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.
Current guidelines recommend that most adults shoot for around 15 micrograms (600 IU) of vitamin D per day. (People ages 70 and over, the sole outliers, have an alternative target of 20 micrograms per day). Vitamin D levels of 50 nmol/L, or 20 nanograms per microliter (ng/mL), or higher are “generally considered adequate for bone and overall health in healthy individuals,” all the way up to 125 nmol/L (50 ng/mL), according to the National Institutes of Health’s Office of Dietary Supplements (ODS). Meanwhile, levels of less than 50 nmol/L (20 ng/mL) are considered inadequate, and levels of less than 30 nmol/L (12 ng/mL) are considered deficient.
Under these standards (which are disputed by some practitioners), low calcium stemming from low vitamin D could start to become a problem once your vitamin D falls below that 50 nmol/L mark, according to Dr. Gover. Because that amounts to an inadequacy, “it can certainly lead to impaired absorption of calcium,” she says.
With that said, Dr. Gover thinks that significant changes are more likely to appear around 30 nmol/L, since that’s toward “the extreme end” of low vitamin D. While low calcium can be asymptomatic, especially when mild or chronic, according to the National Institutes of Health, “you might even see some symptoms, including bone pain or muscle cramps,” she says.
For context, low vitamin D is hardly rare. By some estimates, around two thirds of the US population is low on vitamin D to some degree, though certain demographics (like Black people and young people) are more affected than others. Dr. Gover herself reports she sees people measuring lower than 50 nmol/L “not infrequently” in the winter. In a 2022 Frontiers in Nutrition study that examined data spanning 2001 to 2018 from the CDC’s National Health and Nutrition Examination Surveys, researchers found that 2.6% and 22% of Americans older than one year had vitamin D levels below 25 nmol/L and between 25 and 50 nmol/L, respectively.
What you can do to raise your calcium levels
Protecting your vitamin D and calcium levels is paramount for one demographic in particular: older women (say, those over 50), who are susceptible to bone loss and osteoporosis due to the hormonal changes that take place during menopause. Because of their heightened risk for reduced bone density, “you just want to make sure that you’re optimizing them for bone health by giving them repletion” in the form of supplements as necessary, Dr. Gover says. In a 2009 study of 55 healthy adults published in the Journal of Bone and Mineral Research, researchers found that supplementing with both vitamin D and calcium for two years prevented wintertime bone turnover and bone loss. Prior to that, a 1998 study of older postmenopausal women published in The Journal of Clinical Endocrinology & Metabolism found that 1,000 milligrams per day of calcium supplements staved off wintertime bone loss, even without added vitamin D.
If you’re concerned about your vitamin D and calcium levels, you can ask your doctor for a blood test to measure them. “I check vitamin D routinely as part of the annual physical, and I supplement when it’s low,” Dr. Gover says. “And I do see people feeling better in terms of aches and in terms of mental health.”
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