB, Bhattoa HP, Pludowski P. Determinants of Vitamin D levels from sun exposure: a global perspective. In: Hewison M, Bouillon R, Giovannucci E, Goltzman D, Meyer M, Welsh J, eds. Feldman and Pike's Vitamin D. 5th ed. Volume Two. Academic Press; 2024. Chapter 56.
[3] Schoenmakers I, Jones KS. Pharmacology and pharmacokinetics of Vitamin D. In: Hewison M, Bouillon R, Giovannucci E, Goltzman D, Meyer M, Welsh J, eds. Feldman and Pike's Vitamin D. 5th ed. Volume One. Academic Press; 2024. Chapter 31.
[4] Jablonski NG. Evolution of human skin pigmentation and Vitamin D. In: Hewison M, Bouillon R, Giovannucci E, Goltzman D, Meyer M, Welsh J, eds. Feldman and Pike's Vitamin D. 5th ed. Volume One. Academic Press; 2024. Chapter 2.
[5] Ebeling PR. Vitamin D and osteoporosis. In: Hewison M, Bouillon R, Giovannucci E, Goltzman D, Meyer M, Welsh J, eds. Feldman and Pike's Vitamin D. 5th ed. Volume Two. Academic Press; 2024. Chapter 70.
[6] Darling AL, Lanham-New SA. Defining thresholds for Vitamin D I: scientific rationale for serum 25-hydroxyvitamin D cutoffs of 25 and 50 nmol/L. In: Feldman and Pike's Vitamin D. 5th ed. Volume Two. Academic Press; 2024. Chapter 51.
[7] Vieth R. Defining thresholds for Vitamin D II. In: Feldman and Pike's Vitamin D. 5th ed. Volume Two. Academic Press; 2024. Chapter 52.
[8] Jakobsen J, Jensen MB. Vitamin D in food: compounds, stability, sources. In: Feldman and Pike's Vitamin D. 5th ed. Volume Two. Academic Press; 2024. Chapter 55.
[9] Whiting SJ, Calvo MS. Vitamin D supplement use as a public health strategy to augment diet and sustain population adequacy. In: Hewison M, Bouillon R, Giovannucci E, Goltzman D, Meyer M, Welsh J, eds. Feldman and Pike's Vitamin D. 5th ed. Volume Two. Academic Press; 2024. Chapter 57.
