Encyclopedia entry
kDa (kilodalton)
The kilodalton (kDa) is a unit of molecular weight (one dalton equals one twelfth of the mass of a carbon-12 atom; one kilodalton equals 1,000 daltons). On collagen supplement labels, kDa describes the average molecular weight after hydrolysis. Smaller is better for gut absorption.
Typical kDa across collagen sources
- · Marine (Type I, hydrolysed): 1 to 3 kDa. The smallest molecular weight typically available. Strong gut absorption.
- · Bovine (Type I + III, hydrolysed): 3 to 6 kDa. Good gut absorption.
- · Porcine (Type I + III, hydrolysed): 3 to 5 kDa. Similar to bovine.
- · Whole collagen (not hydrolysed): 300,000+ Da (300 kDa). Triple-helix structure. Limited gut absorption.
- · UC-II (undenatured Type II): Native triple-helix. Used at 40 mg via oral tolerance pathway, not nutritional absorption.
Why smaller kDa absorbs better
The gut absorbs peptides most efficiently as di- and tri-peptides (2 to 3 amino acids). A 2 kDa peptide is roughly 18 to 20 amino acids long and will be broken down further by digestive enzymes before crossing the gut wall. A 6 kDa peptide is 55 to 60 amino acids; more processing is required and absorption rate is lower. The 300 kDa whole-collagen triple-helix is not nutritionally absorbed as collagen at all; it would need to be digested into its constituent amino acids first.
What kDa does not predict
- · Tendon or skin outcome. Outcomes depend on dose and protocol, not on kDa within the hydrolysed range.
- · Taste. Lower kDa peptides are sometimes slightly more bitter.
- · Type composition (I, II, III). kDa is independent of collagen type.
Related: collagen hub · hydrolysed vs native.