Hydrolyzed collagen contains large amounts of highly bioavailable glycine (a non-essential amino acid). Glycine has been shown to naturally reduce anxiety by inhibiting neurotransmitters, such as norepinephrine, that create feelings of panic and anxiety when released (1). Research has even shown that elevated glycine concentrations in the brain can be an effective way to treat anxiety, depression, obsessive-compulsive disorder and other cognitive conditions. In one study, treatment with glycine over 5 years led to a robust reduction of obsessive-compulsive disorder and body dysmorphic disorder signs and symptoms (2).
It has been suggested that the absence of the amino acid tryptophan in collagen could contribute to anxiety, but this would imply that collagen replaces protein in one's diet and that it is the only source of amino acids. In reality, tryptophan is abundant in many common foods, including oats, bananas, dried prunes, milk, cheese, bread, chicken, turkey, and peanuts (3). It is recommended that adults consume between 250 mg and 425 mg of tryptophan per day (~3.5 to 6.0 mg/kg of body weight per day), which is very little and can easily be obtained by eating a balanced diet of whole foods (3).
References:
Hospital News (2021). Nutritional treatments to combat anxiety disorders.
Cleveland, L, DeLaPaz, R, et al. (2010). High-Dose Glycine Treatment of Refractory Obsessive-Compulsive Disorder and Body Dysmorphic Disorder in a 5-Year Period.
Richard, D, Dawes, M, et al. (2009). L-Tryptophan: Basic Metabolic Functions, Behavioural Research and Therapeutic Indications.