Cysteic Acid in Dietary Keratin is Metabolized to Glutathione and Liver Taurine in a Rat Model of Human Digestion

Abstract

Poultry feathers, consisting largely of keratin, are a low-value product of the poultry industry. The safety and digestibility of a dietary protein produced from keratin (KER) was compared to a cysteine-supplemented casein-based diet in a growing rat model for four weeks. KER proved to be an effective substitute for casein at 50% of the total dietary protein, with no changes in the rats’ food intake, weight gain, organ weight, bone mineral density, white blood cell counts, liver glutathione, or blood glutathione. Inclusion of KER in the diet reduced total protein digestibility from 94% to 86% but significantly increased total dietary cysteine uptake and subsequent liver taurine levels. The KER diet also significantly increased caecum weight and significantly decreased fat digestibility, resulting in a lower proportion of body fat, and induced a significant increase in blood haemoglobin. KER is therefore a safe and suitable protein substitute for casein, and the cysteic acid in keratin is metabolised to maintain normal liver and blood glutathione levels.

The Ability of Exercise-Associated Oxidative Stress to Trigger Redox-Sensitive Signalling Responses

“…Specifically, supplementation with thiol-donors such as N-acetyl cysteine (NAC), whose ability to attenuate reduction of glutathione may prevent systemic oxidative stress and hence increase performance-related parameters such as power output and time-to-exhaustion in athletes [ 92 , 93 ], but possibly without preventing the original local stimulus which initiates redox-sensitive cell signalling responses. Accordingly, given that in some individuals oral or systemic administration of NAC can be poorly tolerated [ 2 ], alternative dietary sources of thiols such as taurine and hydrolysed keratin have been recommended [ 2 , 112 ]. However, it should be noted that in another study, NAC supplementation did conform to the hormesis principle by attenuating early adaptive responses to exercise in human skeletal muscle [ 88 ].…”

The effect of chronic soluble keratin supplementation in physically active individuals on body composition, blood parameters and cycling performance

“…Without prior published work it is difficult to provide a comparison. However a preceding rodent study [ 9 ] comparing CAS and KER supplementation suggested that KER may promote LM gain, although the results did not reach significance. There is an earlier published report that increasing the sulfur amino acid content (specifically CYS and methionine) above a CAS-based diet, in rodents at least, maximises weight gain [ 21 ].…”

“…The study used a randomized, blinded, balanced crossover design, with two four-week intervention periods (INT1: intervention 1; INT2: intervention 2) involving supplementation with either KER or a low-cysteine protein (“gold standard” sodium caseinate, CAS [ 9 ]). The interventions were separated by a minimum of 8 weeks to allow wash-out of any remaining traces of the supplement from the participant.…”

“…We have previously shown that ingestion of up to 40 g per day of soluble KER is safe and palatable when supplementing a balanced diet in humans [ 8 ]. Further, a rodent study by Wolber et al (2016) indicated that the consumption of KER has potential benefits other than simply providing a cheap protein source (poultry feathers) [ 9 ]. Following the substitution of 50% of a casein (CAS) protein diet with KER in male Sprague–Dawley rats over a four-week period, blood profiles exhibited significant increases (~ 5%) in haematocrit (Hct) and haemoglobin content (Hb).…”

“…Therefore, using physically active healthy individuals, the purpose of this study was to compare the effects of 4 weeks of cycle training combined with KER or CAS supplementation on anthropometric, blood, cardiorespiratory and performance variables. Based on the observations of a study using a rodent model [ 9 ] and applying these to a repeated-measures cross-over design, we hypothesized that supplementation with KER would result in an increased lean to fat mass ratio, a more favourable blood profile, and consequently improved performance during cycling exercise when compared to supplementation with the same amount of CAS.…”