Responsicle to Sandrine Thuret’s TED talk “You can grow new brain cells. Here’s how”
Thanks for the talk, Sandrine, it was brilliantly structured and delivered. As a layperson and armchair neurologist, I’ll confess that I always assumed that we could grow new neurons as adults. That this was up for debate was a surprise.
In the exercise with the audience you outlined a few areas that either increased or decreased neurogenesis, and most of the items fell reassuringly along the lines of conventional wellness wisdom: exercise, sex, and learning is good; stress, sleep deprivation and aging are bad.
But what was worth delving into more is the dietary effects. Specifically that the ‘good’ list contained:
- Omega-3 fatty acids
- Calorie restriction
- Intermittent fasting
- Crunchy foods
- And other vitamins.
while the ‘bad’ list detailed:
- High saturated fat
- High sugar
- Soft foods
- Vitamin deficiencies
On the face of it, these fall along the lines of accepted modern medical wisdom. But I’m more interested in why this might be from an evolutionary perspective. If we assume that this has been the same for all humans for the past million years, what purpose might this have served?
The human brain is enormously expensive in terms of calories, consuming about 15% of our total energy but weighing only about 2% of our total body weight. On a 2,000 calorie daily diet, these numbers suggest the brain consumes about 300 calories, almost 250 calories more than its weight would predict. That’s a pretty big requirement for an early human diet, where every morsel had to be hunted, plucked, or dug by hand.
So why would fewer calories lead to more neurons? Why would softer foods lead to fewer neurons?
Looking across the two lists and disregarding vitamins and plant toxins such as resveratrol, ethanol, and caffeine, two patterns emerge. The first pattern is associated with a low-calorie diet: caloric restriction, intermittent fasting, and plant-based. In premodern times before deep fryers, crunchy foods were likely fresh vegetables and some insects, both low in calories. Flavonoids are common to many plants, and blueberries hail from an ancient and hardy fruit family, native to many parts of the Northern Hemisphere including cold climates. Omega-3 fatty acids are (mostly) found in fish, a relatively low-calorie animal protein source. People who had this type of low-calorie and inconsistent diet probably lived in nutrient-poor environments, and likely faced starvation and malnutrition on a regular basis.
Conversely, the neuro-suppressant diet of high saturated fat, high sugar, and soft foods suggests a well-fed populace with easy access to food sources. Saturated fat primarily comes from high-calorie foods such as mammalian animal protein sources (cows, sheep, etc.), selected high-energy fruits (coconuts, cacao), and nuts. A high-sugar diet would likely have occurred in areas with abundant fruit trees and vines, such as tropical forests. Meats and fruits are far softer than vegetables and insects, and they are higher in calories as well. The soft texture of these foods may signal the brain that they are high in calories.
Recent research in animals has linked caloric restriction to longer lifespans, and delaying the aging process. This might suggest that in addition to neurogenesis, harsh nutritional conditions also delay aging.
So what might account for this effect? Why would people at risk of starvation generate more neurons and live longer than those with no food shortages?
There might be an evolutionary reason related to care and oversight of offspring. Past the reproductive age, most animals begin the period of senescence (aging). Delaying senescence is unlikely to increase the reproductive capacity of an individual, but it could impact the ability of existing offspring to survive.
And this might explain what’s going on. Early humans in food-deficient environments may have needed to care for offspring much later in life, assisting them in the basics of food acquisition. This could have taken the form of teaching them how to hunt difficult or dangerous prey, which plants to consume or avoid, and how to make toxic foods non-toxic.
For example, Hákarl is an Icelandic delicacy with a 5-month preparation that involves leaving shark meat to drain and decay underground with special soil types for months before being air-cured and consumed. The fresh shark meat is so toxic with tremendously high ammonia levels that only decomposition renders it edible. With its strong smell of ammonia and putrescence, Hákarl regularly makes the list of world’s foulest delicacies, dedicated foodies use it to test their mettle, and tourists are served it alongside a bucket for the near-inevitable retching. Times must have been very tough for early Icelanders to rely on such strong-smelling, rotten food for nutrition. Also, the complex detox process may have created a precarious situation where one misstep could spell death. It would have been important for detailed instructions to be passed down from generation to generation, a situation that favored having elders in the family that could supervise the preparation, lest the family inadvertently poison themselves.
This need for elders well into offspring’s adulthood may explain both lengthened lifespans as well as heightened neurogenesis. Elders in nutrient-poor environments may have been relied upon to oversee complex and dangerous food acquisition and preparation, favoring cognitively-sharper, longer-living individuals. Perhaps the elders’ own bodies signaled this need by ‘turning up’ neurogenesis and ‘turning down’ the aging process to live longer and be more useful to their families. Their families, in turn, may have survived longer and been able to reproduce more.
If so, this is yet another of life’s evolution miracles, a brilliant mechanism to help offspring acquire food and avoid starvation.
I welcome any reactions or thoughts you have.