Researchers in Germany recently published one of those studies that, now and then, make me question my core beliefs. I’m a supplement skeptic, but I try not to let that identity prevent me from assimilating new data. And if there’s one supplement whose possible benefits I’ve been on the fence about in recent years, it’s vitamin D.
The new study, which appears in the European Journal of Applied Physiology, is part of a major initiative to improve the performance of German elite athletes. A research team led by Sebastian Hacker of Justus Liebig University in Giessen studied 474 athletes on German national teams in a range of sports including hockey, table tennis, and three-on-three basketball. They tested vitamin D levels and measured (among other outcomes) handgrip strength.
Here’s the money shot:
This graph shows handgrip strength as a function of 25(OH)D levels, which is how vitamin D status is assessed in the blood. The two dashed lines indicate the thresholds between vitamin D deficiency (below 20 ng/mL), insufficiency (between 20 and 30 ng/mL), and sufficiency (above 30 ng/mL). There have been long debates on where these thresholds should be set, but that’s the current thinking. Note that you’ll sometimes see 25(OH)D levels expressed in nmol/L; to get to those units, multiply the values above by 2.5.
The key point: there’s a clear slope to the line. Higher levels of vitamin D are associated with stronger grip strength, which in turn has been associated with health, longevity, and (less clearly) athletic performance. For every 1 ng/mL increase in 25(OH)D, handgrip strength increases by 0.01 N/kg, which means that going from 20 to 30 ng/mL should boost your strength by about three percent.
The Case for Vitamin D Supplements as a Performance Aid
Vitamin D plays roles in a whole bunch of body systems, including bone health, immune function, and—perhaps most notably for athletes—muscle performance. If you’re truly deficient in vitamin D, there’s no doubt you should get your levels up. But the evidence in the “merely insufficient” range is less clear, even in this data. If you took all the values below 20 mg/mL out of the analysis, would there still be a relationship between vitamin and handgrip strength? It’s not clear.
This isn’t the first time researchers have shown a relationship between vitamin D and strength. In fact, a systematic review published a few months ago pooled data from 28 studies with 5,700 participants and concluded that there’s a positive relationship between vitamin D levels and quadriceps strength. At least, that’s the headline result—but when you look closer, it’s less convincing. The positive relationship was for quad strength when contracting the muscle at a specific speed of 180 degrees per second. But there was no relationship at a slower speed of 60 degrees per second. Worse still, there was a negative correlation for maximal contractions against an immoveable force: higher vitamin D levels were associated with smaller max force.
In other words, we shouldn’t be too quick to assume the new German data is definitive. Instead, it’s another data point in an ongoing debate. Another review, published in September by researchers from Japan, finds “mixed results” in studies on the relationship between vitamin D levels and muscle mass and strength.
Causation or Correlation?
Even if we eventually conclude that there is a positive relationship between vitamin D levels and strength, it doesn’t necessarily follow that we should all start popping vitamin D pills. First of all, there’s the possibility of reverse causation. People who are strong and healthy may choose to spend more time exercising outdoors, which in turn may produce higher vitamin D levels. That’s actually one of the strengths of the new German study: since all the subjects were elite athletes, we can assume that they have similar levels of general fitness and physical activity.
There may also be confounding factors. Back in 2019, Outside contributing editor Rowan Jacobsen wrote a surprising article in which he argued that the benefits of sunlight extend beyond merely raising vitamin D levels, most notably in triggering the release of nitric oxide from your skin into your bloodstream. If that’s the case, then taking vitamin D supplements won’t necessarily fix whatever problems are associated with lack of sunshine.
What we really want are intervention studies, where we give extra vitamin D to people and see if they get stronger. And we don’t want subjects who already have sufficient levels of vitamin D, because they stand to benefit less; instead we want people with insufficient levels. That’s what another new study, this one from Estonia, did.
The Estonian researchers took 28 volunteers with “insufficient” 25(OH)D levels in the low 20s mg/mL. Half of them got a placebo, and the other half took 8,000 IU per day of vitamin D, which eventually got their 25(OH)D levels up to a healthy 57 ng/mL. Both groups did 12 weeks of resistance training, but there were no discernible differences in their results, which were published in the journal Nutrients. Here are the gains in one-rep maximum for various exercises for the two groups:
In fact, the further you dig into the literature, the less convincing the data looks for vitamin D as an athletic supplement. For example, there was a 2019 review in the Journal of the International Society of Sports Nutrition that found no significant benefit of vitamin D supplementation on muscle strength but a trend in the right direction. But even that weak finding was tainted by “key errors in the analytical approach,” according to a reanalysis published last year: the true effect is close to zero.
Of course, vitamin D’s merits as an athletic supplement are distinct from its potential for more general health purposes. Might it be that taking vitamin D supplements helps prevent cancer, heart disease, or type 2 diabetes; increases bone density; or reduces your risk of falls? No, no, no, no, and no, according to a summary of the existing evidence from human trials published in 2021. More than 60 Mendelian randomization studies, which use genetic data to divide people into pseudo-randomized groups with high or low vitamin D levels, have generally found no difference in health outcomes.
Put it all together and the overall case for taking vitamin D supplements doesn’t look very compelling to me—assuming, that is, that you don’t have a genuine deficiency. Defining that threshold is the tricky part. Is it below 20 ng/mL, which health authorities consider deficient? Is it below 30 ng/mL which they label insufficient? Is it somewhere higher or lower or in between? I’m not sure, so for now I’ll hedge my bets: despite all my skepticism, I’m going to arrange to get my levels tested at my next doctor’s appointment.
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