A better way to keep your cool when the heat is on

Posted on July 8, 2021 | Leave a comment

Talk to most any endurance athlete on the matter of staying cool in the heat, and you’ll hear the same advice: ice in the hat, ice on the neck, ice down the bra, even ice vests. Alas, it seems we’ve been doing it wrong.

First, there’s nothing magical about cooling the head – or major vessels leading to the head.  It is true that the hypothalamus regulates temperature, but this structure is deep within the brain and certainly won’t be impacted by some ice in the cap. It is more about cooling the circulating blood that passes the hypothalamus. When body temperature rises, the nervous system redistributes blood flow, sending a larger percentage of it to the skin. This shunting of blood to the periphery promotes heat loss through convection. The brain loathes major change, and you don’t see any such increase in blood flow here with exercise or heat (imagine the headache).  So, our physiology really doesn’t support any unique benefit of ice application to the head.

Silke Koester battling the heat at the Bighorn ultra.

Neither does the research. Both neck cooling and head cooling have come up short in studies. Each approach has been ineffective in lowering body temperature while exercising in the heat (1,2).*

Another enduring notion around cooling stems around the misconception that the more surface area you target with cold, the more heat that dissipates from the body.**  This would be true if heat transfer were uniform across body surfaces, but that has not been supported by the research. Cooling vests provide a nice example here. They can provide some performance gains at shorter durations, but the research has not consistently supported their use as a means of actually lowering body temperature (3). Consider, too, that muscle and fat are good insulators. The direct application of cold to the skin is all about optimizing conductive heat loss, and it’s just not easy for heat to escape through these anatomical layers.  Ice down the bra or shirt hasn’t exactly been studied, but the same principles apply!

Importantly, overwhelming cold can undermine our goals. Very cold temperatures cause vessels to constrict. This slows the flow of cooler blood back to the core – the opposite of what we’re seeking.  Following tips like putting ice under your arms (eeek!) or near the groin may do you more harm than good. The reduction in blood flow triggered by cold may partially explain why ice baths after exercise may impair long-term gains (4).*** If anything, we’re restricting blood flow when it is needed most. And, why are athletes so intent on hindering the inflammatory process that our physiology has so masterfully designed to speed healing?! …closing that can of worms back up.

I’ll cut to the chase. How do we better work with our physiology to stay cool in the heat?

There is a convincing body of evidence showing the palms of the hands, the soles of the feet, and regions of the face are unique portals for dumping heat. The glabrous skin in these regions is anatomically adapted to accommodate large volumes of blood flow – and in turn to dissipate large quantities of heat (5,6,7). What makes these regions so unique is that they have a particularly high concentration of arterio-venous anastomoses (AVAs).  

Quick anatomy tutorial: arteries carry blood away from the heart and veins return blood to the heart. Typically – small, diffuse capillary beds in the tissues allow blood to transition from the arterial side of our circulation back to the venous side. But with AVAs, there is a direct connection between the small arteries and the small veins (see pic!). These AVAs really don’t play a role in nutrient transport, but they’re very adept at transporting heat (7)!

A team of Stanford researchers has published promising findings in this area. In one study, subjects wear a cooling glove on a single hand while exercising at both moderate and high workloads. With the cooling glove, subjects exercised 30-40% longer at high workloads and a striking 50-70% longer at moderate workloads (8). A subsequent study showed hand cooling to speed recovery from heat stress and found the effects to be additive. That is, when cooling was applied to both hands, body temperature recovered even faster (5).

Other teams of researchers have demonstrated that hand cooling between bouts of activity in the heat increases the duration of subsequent exercise and lowers body temperature (9). Yet another group had trained cyclists submerge their hands in water at various temperatures while riding a stationary bike. Once again, hand cooling allowed subjects to ride longer in the heat and keep their body temperatures lower (10).

So, how can we sensibly apply this knowledge? We get creative. At my last ultra in the heat, I ran with anything cold I could get my hands on – or in. I’d dunk my hands in creek crossings for 10-15s and pass frozen otter pops or crushed ice between hands as I left aid stations. That passing back and forth is key with items that cold! Remember, you don’t want to risk constricting the vessels in the AVAs and sabotaging your efforts. If those vessels narrow in reaction to the cold, then you’ve just lost a critical means of heat loss.

What about those small instant ice packs? I could even see them marketed in a similar fashion to the hand warmers used for cold-weather endeavors. Held between your hand and a water bottle, you wouldn’t even notice! Shuffling an ice-filled buff between hands brought some positive results for a friend who just tackled Western States 100. Even hand-held water bottles themselves have hidden potential. As long as you are grasping something cool, blood should travel back to your heart at a slightly lower temperature.

We’re more limited with the feet and face, but we can do our best to keep the cheeks cool. Rather than laying that ice-filled buff around the neck, try periodically pressing it into your cheeks or lips. I see the biggest potential for gains here in cold weather. Our thermoregulatory mechanisms work no different in winter. It may be time to embrace the balaclava. Thanks to the pandemic, I think we all have a healthy collection of buffs that we can tap into as well.**** (last footnote – promise!)

For now, hands are the easiest targets. I do look forward to seeing more practical approaches to AVA cooling roll out for athletes. Until then, our physiology and the research have convinced me to change my own approaches. If you’ve read this far, hopefully you may do the same!

Lastly, I can’t recommend the Huberman Lab podcast enough. This episode tuned me into the benefits of these glabrous ports, and I took a deeper dive into the endurance piece and thermoregulatory research here.

*Still, there’s no denying that some cold ice around the neck or in the hat simply feels amazing in the heat. Such perceptual effects are thought to be behind some of the temporary gains in performance that have been observed (2). So, these approaches are seemingly better than nothing.

*Surface area is a relevant factor when considering evaporative heat loss. We have our sweat to thank for this type, but its effectiveness is really limited by humidity and other factors. The direct application of ice to the skin targets conductive heat loss.

**Pre-cooling, like drinking ice cold drinks or soaking in cool baths before a short event, does have merit and support in the scientific literature. Still, the effects of this heat sink approach don’t appear to last through the hour. Unless you’re racing a crit or a 10k in the heat, the approaches are of little practical benefit. Athletes also have to be cautious not to overdo the cold-water immersion and demand high workloads from muscles that are still cold (2).
Cold baths or showers can be awesome for certain metabolic and nervous system gains, but I skip them after workouts now.

**** For reasons described earlier, we do not lose 40-50% of our body heat through our heads. This myth has been repeatedly debunked, but the advice from generations of well-meaning parents carries on.

REFERENCES

  1. Ansley, L, Marvin, G., Sharma, A, Kendall, M., Jones, D., & Bridge, M. (2008). The effects of head cooling on endurance and neuroendocrine responses to exercise in warm conditions. Physiological Research, 57, 863-72.
  2. Tyler CJ, Sunderland C, Cheung SS. (2013) The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: a meta-analysis. Br J Sports Med, 49(1):7–13.
  3. Bongers CC, Thijssen DH, Veltmeijer MT, et al. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. Br J Sports Med. 2015;49(6):377–384.
  4. Broatch, J. R., Petersen, A., & Bishop, D. J. (2018). The Influence of Post-Exercise Cold-Water Immersion on Adaptive Responses to Exercise: A Review of the Literature. Sports medicine (Auckland, N.Z.), 48(6), 1369–1387. https://doi.org/10.1007/s40279-018-0910-8 https://pubmed.ncbi.nlm.nih.gov/29627884/
  5. Grahn, D., Dillon, J., & Heller, C. (2009). Heat Loss Through the Glabrous Skin Surfaces of Heavily Insulated, Heat-Stressed Individuals. Journal of Biomechanical Engineering. 31,
  6. Grahn, D, Makam, M, & Heller, C. (2018) A method to reduce heat strain while clad in encapsulating outerwear, Journal of Occupational and Environmental Hygiene, 15:8, 573-579, DOI: 10.1080/15459624.2018.1470635
  7. Walløe L. (2015). Arterio-venous anastomoses in the human skin and their role in temperature control. Temperature (Austin, Tex.), 3(1), 92–103. https://doi.org/10.1080/23328940.2015.1088502
  8. Grahn, D., Vinh C., & Heller, C. (2005). Heat extraction through the palm of one hand improves aerobic exercise endurance in a hot environment. Journal of Applied Physiology, 99, 972-978.
  9. Goosey-Tolfrey, V., Swainson, M., Boyd, C., Atkinson, G., & Tolfrey, K. (2008). The effectiveness of hand cooling at reducing exercise-induced hyperthermia and improving distance-race performance in wheelchair and able-bodied athletes. Journal of Applied Physiology, 105, 37–43. doi:10.1152/japplphysiol.01084.2007
  10. Ruddock, A. D., Tew, G. A., & Purvis, A. J. (2017). Effect of hand cooling on body temperature, cardiovascular and perceptual responses during recumbent cycling in a hot environment. Journal of Sports Sciences, 35(14), 1466–1474. https://doi.org/10.1080/02640414.2016.1215501

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Reverse the Impact of Stress on the Brain with Exercise

Posted on January 9, 2020 | Leave a comment

Departing the 2010’s, feelings of stress and worry among Americans were the highest they had been all decade and among the highest in the world [1]. I doubt that drops any jaws. We are a bit obsessed with stress – many trying to rid ourselves of the brute through everything from mindfulness and meditation to yoga retreats and medication. I love the work by Dr. Susan David, who has concluded that all of this energy expended to ‘rid ourselves of stress’ can actually make a person more stressed (dubbed type II stress). Oh, the irony!

Stress is an inescapable aspect of our lives, and it is undoubtedly useful in short-term fight-or-flight situations, helping us better mobilize resources like glucose and oxygen. It’s the prolonged and unrelenting variety of stress that comes at a cost to the brain. In particular, a robust field of research has found that chronic stress shrinks the hippocampus [2, 3]. This region has long been implicated in learning and memory hippocampus_free licensure(episodic & spatial) [2, 4]. As a powerful illustration, atrophy in the hippocampus is the hallmark of Alzheimer’s disease. It is one of the most susceptible brain regions to cell death, and it may be particularly vulnerable to the impact of long-term stress because it has a high concentration of cortisol receptors [5].

So, that’s the bad news. Here’s the good: because the hippocampus exhibits so much adaptive plasticity, the effects produced by chronic stress are largely reversible. Better still, there is a free and readily accessible intervention that can dampen and even reverse these effects. I’ll give you one guess.

Most people have an anecdotal appreciation for the benefits of exercise on stress and get a feel-good buzz after a bout of exercise. What is often underappreciated is that exercise reaps very tangible anatomical and physiological benefits to the brain.  Cue my favorite study [6].

One hundred and twenty older adults were recruited and randomly assigned to 3 days a week of either moderate-intensity aerobic exercise or stretching/toning (control).*  After one year of this intervention, MRI scans showed a 2% increase in hippocampal volume for the aerobic exercise group (see graph)[6]. Exercise grew brains. Unsurprisingly, but still remarkably – the  hippocampal growth corresponded to improved memory function.

Erickson_graph 6.35.41 AM

Notice in the figure how the red line (stretching/control group) slopes down? These data support previous research showing a 1-2% decline in hippocampal volume each year in older adults, free of dementia [8]. That trajectory is inverted with the exercise group (blue line). Getting out the door for a walk 3 days a week effectively reversed age-related loss in the hippocampus by 1 to 2 years. Powerful stuff, exercise.

Now, back to stress. While there haven’t been any exercise interventions in people that look specifically at reversing the effects of stress, it doesn’t take a huge leap of fate to connect the dots here. We know chronic stress shrinks the hippocampus while exercise grows it.** Still, I like to be persuasive, and findings from a recent study in mice is just that.

Researchers at BYU had one group of mice run on wheels (covering ~5k day), while the other group remained sedentary [9]. Half of each group was also exposed to stressful situations – swimming in cold water or walking on an elevated platform. They then looked in the hippocampi of mice to assess a process called long-term potentiation (LTP). Memory formation and recall are most effective when connections between neurons are strengthened over time. LTP is a measure of this connectivity, and ultimately memory.  In the sedentary group, researchers found chronic stress weakened the synaptic connections between neurons, decreasing LTP. The mice allowed to run while exposed to stress had significantly higher LTP, and they performed better on a memory test (radial arm maze) [9]. The research team concluded, “exercise is a viable method to protect learning and memory mechanisms from the negative cognitive impact of chronic intermittent stress on the brain.”

In terms of how exercise supports the hippocampus – we believe it is a combination of increased blood flow to the brain and hippocampus [10] as well as through increased production of BDNF [6, 9, 11]. Brain derived neurotrophic factor (BDNF) is essentially a growth factor for neurons…miracle grow for brain cells.

Ridding ourselves of all the stressors in our lives is not a particularly practical option for most of us. Yet, the negative toll that stress can take is not a phenomenon beyond our control. I think that is downright empowering. So, transitioning into this next decade, resolve to move for your physical health, move for your mental health, and move for your brain health.

*Noteworthy – both groups in the study attended the same number of exercise sessions and therefore had an equal amount of social interactions. This is important because human interactions also have very real benefits on brain function [7] and may be less frequent in the older adult population.  In other words, exercise appears to protect our brains in a way that other interactions cannot. This appears to be true, even if you start moving in your 70s.

**Aerobic training grows brain regions other than the hippocampus. Another exercise intervention in older adults found increased gray and white matter volume in the prefrontal cortex after 6-months [12]. This region is instrumental to executive control functions.

References

[1] Chokshi, N. (2019, April 25). Americans Are Among the Most Stressed People in the World, Poll Finds. The New York Times.  Retrieved from https://www.nytimes.com/2019/04/25/us/americans-stressful.html

[2] Conrad, C. (2010). A critical review of chronic stress effects on spatial learning and memory. Prog Neuropsychopharmacol Biol Psychiatry 34(5): 742-755.

[3] Conrad, C. (2008). Chronic stress-induced hippocampal vulnerability: the glucocorticoid vulnerability hypothesis. Reviews in Neurosciences 19(6): 395-411. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2746750/

[4] McEwen, B., Nasca, C., Gray, J. (2016) Stress effects on neuronal structure: hippocampus, amygdala, and prefrontal cortex. Neuropsychopharmacology Reviews, 41, 3-23.

[5] Kino, T. (2015). Stress, glucocorticoid hormones, and hippocampal neural progenitor cells: implications to mood disorders. Frontiers in Physiology 19. https://www.frontiersin.org/articles/10.3389/fphys.2015.00230/full

[6] Erickson et al. (2011). Exercise training increases size of hippocampus and improves memory. Proceedings of the National Academy of Sciences, 108 (7): 3017-3022.   

[7] Hari, R. & Kujala, M. (2009). Brain basis of human social interactions: from concepts to brain imaging. Physiological Reviews, 89, 453-479.

[8] Raz N, et al. (2005) Regional brain changes in aging healthy adults: General trends, individual differences and modifiers. Cereb Cortex, 15, 1676–1689.

[9] Miller, R. et al. (2018). Running exercise mitigates the negative consequences of chronic stress on dorsal hippocampal long-term potentiation in male mice. Neurobiology of Learning and Memory, 149. DOI: 10.1016/j.nlm.2018.01.008

[10] Burdette J., et al. (2010) Using network science to evaluate exercise-associated brain changes in older adults. Front Aging Neurosci 2:23.

[11] Vaynman S., Ying Z., Gomez-Pinilla F. (2004) Hippocampal BDNF mediates the efficacy of exercise on synaptic plasticity and cognition. European Journal of Neurosciences 20:2580–2590.

[12] Colcombe S., et al. (2006) Aerobic exercise training increases brain volume in aging humans. J Gerontol A Biol Sci Med Sci, 61, 1166–1170.

 

 

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Heat Training May Boost Performance at Altitude

Posted on July 20, 2016 | 1 comment

Rather than escape the blistering heat of summer, there may be good reason to embrace it. For the first time, research has shown that heat acclimation can lead to performance gains similar to that achieved through hypoxic (low-oxygen) training.

The benefits of training at altitude have long been heralded by endurance athletes. Essentially a natural form of doping, the rise in erythropoietin (EPO) is swift in most individuals – helping to boost red blood cells and oxygen-carrying capacity of the blood within 24-48hrs of arriving at altitude.

In the real world, most athletes aren’t fortunate enough to live at or near altitude and don’t want to drop the money to snooze in an altitude tent. Luckily, sweltering summer heat is readily accessible.

UK researchers asked male cyclists to complete a self-paced hypoxic time trial before and after a series of 10 daily 60-minute training sessions in either hot (40oC), hypoxic, or control conditions. In the hypoxic condition, the percent oxygen approximated a target altitude of about 10,000ft.

IMAG0635 - Version 2

Findings, published in Frontiers in Physiology, showed that hypoxic time trial performance and power output were comparably improved for the cyclists who trained in both the low-oxygen and hot environments. Those who trained in the control conditions saw no gains. Similarly, blood results indicated that the 10-day training protocol in both the hot and hypoxic conditions improved cellular tolerance to exercise. Heat shock protein (Hsp) 72 was of particular interest, as it has been linked to delayed tissue injury and cellular resilience to environmental stressors (such as… hypoxia).

Moreover, the authors note, “Heat acclimation induced a greater adaptive stimulus at lower levels of metabolic strain, and in a shorter time frame compared to hypoxic acclimation.” These conclusions were based on the finding that the heat-trained group showed a more efficient aerobic profile, with increased hemoglobin saturation, a higher oxygen pulse (more O2 uptake per heartbeat at rest), and a significantly lower exercise heart rate following acclimation. There was also a smaller increase in body temperature during exercise for the heat-trained cyclists.

The implications here are potentially huge for athletes, suggesting that heat-based training could offer a more proficient means of improving altitude tolerance than equivalent training at actual altitude. An important caveat here is that hypoxic conditions were normobaric. Because pressure wasn’t altered, it’s not yet known whether these findings would hold at natural (i.e., hypobaric) high-altitude environments.

In addition, these researchers went quite hot with their heat-trained group, subjecting them to 40oC (104oF) conditions. Hopefully, the minimum temperatures necessary to reap these physiological benefits are not quite so stifling!

So, if you’re preparing for a high-altitude trek, ultramarathon, adventure race, or any other aerobic event up high, you might want to simply tap into the cardiovascular and cellular benefits provided by the ever-accessible summer heat. You may be a hot sweaty mess, but you’ll be an endurance phenom!

 

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Claims that exercise while expecting might boost brain development

Posted on November 15, 2013 | 3 comments

We’ve come a long way from the days when obstetricians advised women to take it easy during pregnancy.  Today, healthy expecting mothers are encouraged to stay active for reasons ranging from decreased back pain and sleep problems to easier post-partum recovery and reduced risk of child obesity.  Now there is preliminary research to suggest that exercising during pregnancy might even give your child’s brain a jump-start on development.

As reported at last week’s Society for Neuroscience (SfN) annual conference, researchers from the University of Montreal randomly assigned women at the start of their second trimester to either an exercise or a sedentary group.  Women in the exercise group were told to perform at least 20 minutes of aerobic exercise three times a week at a moderate intensity.  Although, the study abstract notes that women in this group went well beyond the 60-minute per week minimum and exercised an average of 117 minutes each week.

A mere 8 to 12 days after delivery, researchers invited all mothers and their newborns back to the lab. Babies were fitted with specialized caps made up of soft electrodes that detect electrical activity in the brain (a tool called electroencephalography (EEG)). Scientists waited for the newborns to fall asleep on their mother’s lap and then monitored how their unconscious brains responded to both familiar and novel sounds – a proxy for auditory memory.

“This is important to look at at this stage in their development, because the ability to discriminate sounds is the basis of learning to speak and to understand the sounds around you,” notes Elise Labonte-LeMoyne from the research team.

The researchers reported that the babies whose mothers had exercised exhibited more mature patterns of cerebral activation. They also suggested that this could indicate their brains are developing more rapidly. The study’s lead researcher, Dr. Dave Ellemberg said in a statement, “We are optimistic that this will encourage women to change their health habits, given that the simple act of exercising during pregnancy could make a difference for their child’s future.”

Before the media gets ahold of this news and conveys the message to expecting mothers that they can exercise their unborn children straight to the Ivy Leagues (woops, too late!), there are some important limitations to emphasize about this initial research. Although the findings were presented at the reputable Society for Neuroscience’s (SfN’s) annual meeting, the methods and findings have not yet undergone the rigors of a peer-review, and have not yet been published in a journal.

Hence, it’s difficult to really assess the quality of this work, as well as its generalizability.  Among other things, we know little about how scientists monitored or determined exercise intensity in the exercise group. It’s also not clear how sedentary the “sedentary” group really was. Did the women exercise up until delivery? We’re not sure of that either. Also, the only report that I saw on the sample size indicates that only 18 women participated.  It’s a start, but hardly conclusive.

Finally, long-term conclusions about the capability of a person’s brain can hardly be drawn based on the results from a single test of auditory memory that is given less than two weeks after entering the world.  To see if the benefits of prenatal exercise do have any lasting effects, Dr. Ellemberg and his colleagues plan to track the infants’ cognitive, motor, and language development until they are at least 1 year old.  I look forward to seeing those published results!

While these exercise findings are both novel and encouraging, they should – as with any science – be interpreted with caution.

[Here’s a nice video from the University of Montreal’s kinesiology lab group for anyone who speaks French 🙂 or might be interested in some visual footage of the study’s methods].

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Exercise to Fend Off Daily Stress

Posted on March 6, 2013 | 3 comments

At any given point in time, most people are feeling stressed out about something. Unfortunate though it is – having stress in your daily life is often unavoidable. But being stressed is something within your control. stress_zebra stripes

One way to exercise that control is to — exercise. A single bout of exercise may offer protection against the emotional events and stress that we encounter later in the day.  That’s the conclusion of new research from the University of Maryland (yes, I’m biased, but this is good work). The immediate ability of physical activity to boost mood and ease anxiety is well established. However, this study found that the stress-busting effects of exercise endure after we’ve left the gym, and more importantly, in the face of subsequent psychological stress. In other words, getting up and moving may make us more resilient to the effects of stress.  If we know there’s no escaping a stressful event, we can at least prepare ourselves to better handle it by breaking a little sweat earlier in the day.

The study, published in the February issue of Medicine and Science in Sports, measured how anxiety levels in college students differed following 30 minutes of moderate intensity cycling (described as “somewhat hard”) and 30 minutes of seated rest. To assess anxiety, students completed the State-Trait Anxiety Inventory (STAI). This tool essentially gathers a snapshot of a person’s emotional state by having them rate subjective feelings of tension, apprehension, nervousness, and worry. On one day, participants answered questions on the STAI before exercise, 15 minutes after exercise, and finally – after viewing a series of images, known to provoke emotion. These include highly arousing unpleasant, pleasant, and neutral pictures. On a separate day, students followed the same procedure with the resting condition.

Researcher J. Carson Smith found that once the emotional picture-viewing period ended, anxiety state rose back up to baseline levels following the resting condition but remained low following the exercise. As expected, exercise and seated rest were equally effective in reduced anxiety levels initially. Smith explains, “Even though you can feel better by just sitting and doing nothing, you’re probably not doing yourself as much good as you would be if you were to exercise to receive that same anti-anxiety effect.”

Americans between 18 and 33-years-old – the so-called Millennials – are more stressed than the rest of the population, according to a recent report from the American Psychological Association.  However, they are also more likely than other generations to turn to sedentary activities to cope with this stress.  Listening to music was most widely reported (59% of young adults), but playing video games, surfing the Internet, and eating were also common ways of managing daily stress. These findings should encourage people to adopt physically active coping mechanisms in lieu of such sedentary ones.

It is still unclear exactly how long the anxiety-reducing effects from exercise last, but this study found benefits on mood approximately one hour after the exercise stopped. The duration of these effects, as well as the intensity and length of exercise needed to optimize them, has also yet to be established.

It is popular belief that endorphins are to thank for these effects, but Smith cautions that very little evidence supports a role of endorphins here. Instead, he hypothesizes that “the neural networks related to reward and motivation are being modulated and tuned such that there is a resistance to the effects of the accumulation of stress.” The use of neuroimaging will shed more light on whether and how these neural networks, or interconnected groups of brain cells, may be systematically impacted by exercise.

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23 and 1/2 hours

Posted on January 3, 2013 | Leave a comment

I thought that the advent of the new year was a perfect time to revive this wonderful visual lecture. Here, Dr. Mike Evans addresses the importance of exercise and the countless health benefits it brings. Worth the watch!  

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Endurance May Have Shaped the Evolution of the Brain

Posted on January 1, 2013 | Leave a comment

With few exceptions, it’s believed that humans are the only species in the animal kingdom capable of running a marathon. Horses are natural sprinters, sled dogs can be bred and diligently trained for feats such as the Iditarod, and while pronghorn antelope can maintain speeds of 40mph, it is thought they cannot do so much beyond 30 consecutive minutes.  Why do we have such a rare capacity for endurance? Is it simply a lucky by-product of our ability to walk on two legs?

exercise_brain evolutionA landmark 2004 paper in the journal Nature, written by Daniel Lieberman (Harvard) and Dennis Bramble (University of Utah), hypothesized that our bodies evolved as they did largely because of the need to run long distances. In addition to longer legs, shorter toes, broader shoulders, and the ability to shed heat through sweat, adaptations enabling trunk and head stabilization, balance, and powerful spring-like mechanics (such as the Achilles tendon) would have all enhanced endurance running faculties. These traits may have been specifically selected for because running would have benefited hunting, foraging, and scavenging abilities. It’s even been hypothesized that early ancestors brought down large prey by relentlessly chasing them to the point of exhaustion. Greater access to food energies (like protein-rich animal sources) would have meant greater reproductive success. Thus, adept early runners would have passed along their DNA. In this way, endurance shaped our bodies.

Importantly, Lieberman and Bramble concluded that our endurance running capabilities likely began during the early evolution of the genus Homo.  The first members of this genus, which originated about 2 million years ago, also stood apart from predecessors because brain size was beginning to rapidly increase (see figure at bottom). The onset of endurance capabilities and this increase in cranial capacity have long been considered two separate phenomenon.  However, a new theory suggests that physical endurance may not have only shaped the human body, but also driven the growth & development of the human brain.

Proposed by anthropologists David Raichlen (University of Arizona) and John Polk (University of Illinois), this theory explains how adaptations intended to improve endurance would have secondary benefits on brain capacities. Aerobic abilities would have become increasingly advantageous as our ancestors – beginning with Homo erectus – shifted to a highly active hunting and gathering lifestyle.

Raichlen and Polk’s paper, available in this month’s publication of Proceedings of the Royal Society Biology, pulls from several lines of research. Among them were evolution, or artificial selection, experiments. Mice were systematically bred for either high aerobic capacity (VO2max) or for their inclination to run (lots of time on the wheel). Mice who naturally expressed either trait were interbred for several generations. In both cases, the resulting line of mice exhibited markedly high baseline levels of certain growth factors and neurotrophins. These compounds (i.e., VEGF, IGF-1, & BDNF) boost aerobic abilities by enhancing metabolic regulation and oxygen transport.

But, these scientists weren’t just breeding running superstars. These mice showed increased brain growth and even superior performance on cognitive tasks. The same growth factors and neurotrophins that promote cardiovascular endurance also boost the growth and health of neurons in the brain. Foremost among these is brain-derived neurotrophic factor (BDNF) – a critical link in how exercise benefits brain structure and function in humans. If the changes that took place over the course of several generations were applied on an evolutionary scale, it is not difficult to see how adapting a highly active lifestyle could have driven brain growth 2 million years ago.

None of this is to suggest that the evolution of an active lifestyle was the only factor that boosted brain size and function. Selection pressures acting specifically on cognitive abilities (rather than athletic ones) may have also played a part. In fact, this has been the prevailing theory until now. For example, improved spatial and relational memory, as well as planning abilities, would have benefited long-distance foraging and the coordination of group hunts. Social skills needed to interact with others would have also required higher-order patterns of thought. Numerous factors likely contributed to human brain evolution, but this theory is the first to suggest that an active lifestyle – in and of itself – was among them.

So, what does all of this mean for us? It suggests that if movement helped mold the structure and function of our brains, then our brains likely still require regular physical activity to function optimally. Substantial research now exists to support the idea that regular exercise leads to more robust cognitive abilities in both children and adults.  But, could it be that these studies are not actually demonstrating that exercise boosts cognitive abilities, but that the absence of exercise adversely affects them? By adapting sedentary lifestyles, so far removed from that of our hunter-gatherer ancestors, it may be that more than our waistlines suffer.

evolution_brain

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Chasing the Blues Away

Posted on December 11, 2012 | Leave a comment

We are bombarded with information that partitions exercise and movement into isolated entities. We learn about heart rate zones, optimal step counts, and the strengths and limitations of maximal oxygen consumption (VO2max). We analyze power output and revolutions per minute, and critique stride length and leg turnover.  Yet, what ultimately brings so many of us back to the physical activities of our choosing is simply the fact that they make us feel good.

tiggereeyore

Indeed, the ability of exercise to lift spirits is quite profound and well established in the scientific literature.  There is now evidence that both aerobic and anaerobic exercise (namely moderate-intensity weight lifting) have the ability to significantly increase positive feelings, energy, and calmness, while reducing negative emotions and tiredness.  Yet, how is it that moving our bodies has any influence at all on our psychological state?

Numerous theories have surfaced over the years to account for the beneficial effects of exercise on mood.  Some have a psychological basis, while others stem from more biological roots. I’ve highlighted some of the mechanisms below that have received the most support from human and animal studies. These same theories also explain how exercise can reduce symptoms of chronic mood disorders, like depression.

Biological mechanisms

  • Elevated Endorphins:  There is significant evidence to show that endorphins are secreted as a result of exercise.  The word endorphin comes from endogenous + morphine, which loosely translates to internally produced pain-killers.  While these chemicals do not act directly on the brain (cannot pass blood-brain barrier), they do have a calming effect in both humans and animals. In support of this mechanism, several studies have found that when endorphins are blocked, exercise does not have the same ability to relieve stress and anxiety.  Decreased endorphin levels may also explain why habitually active people often become irritable and restless when deprived of exercise.  ahem.
  • Monoamine Hypothesis: Monoamines refer to a group of neurotransmitters that include norepinephrine (NE) and serotonin (5-HT).  Common anti-depressant drugs (Prozac, Zoloft, Paxil, etc.) function by regulating the release of these chemicals in our brains.  Substantial research suggests that physical activity modifies the release of NE and 5-HT in much the same way, making  these neurotransmitters more readily available in synapses.
  • BDNF Hypothesis: This theory primarily speaks to the long-term effects of exercise and may be especially significant in explaining how exercise can help depression. Brain-derived neurotrophic factor (BDNF) promotes neural growth and generally works to keep neurons healthy.  Some have described it as “miracle grow” for the brain, and it is key to many of the topics I blog about.  It is relevant here because BDNF also has antidepressant effects.  Both antidepressant drugs and exercise increase levels of BDNF in the hippocampus.  This region is classically associated with learning and memory, but has also been found to play a role in depression.  More on these important relationships later!
  • Other biological theories include the reduction of cortisol (a stress hormone) and better regulation of the hypothalamic-pituitary-adrenal (HPA) axis.  These have strong implications for mood, but more directly impact stress and anxiety.

Psychological mechanisms

  • Sense of Mastery and Self-efficacy:  Because exercise is often viewed as a challenging activity, the ability to commit to it on a regular basis often results in a sense of control, accomplishment, and satisfaction.  Merely setting fitness goals can also fuel a sense of purpose.  These can all contribute to both immediate and long-term boosts in mood.
  • Distraction:  Exercise can serve as a diversion from negative thoughts and other unfavorable stimuli (overflowing inboxes, screaming children…).  By forgetting our worries and focusing on our physical movements, we can temporarily alleviate stress and break cycles of unconstructive thinking.
  • Exposure to Nature:  Physical exercise often gives people a reason to get outside, and as discussed in my last entry – doing so can positively impact mood states.
  • Social Factors:  Interacting with others is important for our overall mental well-being. While some prefer the solitude of exercising alone, exercising with others can foster a reciprocal sense of support.  You may help another reach their fitness and performance goals, and someone else may do the same for you. An additional benefit of exercising with people is that you are less inclined to skip a workout if another is depending on you to be there.  Unsurprisingly, this increased adherence to exercise equates to more of it!

Most likely, the benefits of exercise on mood result from a combination of many of these theories, as well as underlying genetic factors.  Regardless of how exactly it is that exercise makes us feel good, I suspect most of us are just glad that it does!  There may be hope for Eeyore yet.

Comments and questions always welcome!

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