Why winter sun stops working
Skin makes vitamin D when it absorbs a specific band of ultraviolet light, UVB. In winter the sun sits lower in the sky, and at northern latitudes too little UVB reaches the ground. A 1988 study exposed skin samples to sunlight on clear days and found that in Boston no vitamin D was made from November through February.
New York City sits at about 41°N, just south of Boston. Philadelphia, Chicago and Denver are at similar latitudes. Sunlight through a window does not help either, because glass blocks UVB.
| Location | Latitude | Months with no vitamin D made |
|---|---|---|
| Edmonton, Canada | 52°N | October through March |
| Boston | 42°N | November through February |
| Sites at 34°N and 18°N | 34°N, 18°N | None; production continued in mid-winter |
Food fills only part of the gap
Few foods are naturally rich in vitamin D. According to the NIH, 3 ounces of cooked sockeye salmon provides about 570 IU, and a cup of fortified milk about 120 IU. Reaching 600 IU a day from food alone takes deliberate planning.
That is why winter vitamin D intake often comes down to fortified foods and supplements.
Who tends to run low
The NIH lists groups more likely to fall short: older adults, whose skin makes less vitamin D; people with darker skin; people who spend little time outdoors; people with obesity or who have had gastric bypass surgery; people with conditions that limit fat absorption, such as Crohn's or celiac disease; and breastfed infants.
What the 2024 Endocrine Society guideline says
In 2024 the Endocrine Society reviewed vitamin D for preventing disease in people without a clear medical need for it. For healthy adults under 75, the panel suggested against taking more than the standard intake set by the National Academies, because the trials did not show added benefit.
The panel also suggested against routine blood tests for vitamin D in the groups it studied, including people with darker skin or obesity. For adults 50 and older who do take vitamin D, it favored a smaller daily dose over large weekly or monthly doses.
| Group | Guideline suggestion |
|---|---|
| Healthy adults under 75 | Stay at the standard daily intake (600 IU); higher doses are unlikely to add benefit |
| Adults 75 and older | Vitamin D above the standard intake, which may lower mortality risk |
| Children and teens 1–18 | Vitamin D above the standard intake, to help prevent rickets and may reduce respiratory infections |
| Pregnancy | Vitamin D above the standard intake, which may lower some pregnancy risks |
| High-risk prediabetes | Vitamin D above the standard intake, which may slow progression to diabetes |
How this applies to our 2,000 IU softgel
DECLARED Vitamin D3 provides 50 mcg (2,000 IU) per softgel. That is more than three times the 600 IU adult intake, and half the 4,000 IU upper limit for adults.
Under the 2024 guideline, a healthy adult under 75 is unlikely to benefit from more than 600 IU. Some people take more on a clinician's advice, for example after a low test result. How much is right for you is a decision for you and your clinician, not a label.
Whatever you take, add up every source: standalone vitamin D, multivitamins, calcium-plus-D products and fortified foods all count toward the upper limit. Our guide to vitamin D on the label walks through the math.
Too much is possible
Very high vitamin D raises blood calcium, which can cause nausea, vomiting, muscle weakness, confusion and kidney stones, and in extreme cases kidney failure. According to the NIH, this almost always comes from taking too much in supplements, not from sun or food.
This article explains research findings; it does not diagnose, treat, or prescribe.
Sources
- Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab, 1988
- Demay MB, et al. Vitamin D for the prevention of disease: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 2024
- NIH Office of Dietary Supplements — Vitamin D (Health Professional Fact Sheet)
- NIH Office of Dietary Supplements — Vitamin D (Consumer Fact Sheet)
This material is for general educational purposes only and is not medical advice. Consult a qualified healthcare professional about your individual circumstances.