German researchers recently published one of those studies that, from time to time, makes me question my core beliefs. I’m an extra skeptic, but I try not to let that identity stop me from absorbing new data. And if there’s one supplement whose possible benefits I’ve been hesitant about in recent years, it’s vitamin D.
The new study, which appears in the European Journal of Applied Physiologyis part of a major initiative to improve the performance of Germany’s elite athletes. A research team led by Sebastian Hacker from Justus Liebig University in Giessen studied 474 German national team athletes in various sports, including hockey, table tennis and three-on-three basketball. They tested vitamin D levels and measured (among other outcomes) grip strength.
Here is the money plan:
This graph shows grip strength versus 25(OH)D levels, which helps assess vitamin D status in the blood. The two dotted 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 has been long debate about where these thresholds should be set, but this is the current thinking. Note that you will sometimes see 25(OH)D levels expressed in nmol/L; To access these units, multiply the values above by 2.5.
The key point: the line is clearly inclined. Higher levels of vitamin D are associated with stronger grip strength, which in turn has been linked to health, longevity, and (less clearly) athletic performance. For every 1 ng/mL increase in 25(OH)D, grip strength increases by 0.01 N/kg, meaning that going from 20 to 30 ng/mL should increase your strength by about three percent .
The case for vitamin D supplements as a performance aid
Vitamin D plays a role in many body systems, including bone health, immune function and, perhaps most notably for athletes, muscle performance. If you are truly vitamin D deficient, you should definitely increase your levels. But the evidence in the “simply insufficient” range is less clear, even in this data. If you removed all values below 20 mg/mL from the analysis, would there still be a relationship between the vitamin and grip strength? It’s not clear.
This isn’t the first time researchers have demonstrated a relationship between vitamin D and strength. In fact, a systematic review published a few months ago pooled data from 28 studies involving 5,700 participants and concluded that there is a positive relationship between vitamin D levels and quadriceps strength. At least that’s the main result, but when you look more closely, it’s less convincing. The positive relationship was for the strength of the quads when contracting the muscle at a specific speed of 180 degrees per second. But there was no relationship at speeds below 60 degrees per second. Worse still, there was a negative correlation for maximal contractions versus stationary force: higher vitamin D levels were associated with lower maximal force.
In other words, we should not be too quick to assume that the new German data is definitive. Instead, it’s another data point in an ongoing debate. Another review, published in September by Japanese researchers, finds “mixed results” in studies on the relationship between vitamin D levels and muscle mass and strength.
Causation or correlation?
Even if we ultimately conclude that there is a positive relationship between vitamin D levels and strength, that doesn’t necessarily mean we should all start taking vitamin D pills. First of all, there is the possibility of reverse causality. Strong, healthy people may choose to spend more time exercising outdoors, which can produce higher vitamin D levels. This is one of the strengths of the new German study: since all the subjects were elite athletes, it can be assumed that they have similar levels of general fitness and physical activity.
There may also be confounding factors. In 2019, Out Editor-in-Chief Rowan Jacobsen wrote a surprising article in which he claims that the benefits of sunlight go beyond simply increasing vitamin D levels, including triggering the release of nitric oxide from your skin in your blood circulation. If this is the case, taking vitamin D supplements will not necessarily solve the problems associated with lack of sunlight.
What we really want are intervention studies, where we give people a vitamin D supplement and see if they get stronger. And we don’t want subjects who already have sufficient levels of vitamin D, because they would benefit less; rather, we want people whose levels are insufficient. That’s what another new study did, this one from Estonia.
The Estonian researchers took 28 volunteers with “insufficient” levels of 25(OH)D in the 20 mg/mL range. Half of them received a placebo and the other half took 8,000 IU per day of vitamin D, which ultimately increased their 25(OH)D levels to 57 ng/mL. Both groups underwent 12 weeks of resistance training, but there was no discernible difference in their results, which were published in the journal Nutrients. Here are the gains in one repetition maximum for different exercises for both groups:
In fact, the deeper you delve into the literature, the less compelling the data on vitamin D as a sports supplement becomes. For example, a 2019 study published in the Journal of the International Society of Sports Nutrition found no significant benefit of vitamin D supplementation on muscle strength, but a trend in the right direction. But even this weak result was marred 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 a sports supplement are distinct from its potential for more general health purposes. Taking vitamin D supplements may help 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 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 in vitamins. D levels generally showed no differences in health outcomes.
Putting all this together, the case for taking vitamin D supplements doesn’t seem very compelling to me, assuming, of course, that you don’t have a true deficiency. Setting this threshold is the tricky part. Is it below 20 ng/mL, which health authorities consider deficient? Is it below 30 ng/mL, which they call insufficient? Is it somewhere higher, lower, or somewhere in between? I’m not sure, so for now I’ll bet: Despite all my skepticism, I’ll have my levels tested at my next doctor’s appointment.
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