Journal Club: Cacao-rich diet and age-related hearing loss, in mouse model.

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Today's journal article

Heredia-Molina RF, Riestra-Ayora JI, Vasallo IJT, Sanz-Fernández R, Sánchez-Rodríguez C. Sirtuins mediate the reduction of age-related oxidative damage in the cochlea under a cocoa-rich diet. 

Why I picked this article

Many people notice that following conversations in busy places gets harder with age. These are age-related changes to the inner ear, which lead to age-related hearing loss. The inner ear is a very delicate organ, where cells in the inner ear must move ions and energy to be able to convert sound into nerve signals. Reactive oxygen species can cause oxidative damage to proteins that carry out those essential roles, and eventually damage cells in the inner ear, causing hearing loss. 

To prevent damage caused by reactive oxygen species, much research has focused on antioxidant compounds. Cocoa beans from the Theobroma cacao trees contain polyphenols, including flavanols. Polyphenols are powerful antioxidants. The health benefits of cacao for its antioxidant properties have been demonstrated against various stress-related, age-related diseases. 

This research investigated whether a cocoa-rich diet would have any impact on a mouse model of age-related hearing loss. 

From pixabay.com (by allybally4b)

Some of the research findings

Animal model: 
  • Adult C57BL/6J mice, both sexes. 
    • 6-month-old: mature 
    • 14-month-old: getting old
    • 22-month-old: very old for lab mice
  • Dietary intervention: 
    • Standard chow versus cocoa-supplemented chow calibrated to 8.2 mg/kg/day cocoa powder.
    • This cocoa powder dose is equivalent to ≈5 g/day human equivalent for 70 kg by allometric scale conversion according to the publication. 
    • Cocoa was blended at 8.2 g/kg feed; average intake ≈25 mg cocoa/mouse/day. 
Output measures: 
  • Protein/mRNA levels for SIRT1, SIRT3, FOXO3, p53 were measured in whole-cochlea homogenates. These proteins are all known to be important for antioxidant defence: 
    • SIRT1 & SIRT3 are Sirtuins family of (NAD +)-dependent class III histone deacetylases. These proteins play important roles in oxidative response. 
    • SIRT1 regulates ageing processes and antioxidant defence, together with FOXO3a and p53. 
    • SIRT3 is found in the mitochondria, the cell's energy powerhouse, to help mitochondrial antioxidant defense, with SOD2 and IDH2. 
  • Glutathione peroxidase (GSH-Px) & catalase (CAT) activities are also monitored as signs of antioxidant defence. 
Key findings: 
  • The inner ear (cochlea) analysis showed that the cochleae from the cocoa-rich diet group looked healthier at 14-month and 22-month timepoints. Auditory neurons and sensory cells appeared to be more preserved. 
    • Cochleae from cocoa-fed mice showed higher SIRT1 and SIRT3 levels. 
    • Cochleae from cocoa-fed mice also had increased levels of FOXO3.
    • Cochleae from cocoa-fed mice had lower levels of p53.
    • Effects were most pronounced at 22 months. 
  • There were some differences between males vs females: 
    • FOXO3 were higher in males for 60 and 14-month time periods
    • SIRT1 was higher in males, but SIRT3 was lower in males at the 22-month timepoint. 
  • The signs of oxidative damage were less in cocoa-fed animals. 
  • Red fluorescent probe dihydroethidium (DHE; Calbiochem, San Diego, USA) was used to measure the reactive oxygen species. 
    • The reactive oxygen species level was significantly lower in aged cocoa-fed mice compared to the age-matched controls.
    • Fluorescent intensity was used to assess the quantity. 
Overall, the signs of oxidative damage in the inner ear were reduced in the cocoa-rich diet with heightened antioxidant defence. 

Haruna's takeaway

The health benefits from polyphenols in cacao-rich products, like dark chocolate, for ageing have been known and talked about for some time. I guess we can include hearing to be one of them. Dietary and lifestyle factors tend to lead to crude generalisations that everything that's good for the rest of the body is good for hearing. One potential detail from this study is that the equivalent dose of cocoa powder has been estimated to be 5g per 70kg adult human. I don't understand how the allosteric conversion works, and also acknowledge that the mouse digestive system and native diet through evolution are very different to those of humans as a major limitation. But assuming this is right, 5 g is a teaspoon size. If you eat 80% cocoa dark chocolate, it will be a small piece and highly feasible to consume? 

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This is Haruna's 98/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.