Can Road Salt and Other Pollutants Disrupt Our Circadian Rhythms?
By Jennifer Marie Hurley
Every winter, local governments across the U.S. apply millions of tons of road salt to keep streets navigable during snow and ice storms. Runoff from melting snow carries road salt into streams and lakes, and causes many bodies of water to have extraordinarily high salinity.
At Rensselaer Polytechnic Institute, my colleague Rick Relyea and his lab are working to quantify how increases in salinity affect ecosystems. Not surprisingly, they have found that high salinity has negative impacts on many species. They have also discovered that some species have the ability to cope with these increases in salinity.
But this ability comes at a price. In a recent study, Rick and I analyzed how a common species of zooplankton, Daphnia pulex, adapts to increasing levels of road salt. We found that this exposure affected an important biological rhythm: The circadian clock, which may govern Daphnia's feeding and predation avoidance behaviors. Since many fish prey on Daphnia, this effect could have ripples throughout entire ecosystems. Our work also raises questions about whether salt, or other environmental pollutants, could have similar impacts on the human circadian clock.
Daphnia pulexBrian Mattes, CC BY-ND
Daily biological rhythms and the circadian clock
In studying how road salt affects aquatic ecosystems, the Relyea lab showed that Daphnia pulex can adapt to handle moderate exposures in as little as two and a half months. These levels ranged from 15 milligrams of chloride (a building block of salt) per liter of water to a high of 1,000 milligrams per liter—a level found in highly contaminated lakes in North America.
However, an organism's ability to adapt to something in its environment can also be accompanied by negative trade-offs. My lab's collaboration with Rick's began in an effort to identify these trade-offs in salt-adapted Daphnia.
In my lab, we study how our circadian rhythms allow us to keep track of time. We investigate how the molecules in our cells work together to tick like a clock. These circadian rhythms allow an organism to anticipate 24-hour oscillations in its environment, such as changes from light (daytime) to dark (nighttime), and are essential to an organism's fitness.
Rick and I hypothesized that adaptation to high salinity could disrupt Daphnia's circadian rhythms based on recent evidence showing that other environmental contaminants can disrupt circadian behavior. One important behavior in Daphnia that may be controlled by the circadian clock is the diel vertical migration—the largest daily biomass migration on Earth, which occurs in oceans, bays and lakes. Plankton and fish migrate down to deeper water during the day to avoid predators and sun damage, and back up toward the surface at night to feed.
Echogram illustrating the ascending and descending phases of diel vertical migration, in which organisms ascend and descend through the water column. The color scale reflects acoustic scattering by concentrations of organisms at different depths. DEEP SEARCH—BOEM, USGS, NOAA
Given what we know about circadian function, it would be logical to assume that exposure to pollution would not affect an organism's circadian rhythms. While circadian clocks can incorporate environmental information to tell the time of day, they are heavily buffered against most environmental effects.
To understand the importance of this buffering, imagine that the timing of an organism's day length responded to environmental temperature. Heat speeds up molecular reactions, so on hot days the organism's 24-hour rhythm could become 20 hours, and on cold days it might become 28 hours. In essence, the organism would have a thermometer, not a clock.
Adaptation to pollution affects key circadian genes
To determine whether clock disruption is a trade-off to pollutant adaptation, we first had to establish that Daphnia is governed by a circadian clock. To do this, we identified genes in Daphnia that are similar to two genes, known as period and clock, in an organism that serves as a circadian model system: Drosophila melanogaster, the common fruit fly.
We tracked the levels of period and clock in Daphnia, keeping the organisms in constant darkness to ensure that a light stimulus did not affect these levels. Our data showed that the levels of period and clock varied over time with a 24-hour rhythm—a clear indication that Daphnia have a functional circadian clock.
We also tracked the same genes in populations of Daphnia that had adapted to increased salinity. Much to my surprise, we discovered that the daily variation of period and clock levels deteriorated directly with the level of salinity the Daphnia were adapted to. In other words, as Daphnia adapted to higher salinity levels, they showed less variation in the levels of period and clock over the day. This demonstrated that Daphnia's clock is indeed affected by pollutant exposure.
We currently don't understand what causes this effect, but the relationship between salinity levels and decreased variation in the levels of period and clock offers a clue. We know that exposure to pollutants causes Daphnia to undergo epigenetic regulation—chemical changes that affect the function of their genes, without altering their DNA. And epigenetic changes often show a gradual response, becoming more pronounced as the causal factor increases. Therefore, it is likely that high salinity is inducing chemical changes through these epigenetic mechanisms in Daphnia to suppress the function of its circadian clock.
The broad effects of circadian clock disruptions
We know that environmental conditions can affect what the clock regulates in many species. For example, changing the sugar that the fungus Neurospora crassa grows on changes which behaviors the clock regulates. But to our knowledge, this study is the first to show that genes of an organism's core clock can be directly impacted by adapting to an environmental contaminant. Our finding suggests that just as the gears of a mechanical clock can rust over time, the circadian clock can be permanently impacted by environmental exposure.
This research has important implications. First, if Daphnia's circadian clock regulates its participation in the diel vertical migration, then disrupting the clock could mean that Daphnia do not migrate in the water column. Daphnia are key consumers of algae and a food source for many fish, so disrupting their circadian rhythms could affect entire ecosystems.
Second, our findings indicate that environmental pollution may have broader effects on humans than previously understood. The genes and processes in Daphnia's clock are very similar to those that regulate the clock in humans. Our circadian rhythms control genes that create cellular oscillations affecting cell function, division and growth, along with physiological parameters such as body temperature and immune responses.
The human circadian clock regulates the cycles of many bodily functions.NIH
When these rhythms are disrupted in humans, we see increased rates of cancer, diabetes, obesity, heart disease, depression and many other diseases. Our work suggests that exposure to environmental pollutants may be depressing the function of human clocks, which could lead to increased rates of disease.
We are continuing our work by studying how the disruption of Daphnia's clock affects its participation in the diel vertical migration. We are also working to determine the underlying causes of these changes, to establish whether and how this could happen in the human brain. The impacts we have found in Daphnia show that even a simple substance such as salt can have extremely complex effects on living organisms.
Reposted with permission from our media associate The Conversation.
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By Beth Ann Mayer
Since even moderate-intensity workouts offer a slew of benefits, walking is a good choice for people looking to stay healthy.
How to Rock Your Walk<p>Walking isn't just fun and healthy. It's accessible.</p><p>"Walking is cheap," says Dr. John Paul H. Rue, a sports medicine doctor at <a href="https://mdmercy.com/" target="_blank">Mercy Medical Center in Baltimore</a>. "You can do it anywhere at any time; [it] requires little to no special equipment and has many of the same cardio benefits as running or other more intense workouts."</p><p>Want to up your walking game? Try the tips below.</p>
Use Hand Weights<p>Cardio and strength training can go hand-in-hand when you add weights to your walk.</p><p>A <a href="https://journals.lww.com/acsm-msse/Fulltext/2019/03000/Associations_of_Resistance_Exercise_with.14.aspx" target="_blank">2019 study</a> found that weight training is good for your heart, and <a href="https://www.mayoclinicproceedings.org/article/S0025-6196(17)30167-2/abstract" target="_blank">research</a> shows it reduces the risk of developing a <a href="https://www.healthline.com/health/nutrition-metabolism-disorders" target="_blank">metabolic disorder</a> by 17 percent. People with metabolic disorders have a higher chance of being diagnosed with high cholesterol, high blood pressure, and diabetes.</p><p>Rue suggests not carrying weights for your entire walk.</p><p>"Hand weights can give you an added level of energy burning, but you have to be careful with these because carrying [them] over a long period of time or while walking could actually lead to some overuse injuries," he says.</p>
Make It a Circuit<p>As another option, consider doing a circuit. First, put a pair of dumbbells on your lawn or somewhere in your home. Walk around the block once, then stop and do some bicep curls and tricep lifts before walking around the block again.</p><p>Rue recommends <a href="https://www.healthline.com/health/exercise-fitness/running-with-weights" target="_blank">avoiding ankle weights</a> during cardio workouts, as they force you to use your quadriceps rather than hamstrings. They can also cause muscle imbalance, according to the <a href="https://www.health.harvard.edu/staying-healthy/wearable-weights-how-they-can-help-or-hurt" target="_blank">Harvard Health Letter</a>.</p>
Find a Fitness Trail<p>Strength training isn't limited to weights. You can get stronger by <a href="https://www.healthline.com/health/bodyweight-workout" target="_blank">simply using your body</a>.</p><p>Often found at parks, fitness trails are obstacle courses with equipment for pullups, pushups, rowing, and stretches to build upper and lower body strength.</p><p>Try searching "fitness trails near me" online, checking out your local parks and recreation website, or calling the municipal office to <a href="https://calisthenics-parks.com/" target="_blank">find one</a>.</p>
Recruit a Friend<p>People who workout together stay healthy together.</p><p><a href="https://bmcgeriatr.biomedcentral.com/articles/10.1186/s12877-017-0584-3" target="_blank">One study</a> showed that older adults who exercised with a group improved or maintained their functional health and enjoyed their lives more.</p><p>Enlist the help of a walking buddy with a regimen you aspire to have. If you don't know anyone in your area, apps like <a href="https://www.strava.com/" target="_blank">Strava</a> have social networking features so you can get support from fellow exercisers.</p>
Try Meditation<p>According to the <a href="https://www.nccih.nih.gov/research/statistics/nhis/2017" target="_blank">2017 National Health Interview Survey</a>, published by the National Institutes of Health, meditation is on the rise, and for good reason.</p><p>Researchers <a href="https://pubmed.ncbi.nlm.nih.gov/29616846/" target="_blank">found</a> that mind-body relaxation practices can regulate inflammation, <a href="https://www.healthline.com/health/biological-rhythms" target="_blank">circadian rhythms</a>, and <a href="https://www.healthline.com/health/glucose" target="_blank">glucose</a> metabolism, as well as lower <a href="https://www.healthline.com/health/high-blood-pressure-hypertension" target="_blank">blood pressure</a>.</p><p>"Any form of exercise can be turned into a meditation of some type, either by the surroundings you are walking in, like a park or trail, or by blocking out the outside world with music on your headphones," Rue says.</p><p>You can also play a podcast or download an app like <a href="https://www.headspace.com/headspace-meditation-app" target="_blank">Headspace</a> that has a library of guided meditations to practice while you walk.</p>
Do Fartlek Walks<p>Typically used in running, fartlek intervals alternate periods of increased and decreased speed. These are <a href="https://www.healthline.com/nutrition/benefits-of-hiit" target="_blank">high-intensity interval training (HIIT)</a> workouts, which allow exercisers to accomplish more in less time.</p><p><a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0154075" target="_blank">One study</a> showed that 10-minute interval training improved <a href="https://www.healthline.com/health/metabolic-syndrome" target="_blank">cardiometabolic</a> health, or lowered the risk of heart disease, stroke, and diabetes, just as well as working out at a continuous pace for 50 minutes.</p><p><a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0111489" target="_blank">Research</a> also shows that HIIT workouts increase muscle <a href="https://www.healthline.com/health/fast-twitch-muscles" target="_blank">oxidative</a> capacity, or the ability to use oxygen. To do a fartlek walk, try walking at an increased pace for 3 minutes, slow down for 2 minutes, and repeat.</p>
Gradually Increase Pace<p>A faster walking pace is associated with a lower risk of <a href="https://www.healthline.com/health/copd" target="_blank">chronic obstructive pulmonary disease (COPD)</a> and respiratory diseases, according to a <a href="https://pubmed.ncbi.nlm.nih.gov/30303933/" target="_blank">2019 study</a>.</p><p>Still, it's best not to go from a stroll to an Olympic-worthy power walk in a day. Instead, increase your pace gradually to prevent injury.</p><p>"Start by walking at a brisk pace for about 10 minutes per day, 3 to 5 days per week," Rue says. "Once you've done this for a few weeks, increase your time by 5 to 10 minutes per day until you get to 30 minutes."</p>
Add Stairs<p>You've likely heard that taking the stairs instead of an elevator is a way to add more movement into your daily routine. It's also a way to step up your walking. Stair climbing has been shown to <a href="https://www.sciencedirect.com/science/article/pii/S2211335519301123?via%3Dihub" target="_blank">decrease the risk of mortality</a> and can easily add a bit more challenge to your walk.</p><p>If you don't have stairs in your home, you can often find them outside a local municipal building, train station, or at a high school stadium.</p>
Is Your Walk a True Cardio Workout?<p>Not all walks are equal. A walk that's too leisurely may not provide enough burn to qualify as cardio. To see if you're getting a good workout, try to <a href="https://www.healthline.com/health/how-to-check-heart-rate" target="_blank">measure your heart rate</a> using a monitor.</p><p>"A target goal for a good walking workout heart rate is about 50 to 70 percent of your maximum heart rate," Rue says, adding that maximum heart rate is <a href="https://www.healthline.com/health/fitness-exercise/fat-burning-heart-rate" target="_blank">typically calculated</a> by 220 beats per minute minus your age.</p><p>You can also monitor how easily you can carry on a conversation while you walk to gauge your heart rate.</p><p>"If you can walk and carry on a normal conversation, that's probably a lower intensity walk," says Rue. "If you are slightly breathless but can still have a conversation, that's probably a moderate workout. If you are out of breath and can't talk normally, that's a vigorous workout."</p>
Takeaway<p>By shaking up your routine, you can add excitement to your workout and reap even more rewards than a basic walk provides. Increasing the pace and intensity of a workout will make it more effective.</p><p>Simply pick your favorite variation to add some spice to your next walk.</p>
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