Hydroxyproline ingestion and urinary oxalate and glycolate excretion

Knight J, Jiang J, Assimos DG, Holmes RP · Kidney International

Hydroxyproline, the amino acid that makes collagen collagen, is a precursor of oxalate — and the dose that shows it is the dose people take. Against a matched 30 g whey arm, 30 g of gelatin raised urinary glycolate 5.3-fold and urinary oxalate 43%. The threshold sits low: 5 g and 10 g gelatin loads moved plasma hydroxyproline and urinary oxalate significantly, while 1 g and 2 g did not. The authors estimate hydroxyproline metabolism supplies 5-20% of endogenously derived urinary oxalate at ordinary intakes.

Population

Two groups of healthy volunteers on controlled oxalate diets at Wake Forest: ten (five male, five female, mean age 32) for the gelatin-versus-whey comparison, and a separate six (three male, three female, mean age 28) for the graded gelatin loads

Intervention

30 g gelatin per day, plus separate 1, 2, 5 and 10 g gelatin load experiments

Comparator

30 g whey protein, matched diet

Limitations

A metabolic feeding study in ten and six healthy volunteers respectively, with no clinical endpoint — urinary oxalate is a risk marker, not a stone. It does not establish that anyone taking collagen forms stones, only that the substrate arrives. NIH-funded, no commercial interest.