Size matters

Dr Sabina Brennan is a research psychologist, neuroscientist, filmmaker, award-winning science communicator and author of the No 1 bestseller 100 Days to a Younger Brain. In her guest blog, Dr Brennan shares with us why brain size matters.


Some people are able to tolerate more Alzheimer’s disease-related pathology than others while still maintaining cognitive function. We call this resilience reserve. Using a computer analogy we draw a distinction between brain reserve (the hardware) and cognitive reserve (the software).

1.Sabina Brennan Book Launch_2019_03_13

Dr Sabina Brennan

Brain reserve (the hardware) is the structural stuff: grey matter, white matter and the thickness of your cortex. Brain reserve refers to the actual differences in the brain itself that might explain how one individual has greater tolerance to damage than another.
Beth and Janet have the same amount of Alzheimer’s disease, but their brains are different sizes. Beth has more brain cells, denser brain connections and a larger brain than Janet. This means that Beth has more brain without disease than Janet has. It is not the amount of disease in the brain that accounts for differences in cognitive functioning between people, it is the amount of intact brain. Beth’s bigger brain will be more resilient than Janet’s to the effects of the same amount of disease pathology.

To put it simply: brain size matters. The larger your adult brain is, the longer you can resist the impact of disease on your functioning.

As the disease progresses the amount of diseased brain will increase and the amount of intact brain will decrease until a threshold is reached where the intact brain can no longer maintain normal cognitive functioning. Of course, the less Alzheimer’s disease pathology you have the better. However, if you do develop pathology the good news is that a brain healthy lifestyle can build brain reserves which will contribute to resisting its effects.

Cognitive reserve (the software) refers to the flexibility of brain networks in the face of disruption caused by ageing, injury or disease. Ben and Doug have the same amount of hardware (brain reserve). Ben can tolerate more disease-induced brain changes because the capacity of his underlying software (cognitive reserve) differs from Doug’s in a way that allows his brain to cope with or adapt to the disruptions.

Taking brain and cognitive reserve together let’s consider Jake and Peter, two fifty-five-year-old men. Jake has high reserve (high resilience) and Peter has low reserve (low resilience). Both begin to develop the pathology of Alzheimer’s disease in their brains at the same time. Both die at the age of seventy-five. Peter’s cognitive function gradually declines, going from mild through moderate, to severe and ultimately to his death. In contrast, Jake, won’t manifest any perceptible symptoms. Even though the disease pathology is still progressing in Jake’s brain his high levels of reserve allow his brain to cope with and compensate for the physical damage that is occurring until his death at seventy-five.

Had Jake lived for longer his reserves would eventually have been exhausted and he would have manifested dementia symptoms. However, unlike Peter, Jake’s decline would be dramatic and severe. Like falling off a cliff edge. Jake would have experienced a precipitous drop in his cognitive functioning, bypassing the mild and moderate stages. Reserve built through brain healthy life choices allows people like Jake to spend a greater proportion of their life living independently in possession of their cognitive faculties and a smaller proportion of life with functioning devastated by this disease.
Your brain has the capacity to build reserves. Adopting a brain healthy lifestyle is like investing in brain capital that you can cash in at some point in the future to cope with or compensate for damage, disease or decline. It will also optimise your brain function in the here and now. Here are my top tips for building reserves

  • Cherish sleep
  • Manage Stress
  • Stay socially engaged
  • Go mental (challenge yourself, learn new things, embrace new experiences)
  • Love our heart
  • Get physically active
  • Adjust your attitude (be positive, enjoy life and keep smiling)

About 100 Days to a Younger Brain – maximise your memory, boost your brain health and defy dementia.

Jacket_Insta100 Days to a Younger Brain delivers, in clear everyday language, the basics on how your brain works and how to keep your brain healthy. The good news is that the life style changes, activities and attitudes that boost brain health can easily be incorporated into your daily life. The book is grounded in scientific research and filled with really practical tips to help you to do that. While there are generic tips that we can all follow to keep our brains healthy 100-Days to a Younger Brain acknowledges that your brain is unique, shaped by your life experiences and life choices. As you work through this life-changing programme you will gain a clear picture of the current state of your brain health and insight into what you are doing right and what needs fixing. Armed with this information you will set your own personal goals and create a bespoke brain health plan to optimise your brain function, slow brain ageing and minimise the impact of brain injury and brain disease

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