Indian studies have found low vitamin D levels in a majority of the people they tested, across cities, age groups and income levels. For a country with this much sun, that surprises people every time. The reason is that vitamin D is not made from sunlight in general. It is made from a narrow band of ultraviolet light — UVB — reaching bare skin, and most of the way modern Indians live keeps UVB away from skin.
Why the sun is not enough
- UVB does not pass through window glass. Sun on your arm in a car, an office or a balcony behind glass produces no vitamin D.
- UVB is strongest around midday. Early-morning and late-evening sun, which is when most people are outdoors, carries little of it.
- Air pollution and haze absorb UVB before it reaches the ground, which is one reason urban levels tend to be lower than rural ones.
- Clothing that covers the arms and legs, and sunscreen, both block it — for good reasons of their own.
- Melanin, the pigment in darker skin, is a natural UVB filter. The same exposure produces less vitamin D in darker skin than in lighter skin.
- Very few Indian foods contain meaningful vitamin D. Fortified milk and oils help, but a typical vegetarian diet provides little.
What the blood test measures
The test your doctor orders is 25-hydroxyvitamin D, written 25(OH)D, reported in ng/mL in most Indian labs (some use nmol/L; multiply ng/mL by 2.5 to convert). It reflects your stores over the previous few weeks. The complication is that there is no single agreed cut-off. Two widely used sets of ranges:
| 25(OH)D, ng/mL | US Institute of Medicine (2011) | Endocrine Society (2011) |
|---|---|---|
| Below 12 | Deficient | Deficient |
| 12 – 20 | Inadequate for some people | Deficient |
| 20 – 30 | Sufficient for nearly everyone | Insufficient |
| 30 and above | Sufficient | Sufficient |
So a reading of 24 ng/mL is 'sufficient' by one guideline and 'insufficient' by the other. Your lab report will print one set of ranges; it is worth knowing which. What both agree on is that below 12 ng/mL is genuinely deficient, and that levels above about 100 ng/mL are too high — vitamin D can be overdone, which is the strongest argument for testing before taking large doses.
Maintenance dose versus correction dose
ICMR-NIN's 2020 recommendation for adults is 600 IU of vitamin D a day, with the note that it assumes limited sun exposure. That is a maintenance figure: enough to hold a normal level steady. It is a very different thing from the 60,000 IU once-a-week course that doctors commonly prescribe for a few weeks to bring a low level up. Both are legitimate; they answer different questions. A daily 400–600 IU product is for keeping a level. A weekly 60,000 IU product is for correcting one, and it should follow a test, with a dose and duration set by the doctor.
D3 or D2, and why the meal matters
Cholecalciferol (D3) is the form the skin makes and the one most supplements use; ergocalciferol (D2) comes from plant and fungal sources. At the same dose, D3 raises and holds blood levels somewhat better. Either way, vitamin D is fat-soluble: it is absorbed along with dietary fat, which is why the standard advice is to take it with a meal rather than on an empty stomach, and why the form of the product — an oil-based solution versus a dry granule dissolved in water — affects how much of the dose gets in.
Who should test
Anyone who spends the working day indoors, anyone with persistent tiredness or muscle aches without another explanation, pregnant and breastfeeding women, older adults, and anyone about to start a high-dose course. The test is inexpensive and available at every diagnostic chain. Testing once, correcting if needed, and then maintaining is a far better plan than taking high doses indefinitely on the assumption that you must be low.
Sources
- 1.Nutrient Requirements for Indians, ICMR-NIN, 2020 — vitamin D allowance of 600 IU/day for adults
- 2.Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press, 2011
- 3.Holick MF et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab, 2011
General nutrition information, not medical advice. Nutraceuticals are not intended to diagnose, treat, cure or prevent any disease. Speak to your doctor before starting a supplement if you are pregnant, breastfeeding, on medication or managing a condition.