There is increasing evidence that adult humans continue to grow new neurons in their brains [1], even up to the age of 100 years [2] and presumably beyond. Neurogenesis, or growth of new neurons, has been studied in adult rats and other mammals [e.g., 3]. Evidence of adult neurogenesis overturned the prevailing belief that the adult brain was fixed and incapable of regeneration. As these findings become more popularized, what will this mean for our concepts of aging?
It was long believed that brain development was completed around the age of 6 years old. This had disastrous consequences for people born with phenylketonuria (PKU) an inborn error of metabolism in which people cannot break down the amino acid phenylalanine. Phenylalanine is a neurotoxin that readily crosses the blood-brain barrier. What is remarkable is that PKU can be completely controlled by a very restrictive diet limited in phenylalinine. Children with PKU who are untreated will have significant cognitive deficits and have to be institutionalized as adults. Since the 1960s every baby in North America has been tested at birth for PKU.
Until the 1980s the standard recommendation was to discontinue treatment of children with PKU after the age of 6 years old. Subsequently researchers found a severe decline in intellectual function in children who had gone off diet [4]. The current recommendation for people with PKU is that they stay on diet their entire lives.
Assumptions regarding the age at which the brain is “complete” has had consequences for people living with PKU. As the evidence of neurogenesis in adult humans emerges, how will that affect the way we think about age?
Given that people continue to learn and grow through their lives it actually seems a bit preposterous to believe that brain development would ever be complete. Setting aside neurogenesis for a moment, there seems to be a trend among people who are anxious about cognitive decline to engage in more cognitive exercises (e.g., sudoku, crossword puzzles). There is a “use it or lose it” mentality about cognition. The idea of neurogenesis is radical in the face of the belief that aging is essentially and only a process of deterioriation.
Ironically, it is not cognitive exercise that promotes neurogenesis, but rather physical exercise [5, 6]. I am starting to develop a different image of healthy aging. Rather than picturing the slow cognitive decline that seniors try to hold off through mind-exercises, I imagine engaged elderly people who are physically active. I think of walking groups (for those who are able) , or water exercises, or dance classes. Anything to get the heart pumping. What’s good for the heart, they say, is good for the brain.
Will we start to think of the elderly as more capable (old dogs, new tricks)? Will the idea of neurogenesis provide hope for those experiencing cognitive decline? Ultimately, how might long-term care facilities adjust their programming to accommodate the developing brains of their octogenarian residents?
[1] Eriksson PS et al. (1998). “Neurogenesis in the adult human hippocampus” Nature Medicine 4, 1313 – 1317 doi:10.1038/3305
[2] Knoth R, Singec I, Ditter M, Pantazis G, Capetian P, et al. (2010). “Murine Features of Neurogenesis in the Human Hippocampus across the Lifespan from 0 to 100 Years”. PLoS ONE 5(1): e8809. doi:10.1371/journal.pone.0008809
[3] Reynolds, BA; Weiss (1992). “Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous system”. Science (New York, N.Y.) 255 (5052): 1707–10. doi:10.1126/science.1553558
[4] Seashore, MR et al. (1985). “Loss of intellectual function in children with phenylketonuria after relaxation of dietary phenylalanine restriction”. Pediatrics 75(2): 226-32.
[5] van Praag H, Shubert T, Zhao C, Gage FH (September 2005). “Exercise enhances learning and hippocampal neurogenesis in aged mice”. J. Neurosci. 25 (38): 8680–5. doi:10.1523/JNEUROSCI.1731-05.2005
[6] Pereira AC et al. (2007) “An in vivo correlate of exercise-induced neurogenesis in the adult dentate gyrus” Proc Natl Acad Sci U S A. 104(13): 5638–5643. doi: 10.1073/pnas.0611721104
Neuroethics at the Core 
Thank you for reminding us that exercise is an important factor in healthy aging. It’s an important message to get out there! Interestingly, there is also a body of evidence that suggests that cognitive training and enrichment can also positively impact neurogenesis.
All things considered, it seems the best way to ensure healthy aging is to complete crossword puzzles *while* running. 🙂
Excellent advice. Apart from its continued ability to grow new neurons, it’s also been discovered that the brain is the only part of the body that has DNA which can change. As we go through new experiences our brain responds by adapting with new structures and pathways. Very exciting!
Martin
I recently saw a report that stated life expectancy could increase by as much as 30 years in the next few decades. How amazing would that be? Of course the economic and social challenges may be scary. In the context of your article Elana I can’t help but think that for those not able to exercise mobility scooters are a great way to at least get out of the house and interact with others which reduce the risk of depression.
Kevin
Very interesting stuff! Almost every health article will tell you that exercise is good for your brain in many ways. I didn’t know though that it could actually help your brain continue to build itself throughout your entire life. It is truly amazing to me the complexity of the human brain, and how little we actually know about it. My belief is that when the brain is more fully understood, amazing things will happen in the areas of psychotherapy, life expectancy, etc. etc.