Now, or close to it. US vitamin D levels peak in August and September and fall from there until February, so starting in early autumn means holding onto a high rather than recovering from a low.
Most people do the opposite. They start in January, when they already feel the effects, by which point their level has been falling for four months.
WHEN DO VITAMIN D LEVELS DROP IN THE FALL?
Immediately after the late-summer peak, and then continuously.
Serum vitamin D in the United States peaks in August and September, roughly eight weeks after the summer solstice, and reaches its lowest point in February. The lag exists because your level reflects accumulated recent exposure rather than today's weather.
That lag is the part worth understanding. The sun has already passed its highest point months before your level peaks, and your level keeps falling for weeks after the sun starts climbing again in spring.
| Month | What is happening to your level |
|---|---|
| August to September | Annual peak |
| October to November | Falling; above 37°N, skin synthesis is ending or has ended |
| December to January | Falling; no meaningful synthesis at higher latitudes |
| February | Annual low |
| March to July | Climbing back |
WHEN SHOULD YOU START TAKING VITAMIN D FOR WINTER?
September, if you want to be at a stable level before the deficit builds.
The reasoning is arithmetic. Circulating 25-hydroxyvitamin D has a half-life of about 15 days according to the NIH Office of Dietary Supplements, and levels reach a new steady state after four or five half-lives. That puts eight to twelve weeks between starting and arriving.
Start in September and you are at a new steady state by late November, which is when synthesis has stopped across most of the country and the drop would otherwise be steepest. Start in January and you reach that same point in March, having spent the whole winter climbing.
Starting in September holds a peak. Starting in January climbs out of a hole.
WHEN DOES YOUR SKIN STOP MAKING VITAMIN D?
It depends on latitude, and for much of the US the answer is earlier than people expect.
Webb, Kline and Holick measured cutaneous synthesis in winter sunlight and found that skin in Boston, at 42.2°N, produced no measurable previtamin D3 from November through February. In Edmonton, at 52°N, the window ran October through March.
Most of the US population lives above 37°N. For a large share of the country, the sun stops being a source somewhere in October or November regardless of how much time you spend outside. We covered whether you can get enough from the sun in more detail, and the same latitude effect is one of the main reasons behind who ends up low in the first place.
WHY DOES STARTING IN JANUARY NOT WORK AS WELL?
Because of where you are starting from, not because supplementation works differently in winter.
By January you are four to five months past your peak with no synthesis replacing what you have used. Starting then still works, and it still takes the same eight to twelve weeks, but you spend those weeks climbing back toward where you were in September rather than staying there.
The other problem is that January starts are usually reactive. Something prompts them, which means the low period has already happened.
DO YOU NEED MORE VITAMIN D IN WINTER THAN SUMMER?
Not a different dose, necessarily. A different reliance.
In summer, sunlight contributes something even if it rarely contributes enough. In winter above 37°N it contributes nothing for months, so the same daily amount is carrying the entire load rather than topping up.
That is also why stopping in summer and restarting in autumn works against you. Taking it through late summer adds to your annual high, and we went through whether you need it in summer separately. Year-round is simpler and steadier than seasonal cycling.
SHOULD YOU TEST BEFORE STARTING?
It is useful if you want to know where you actually stand, and a late-summer test is the most flattering reading you will get all year, which is worth knowing when you interpret it. A serum 25-hydroxyvitamin D test is a standard panel any provider can order.
If you test and it comes back low at your annual peak, that is meaningful information. We covered what to do with a low result separately.
WHAT TO DO ABOUT IT
Pick a daily amount, start now rather than in January, and take it through the winter without gaps.
Consistency is the part that decides whether any of this matters, and it is where most people fall down around week three. D3X is liquid D3 and K2 in organic olive oil, so the fat that drives absorption comes with every dose.* Three ingredients, one dropper a day, into coffee or food or straight.
The window between now and the February low is about five months. The 3-month plan covers the steepest part of it and ships on its own, so staying consistent is not something you have to remember in December.
Sources
Webb AR, Kline L, Holick MF. "Influence of season and latitude on the cutaneous synthesis of vitamin D3." Journal of Clinical Endocrinology and Metabolism, 1988.
Kasahara AK, Singh RJ, Noymer A. "Vitamin D (25OHD) serum seasonality in the United States." PLOS ONE, 2013.
Frequently Asked
When should you start taking vitamin D for winter?
September is the practical answer. Levels reach a new steady state after eight to twelve weeks, so starting in early autumn means arriving at a stable level by late November, when skin synthesis has stopped across most of the US.
When do vitamin D levels peak and bottom out?
In the United States, serum vitamin D peaks in August and September, roughly eight weeks after the summer solstice, and reaches its lowest point in February. Your level reflects accumulated recent exposure rather than current weather, which is why the peak lags the solstice.
When does your skin stop making vitamin D in winter?
It depends on latitude. Webb, Kline and Holick found skin in Boston at 42.2°N produced no measurable previtamin D3 from November through February, and in Edmonton at 52°N that window ran October through March. Most of the US population lives above 37°N.
Is it too late to start taking vitamin D in January?
No, but you start from a lower point. By January you are four to five months past your annual peak with no synthesis replacing what you have used, so the same eight to twelve weeks is spent climbing back rather than holding a high.
Do you need more vitamin D in winter than in summer?
Not necessarily a different dose, but a different reliance. In summer sunlight contributes something even if rarely enough. Above 37°N in winter it contributes nothing for months, so the same daily amount carries the whole load instead of topping up.
Should you stop taking vitamin D in summer?
It works against you. Taking it through late summer adds to your annual high, while stopping and restarting in autumn means beginning from a level that has already been falling for weeks. Year-round intake is steadier than seasonal cycling.
