Here in the northern hemisphere, winter famously contributes to widespread vitamin D deficiency as sunlight exposure decreases. The trend is “very marked in clinical practice,” Mary Gover, MD, an internal medicine doctor at Montefiore Einstein Advanced Care in New York City, tells SELF.
What you might not know, however, is that vitamin D isn’t the only critical nutrient that takes a dive this time of year: Calcium can also be impacted, according to Dr. Gover, and a drop in the bone-building mineral can potentially cause issues as well.
Because the full explanation is a little complicated, we’ll break it down step-by-step below. Here’s why wintertime calcium deficiency can happen, who is most likely to be affected, and what countermeasures you can take.
Why calcium levels can drop in the winter
Vitamin D plays an important role in regulating a number of essential minerals in your body, including magnesium, phosphorus—and calcium. Namely, “you need appropriate vitamin D levels to absorb calcium from your diet,” Dr. Gover says. Without it, you wouldn’t absorb enough in the gut—only around 10% to 15% of your calcium intake, compared to 30% or 40% otherwise.
Because low vitamin D is more common in the winter, low calcium is more likely in turn. Multiple studies have found that blood levels of 25(OH)D, the primary indicator of vitamin D status, are down in winter compared to other seasons. Studies have also found that calcium absorption declines as well, though the difference isn’t always statistically significant.
However, Dr. Gover has some words of reassurance: “The vast majority of the time when someone has low vitamin D, they don’t have low calcium as well,” she says. “You have to have a very low vitamin D level to have that downstream effect.”
Calcium deficiency, a.k.a. hypocalcemia, is defined as having blood levels of the mineral below 8.5 milligrams per deciliter (2.12 nanomoles per liter). While low vitamin D is a major cause, it’s not the only one: Other potential triggers include pancreatitis, kidney dysfunction, parathyroid issues, and certain medications (like the antibiotic rifampin, the antiseizure drugs phenytoin and phenobarbital, and steroids).
When to look out for calcium deficiency
So how low does your vitamin D have to be to qualify as “very low”? To pinpoint the vitamin D level at which calcium issues can arise, we need to explain how much vitamin D you need in the first place.