A silent epidemic is going around India, from its farms to its high rises, even though the solution is just on the other side of the door: vitamin D deficiency.

“Vitamin D deficiency in India has reached epidemic proportions,” says Dr. Prateek Kumar Gupta, senior consultant, orthopedics, at Sir Ganga Ram Hospital in New Delhi. “Almost every third person that gets tested for vitamin D levels in my practice comes back with a positive for deficiency.”

“Epidemic proportions” is a phrase that often gets tossed around carelessly, but if anything, his estimate falls short: According to this 2014 report, 79% of Indians have vitamin D deficiency. To put it in perspective, India’s prevalence of vitamin D deficiency is as bad (if not worse) than the UK’s — a country known for its cloud cover as much as India is known for its sun.

A quick refresher on the sunshine vitamin’s role in your body: Your body produces vitamin D when bare skin is exposed to the sun’s ultraviolet B rays. You need it to help your body absorb calcium, which keeps your bones and teeth strong. No amount of calcium in your diet will help you unless you also have enough vitamin D.

But herein lies the problems for Indians; the average Indian skin tone has a high melanin content, which acts a little like an inbuilt suncreen protecting the skin from all of the sun’s rays — including the UVB rays needed to produce vitamin D. That’s why vitamin D deficiency in India crosses class lines, though the trappings of modern living that so often keep us out of the sun exacerbate these natural effects further for the middle and upper classes.

Vitamin D can be consumed in food, though this method of intake can’t compete with sunlight as a source. And vegetarians are particularly vulnerable, as all foods with naturally occurring vitamin D are non-veg.

Vitamin D deficiency in India is the most under-diagnosed and under-treated nutritional deficiency in the world. Unfortunately, it’s also a serious one: As if a calcium deficiency resulting from a vitamin D deficiency isn’t serious enough (in the long term, it can lead to osteoporosis), there are also a growing number of studies linking low levels of vitamin D to cancer, diabetes, cardiovascular diseases, tuberculosis, and multiple sclerosis.

Vitamin D deficiency in India can take root from an early age. While vitamin D drops are prescribed for newborns as a matter of course (their skin being too sensitive for sun exposure in the first few months) in other countries, here, this isn’t part of protocol, Dr. Gupta says. He suggests new parents consult their baby’s pediatrician about whether a supplement is necessary from birth.

But there is good news.

“Unless there’s a family history, or a severe deficiency that could result in rickets or osteomalacia, vitamin D deficiency doesn’t manifest easily in children,” says Dr. Gupta. “It usually becomes more apparent in teenagers and adolescents.”

This means there’s time for correction in the form of daily sun exposure and, to some extent, diet. Dr. Gupta advises at least 30 to 45 minutes in the sun every day, ideally in the morning, when the rays are not too harsh. He also suggests a 10-minute break in the sun around midday.

Dr. Gupta also advises being on the watch for common symptoms of vitamin D deficiency, which are subtle and easily mistaken for other problems: body aches, fatigue and lethargy. Or, a trivial injury that results in a fracture or minor crack in the bone, could be a tip-off of a calcium deficiency (caused by a vitamin D deficiency). A slow-to-heal wound, too, could be an indication.

If a deficiency is diagnosed it’s typically through a 25-hydroxy vitamin D blood test; more than 30 nanograms/milliliter is optimal, 20 to 29 is insufficient and less than 20 is deficient. Vitamin D deficiency treatment involves prescription supplements or specific foods rich in vitamin D, such as oily and fatty fish, beef, pork, egg yolks and, where available, fortified milk and milk products.