Keller 2019: shilajit, strength and fatigue trial

This trial, run at the University of Nebraska-Lincoln, tested whether shilajit helps muscles hold on to their strength when they get tired. It is one of the better-designed shilajit studies: randomised, double-blind, with a placebo group and two doses.

The headline finding, though, is narrower than it is often reported. Shilajit did not increase strength. The higher dose reduced how much strength men lost during a hard, fatiguing exercise test, and that effect was clear only in the stronger half of the group.

What did the Keller 2019 study do?

Sixty-three recreationally active men, average age 21, were randomly assigned to take 250 mg of shilajit, 500 mg, or a placebo capsule once a day for 8 weeks, 21 men per group. Before and after, they did a strength test, then 100 maximal leg extensions on an exercise machine in two sets of 50, then the strength test again. Blood was taken to measure hydroxyproline, a marker of collagen breakdown (Keller et al., 2019).

What did it find?

For the whole group of 63, the 500 mg group lost less strength after the fatiguing exercise than the 250 mg group. The paper does not report a significant difference between 500 mg and placebo across the whole group.

The researchers then split each group at the halfway point by starting strength. In the stronger half, men taking 500 mg lost an average of 8.9% of their strength after the fatiguing exercise, compared with 17.0% on 250 mg and 16.0% on placebo. The 250 mg dose did no better than placebo.

A similar split by starting hydroxyproline level found that, among men with higher levels at the start, those on 500 mg ended with lower hydroxyproline (1.5 μg/mL) than those on 250 mg or placebo (both 2.4 μg/mL). The authors read this as less collagen breakdown in connective tissues such as tendons.

Eight weeks of shilajit had no effect on maximum strength before fatigue, peak torque or body weight. No adverse events were reported.

What does this mean?

It hints that 500 mg a day of this extract might help muscles resist fatigue in people who are already fairly strong, and that 250 mg is too little. It fits with an earlier study from Ohio State University in which shilajit switched on genes linked to muscle repair and connective tissue (see Das 2016), and with a later trial from the same Nebraska lab that found higher levels of a collagen-building marker (see Neltner 2024).

What doesn’t it show?

• That shilajit builds strength or muscle. Maximum strength and body weight did not change.
• That the benefit is reliable. The clearest result came from splitting the groups after the fact, leaving about 10 men per group in the stronger half. Subgroup findings like this often fail to hold up when tested again.
• That it helps in real sport. The test was 100 maximal leg extensions on a laboratory machine. Whether that translates to running, lifting or team sports is unknown.
• That it works for women or older people. Everyone was a healthy young man.
• That other products would work. Only PrimaVie was tested.

How much should we trust it?

It is a careful trial with a weak headline. The design is sound and the paper is open about its methods, but the main positive result is in a subgroup, the study was funded by the extract’s maker, and no independent team has repeated it. The authors themselves note that their sample was a convenience sample of healthy young men and that the protocol used only one type of muscle action.

That is why we grade shilajit for exercise performance as weak.

Where does this study fit?

This trial is the main controlled evidence on our shilajit and exercise performance page, and its comparison of 250 mg and 500 mg informs our dosage advice. For other trials, see the research library.