How Much Vitamin D Do You Really Need?

How much vitamin D do you need to ensure strong, healthy bones? Unfortunately, there isn’t an easy answer.

“It’s incredibly controversial,” says Neil Binkley, MD, a UW Health geriatrician and internationally recognized researcher who leads the UW Osteoporosis Clinical Research Program. “The experts in the field are quite polarized. I try to be somewhere in the middle because I really believe that we simply don’t know.”

Why all the controversy about “the sunshine vitamin”? Doctors determine whether an individual’s vitamin D level is normal, insufficient or deficient by measuring serum 25-hydroxyvitamin D in the blood. But the measurement has historically been problematic and difficult to standardize, and experts don’t agree on the optimal level.

Another problem: Randomized trials of vitamin D supplementation haven’t required that participants start with low levels, so it’s difficult to draw clear conclusions from such research. Additionally, there is substantial variation in 25-hydroxyvitamin D increase after individuals begin vitamin D supplementation.

A UW Health clinical trial is studying this individual variability. “If I give you 2,000 IU of vitamin D a day, your level may go up a little, it may go up a lot, or it may go up by something in between,” Binkley explains. “It all depends on how well you absorb and how rapidly you use up vitamin D. You may not absorb vitamin D as well as Jane Doe Average, and therefore you may need a bigger dose, or you may use it up more rapidly, and thus also need more.”

Here’s what we do know about this controversial vitamin:

It’s vital for health. “It’s important for calcium absorption and therefore bone strength, and perhaps for optimal skeletal muscle function as well,” Binkley says. “The reason why we sustain fractures in older age is because of weakening bones and muscles, which can lead to falls. Beyond that, there’s a huge body of literature that shows that vitamin D might be important for a multitude of other things, including infection, cancer and heart disease, though more research needs to be done.”

It’s important at every age. “Our peak bone strength or density is achieved somewhere around age 20 to 25, so if we don’t get strong bones by that age, it’s not going to get better later,” he says. “Deficiency of calcium and vitamin D and overall nutrient intake will come back to bite us later when we’re 50, 60, 70, 80 and 90.” But if you missed the window to maximize your bone strength, don’t give up. Getting enough vitamin D in your golden years can still lower your risk of falls, fractures and osteoporosis.

Sunshine helps. Long before the days of multivitamins, humans made vitamin D the old-fashioned way: through sun exposure to bare skin. “Commonsense says that humans were designed to live outside and make vitamin D, and now we don’t do that. We live inside, work inside, and dermatologists advise us to stay out of the sun, so we’re not making vitamin D the way we were designed to,” Binkley notes. People who still spend a lot of time outside tend to have higher vitamin D levels. Unfortunately, it’s difficult to quantify sun recommendations because skin type, location and time of year can all affect how long it takes your body to generate vitamin D from the sun.

Aim for what nature gives you. Although there’s not enough research to definitively confirm the optimal level of vitamin D, people who are outside a lot tend to have serum 25-hydroxyvitamin D levels around 40 ng/mL. “Aim for that,” Binkley advises. “If you are below that level, it’s probably not an ideal status, and I think it’s likely that having a vitamin D level of 40 rather than 15 is good for your bones and muscles. Not knowing what your vitamin D level is and how rapidly you metabolize it, a reasonable amount is 1,000 to 2,000 IU per day.”

Don’t rely on food alone. You’d have to drink 20 cups of vitamin D-fortified milk to get 2,000 IU per day. “To get enough vitamin D from food is really difficult,” Binkley says. “About the only people who have optimal 25-hydroxyvitamin D levels from food eat large amounts of wild salmon or that type of ocean-going fish.” A multivitamin may have just 400 IU, so a vitamin D supplement is probably a good idea.

Don’t overdo it. Again, aim for what your ancestors may have gotten naturally. “I don’t think it makes sense to take supplements that put you above what normal physiology can achieve,” Binkley says. Super-high levels of vitamin D won’t offer any additional benefit and could even cause harm through a buildup of calcium in the blood, which can lead to kidney damage and other problems. But vitamin D toxicity is very rare, and over-the-counter supplements as high as 4,000 IU should be OK, he says.