Forget 98.6°F. Humans Are Cooling off — Here’s Why
By Kristen Fischer
The "normal" body temperature of 98.6°F (37°C) is actually not so normal. New research finds the average human body temperature of Americans has dropped.
"What everybody grew up learning, which is that our normal temperature is 98.6, is wrong," said Dr. Julie Parsonnet, a professor of medicine as well as health research and policy at Stanford University School of Medicine.
The 98.6°F standard was established by a German doctor in 1851. Recent studies have indicated that's too high; research on 35,000 British people found their average was 97.9°F.
Parsonnet's study published this week in eLife. It found that temperature changes since 1851 reflect a historical pattern instead of an error. They contend the decrease is the result of environmental changes over the past 200 years that have affected human physiology.
Parsonnet looked at data from 1862 through 1930, 1971 through 1975, and 2007 through 2017. It included 677,423 temperature measurements.
The body temperature of men born in the 2000s is 1.06°F degrees lower, on average, than men born in the early 1800s. Women have temps about 0.58°F lower than those born in the 1890s. That means body temperatures declined 0.05°F every decade.
Parsonnet's team also found a decline in temperature since the 1960s, not just since the Civil War.
Body temperature is complicated, Parsonnet says. It can vary not just from person to person but at different times of day and at different ages.
It can be almost half a degree higher in the afternoon than morning. It's much lower in older adults. Your weight and height as well as hot or humid weather can also affect it, Parsonnet says.
In her experience, Parsonnet says at least 75 percent of normal temperatures are below 98.6°F.
"Even in younger adults at the end of the day, when temperatures are at their highest, the temperature still doesn't get up to 98.6," she said. "In the elderly, it would be quite unusual to have a temperature as high as 98.6."
"The threshold for fever is generally held as 1°C above normal," said Dr. Bradley Uren, assistant professor of emergency medicine at Michigan Medicine. "A fever is generally held as 38°C, or 100.4°F."
"It is rare to see a patient at exactly 98.6," he noted. "A temperature is just one piece of information that physicians must and do consider in determining a course of treatment for an individual."
Even though our temperatures can fluctuate, doctors still know what's generally normal. They also know to look for variations in temperature, which can indicate a medical condition, Uren says.
"Physicians must take into account the entirety of a patient's condition in determining if a temperature, either higher or lower, or even within the normal range, is significant," Uren said.
"For example, patients may be treated for infection even with a normal temperature if the other historical and physical findings, and the patient's overall condition, is consistent with infection," he said.
Dr. Amy Mullins, medical director of quality improvement for the American Academy of Family Physicians, notes that the study won't change how patients or doctors should define fevers.
Why Temps Are Falling
It's possible that some of these changes may be because thermometers provide more accurate readings, Parsonnet noted in her study.
However, that doesn't explain all of it. Temperatures declined over time even when her team controlled for different devices.
Susan Yeargin, Ph.D., associate professor of exercise science at University of South Carolina, has tested all types of temperature devices in diagnosing hyper- and hypothermia. They were inaccurate compared to the rectal temperature.
"Each person's unique normothermic body temperature should be considered by healthcare providers. Certain medications can influence a person's regular 'set point' temperature, and obviously hot and cold outdoor temperatures can affect a person's body temperature over time," Yeargin said.
"When healthcare providers give advice to individuals about these medications and environments, their unique normothermic body temperature should be considered," she said.
According to Parsonnet, other reasons for the decline in body temperature over time could be that we're also using less energy and have a lower metabolic rate than in the past.
The reduction may be due to a population-wide decline in inflammation. Generally, inflammation increases our metabolism and raises temperature.
Because of improvements in public health, this could be why inflammation has decreased. The ambient temperatures we live in, thanks to heating and air conditioning, could be factors in lower metabolic rates.
"I think it's most likely because we have much less inflammation in our bodies now than we did when the standard was developed in the mid-19th century," Parsonnet said.
"We have less inflammation because we have far fewer chronic infectious diseases like tuberculosis and periodontal disease, far less recurrent infection, shifts in our microbiomes, and we also have learned how to combat inflammation directly through better diets, and also with things like nonsteroidal drugs and statins," she explained.
In general, humans are physiologically different than we were in the past, Parsonnet says.
Reposted with permission from Healthline.
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By Melissa Gaskill
Two decades ago scientists and volunteers along the Virginia coast started tossing seagrass seeds into barren seaside lagoons. Disease and an intense hurricane had wiped out the plants in the 1930s, and no nearby meadows could serve as a naturally dispersing source of seeds to bring them back.
Restored seagrass beds in Virginia now provide habitat for hundreds of thousands of scallops. Bob Orth, Virginia Institute of Marine Science / CC BY 2.0<p>The paper is part of a growing trend of evidence suggesting seagrass meadows can be easier to restore than other coastal habitats.</p><p>Successful seagrass-restoration methods include <a href="https://www.sciencedirect.com/science/article/abs/pii/S0304377099000078?via%3Dihub" target="_blank">transplanting shoots</a>, <a href="https://onlinelibrary.wiley.com/doi/10.1111/j.1061-2971.2004.00314.x" target="_blank" rel="noopener noreferrer">mechanized planting</a> and, more recently, <a href="https://www.nature.com/articles/s41467-020-17438-4" target="_blank" rel="noopener noreferrer">biodegradable mats</a>. Removing threats, proximity to donor seagrass beds, planting techniques, project size and site selection all play roles in a restoration effort's success.</p><p>Human assistance isn't always necessary, though. In areas where some beds remain, seagrass can even recover on its own when stressors are reduced or removed. For example, seagrass began to recover when Tampa Bay improved its water quality by reducing nitrogen loads from runoff by roughly 90%.</p><p>But more and more, seagrass meadows struggle to hang on.</p><p>The marine flowering plants have declined globally since the 1930s and currently disappear at a rate equivalent to a football field every 30 minutes, according to the <a href="https://www.unep.org/resources/report/out-blue-value-seagrasses-environment-and-people" target="_blank" rel="noopener noreferrer">United Nations Environment Programme</a>. And research published in 2018 found the rate of decline is <a href="https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GB005941" target="_blank" rel="noopener noreferrer">accelerating</a> in many regions.</p><p>The causes of decline vary and overlap, depending on the region. They include thermal stress from climate change; human activities such as dredging, anchoring and coastal infrastructure; and intentional removal in tourist areas. In addition, increased runoff from land carries sediment that clouds the water, blocking sunlight the plants need for photosynthesis. Runoff can also carry contaminants and nutrients from fertilizer that disrupt habitats and cause algal blooms.</p><p>All that damage comes with a cost.</p>
The Value of Seagrass<p>As with ecosystems like rainforests and <a href="https://therevelator.org/mangroves-climate-change/" target="_blank">mangroves</a>, loss of seagrass increases carbon dioxide emissions. And that spells trouble not just for certain habitats but for the whole planet.</p><p>Although seagrass covers at most 0.2% of the seabed, it <a href="https://www.unenvironment.org/news-and-stories/story/seagrass-secret-weapon-fight-against-global-heating" target="_blank">accounts for 10%</a> of the ocean's capacity to store carbon and soils, and these meadows store carbon dioxide an estimated 30 times faster than most terrestrial forests. Slow decomposition rates in seagrass sediments contribute to their <a href="https://www.researchgate.net/publication/238506081_Assessing_the_capacity_of_seagrass_meadows_for_carbon_burial_Current_limitations_and_future_strategies" target="_blank" rel="noopener noreferrer">high carbon burial rates</a>. In Australia, according to <a href="https://onlinelibrary.wiley.com/doi/10.1111/gcb.15204" target="_blank" rel="noopener noreferrer">research</a> by scientists at Edith Cowan University, loss of seagrass meadows since the 1950s has increased carbon dioxide emissions by an amount equivalent to 5 million cars a year. The United Nations Environment Programme reports that a 29% decline in seagrass in Chesapeake Bay between 1991 and 2006 resulted in an estimated loss of up to 1.8 million tons of carbon.</p>
Eelgrass in the river delta at Prince William Sound, Alaska. Alaska ShoreZone Program NOAA / NMFS / AKFSC; Courtesy of Mandy Lindeberg / NOAA / NMFS / AKFSC<p>Seagrasses also protect costal habitats. A healthy meadow slows wave energy, reduces erosion and lowers the risk of flooding. In Morro Bay, California, a 90% decline in the seagrass species known as eelgrass caused extensive erosion, according to a <a href="https://www.sciencedirect.com/science/article/abs/pii/S0272771420303528?via%3Dihub" target="_blank" rel="noopener noreferrer">paper</a> from researchers at California Polytechnic State University.</p><p>"Right away, we noticed big patterns in sediment loss or erosion," said lead author Ryan Walter. "Many studies have shown this on individual eelgrass beds, but very few studies looked at it on a systemwide scale."</p><p>In the tropics, seagrass's natural protection can reduce the need for expensive and often-environmentally unfriendly <a href="https://www.nioz.nl/en/news/zeegras-spaart-stranden-en-geld" target="_blank" rel="noopener noreferrer">beach nourishments</a> regularly conducted in tourism areas.</p><p>Seagrass ecosystems improve water quality and clarity, filtering particles out of the water column and preventing resuspension of sediment. This role could be even more important in the future. By producing oxygen through photosynthesis, meadows could help offset decreased oxygen levels caused by warmer water temperatures (oxygen is less soluble in warm than in cold water).</p><p>The meadows also provide vital habitat for a wide variety of marine life, including fish, sea turtles, birds, marine mammals such as manatees, invertebrates and algae. They provide nursery habitat for <a href="https://wedocs.unep.org/bitstream/handle/20.500.11822/32636/seagrass.pdf?sequence=1&isAllowed=y" target="_blank" rel="noopener noreferrer">roughly 20%</a> of the world's largest fisheries — an <a href="https://www.floridamuseum.ufl.edu/science/seagrass-meadows-harbor-wildlife-for-centuries/" target="_blank" rel="noopener noreferrer">estimated 70%</a> of fish habitats in Florida alone.</p><p>Conversely, their disappearance can contribute to die-offs of marine life. The loss of more than 20 square miles of seagrass in Florida's Biscayne Bay may have helped set the stage for a widespread <a href="https://www.wlrn.org/2020-08-14/the-seagrass-died-that-may-have-triggered-a-widespread-fish-kill-in-biscayne-bay" target="_blank">fish kill</a> in summer 2020. Lack of grasses to produce oxygen left the basin more vulnerable when temperatures rose and oxygen levels dropped as a result, says Florida International University professor Piero Gardinali.</p>
Damaged Systems, a Changing Climate<p>Governments and conservationists around the world have already put a lot of effort into coastal restoration efforts. And that's helped some seagrass populations.</p><p>Where stressors remain, though, restoration grows more complicated. <a href="https://www.rug.nl/research/portal/en/publications/the-future-of-seagrass-ecosystem-services-in-a-changing-world(3a8c56db-7bed-4c9e-ac7f-c72453e2a102).html" target="_blank">Research</a> published this September found that only 37% of seagrass restorations have survived. Newly restored meadows remain vulnerable to the original stressors that depleted them, as well as to storms — and <a href="https://www.ecowatch.com/tag/climate-crisis">climate change</a>.</p>
Seagrass in Dry Tortugas National Park, Florida. Alicia Wellman / Florida Fish and Wildlife / CC BY-NC-ND 2.0<p>In Chesapeake Bay a cold-water species of seagrass is currently hitting its heat limit, especially in summer, according to Alexander Challen Hyman of University of Florida's School of Natural Resources and Environment. As waters continue to warm due to climate change, the species likely will disappear there.</p><p>Climate-driven sea-level rise complicates the problem as well. Seagrasses thrive at specific depths — too shallow and they dry out or are eaten, too deep and there isn't enough light for photosynthesis.</p>
But There’s Good News, Too<p>Luckily, left to its own devices, a seagrass meadow can flourish for hundreds of years, according to a <a href="https://royalsocietypublishing.org/doi/10.1098/rspb.2019.1861" target="_blank" rel="noopener noreferrer">paper</a> published last year by Hyman and other researchers from the University of Florida. The researchers arrived at their conclusion by looking at shells of living mollusks and fossil shells to estimate the ages of meadows in Florida's Big Bend region on the Gulf Coast.</p><p>That area has extensive, relatively pristine seagrass meadows. "Our motivation was to understand the past history of these systems, and shells store a lot of history," said co-author Michal Kowalewski.</p><p>A high degree of similarity between living and dead shells indicates a stable area, while a mismatch suggests an area shifted from seagrass to barren sand. The researchers found that long-term accumulations of shells resembled living ones, suggesting that the seagrass habitats have been stable over time.</p><p>That stability allows biodiversity to thrive, creating conditions where specialist species can survive and flourish, according to Hyman.</p><p>Discovering the long-term stability of seagrass meadows has implications for choosing restoration sites, Kowalewski notes.</p><p>"There must be reasons they thrive in one place, while a mile away they don't and fossil data says they probably never did," he said. "If we remove a seagrass patch, we cannot hope to plant it somewhere else. It's not just the seagrass that is special. The location at which it's found is special, too."</p><p>A better approach is conserving these habitats in the first place, but we're not doing enough of that right now. The UN reports that marine protected areas safeguard just 26% of recorded seagrass meadows, compared with 40% of coral reefs and 43% of mangroves.</p>
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