It would be interesting to study this subset of patients, especially the metabolomics.
And this 2015 study proposed 'that amino acids are being increasingly used to provide an adequate carbohydrate source for the citric acid cycle.'
Elevated 3-methylhistidine was also found in this study from 2007.
Sepsis (infections) increases 3-methylhistidine, as does a zinc deficiency. I believe elevated cortisol increases it as well, but can't find the study at the moment.
Carnitine, taurine (in rats), zinc, and glutamine have been shown to lower or normalize it.
Edit: BCAAs also lower 3-methylhistidine.