Your body temperature is regulated by complex communication with your hypothalamus, vascular system and skin.1 It is a measure of how well you can make and release heat. The objective is to maintain your body at a consistent temperature to provide your organs, enzymes and hormones an optimal environment. When your body temperature rises (hyperthermia) or falls (hypothermia) it can be dangerous or life threatening.2
On average, the temperature taken in your rectum or ear may be slightly higher than one that is taken across your forehead or under your arm. When taken in your mouth it will lie between one taken rectally and one under your arm.
Your temperature may fluctuate during the day and across different areas of your body. For instance, it may rise or fall by 1 degree Fahrenheit (F) depending upon your activity level and environmental temperature. Temperature is also sensitive to hormonal levels, which helps women estimate ovulation by taking their temperature before getting out of bed in the morning.
Body temperature has become an integral part of measuring illness, yet the origin of the generally accepted 98.6 F as a normal temperature is linked to a single study.3 Carl Reinhold August Wunderlich is credited with the clinical application of a thermometer and the determination of a normal temperature.
Normal Temperatures Have Dropped to Lower Than 98.6
Researchers from Stanford University4 undertook a massive study5 to evaluate whether the norm established in 1850 still was accurate. They found the average temperature is not what it once was. Dr. Julie Parsonnet, professor of medicine, says, “What everybody grew up learning, which is that our normal temperature is 98.6, is wrong.”
Personnet and her team evaluated three datasets over different historical periods: from military service medical and pension records of U.S. Army Veterans of the Civil War; from the U.S. National Health and Nutrition Examination Survey from 1971 to 1975; and from the Stanford Translational Research Integrated Database Environment pulled from 2007 to 2017.
In total there were 677,423 measurements from which they developed a model to find trends. They determined that men born in the early to mid-1990s had temperatures averaging 1.06 F lower than men born in the 1800s. The average temperature of women born in the 1990s was 0.58 F lower than that of women from a century earlier.
The numbers reflected an average decrease of 0.05 F every 10 years. The researchers also questioned if the drop was the result of better instrumentation. However, they found a similar decline in temperatures in men in the veteran’s dataset with each decade where the same instruments were used. Parsonnet commented on the results of the study:
“Physiologically, we’re just different from what we were in the past. The environment that we’re living in has changed, including temperatures in our homes, our contact with microorganisms and the food that we have access to. All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re actually changing physiologically.”
The researchers postulated the reductions may be a result of different ambient environments. However, Kenneth Welch, Ph.D., who was not involved in the study, proposed that many of the medications people in the U.S. are taking, such as ibuprofen and statins, affect temperature measurement.6
Thinking along that line, Civil War veterans would not have consistently taken medications as people currently do. Although the results are interesting, Welch doesn’t believe it will make a difference in the day-to-day lives of individuals. He believes an alteration in normal or average temperatures is important to note, but it is crucial to discern when a change may indicate a serious condition.
Is a Fever Still a Fever?
Welch commented that even infections without any significant symptoms may trigger a small rise in temperature. Body temperature increases may be caused by environmental factors, infections, reactions to vaccinations or medication, or even an allergy.7
Rising body temperature is called a fever, even when the increased measurement is not caused by a viral or bacterial infection. However, this type of high temperature doesn’t usually trigger the same body aches or headaches you experience with an infection.
Conversely, hypothermia (low body temperature) is what happens when you lose heat faster than your body can create it. This is a medical emergency.8 As your body temperature drops, your organs and vital systems can’t do their job; this has to be treated immediately or it will be fatal.
To regulate temperature your hypothalamus communicates with your skin, body fluids, salt concentrations, blood vessels and sweat glands9 in a process called thermoregulation.10 When you experience a fever, it’s a natural response to an external stimulus, such as infection, allergy or a reaction to a medication. In other words, something is out of the ordinary.
In adults, a temperature at 103 F or higher is cause for medical attention. However, in infants and toddlers, even low rises in temperature may indicate a serious infection.11 Symptoms that may accompany a fever, depending on the trigger, may include:
Sweating and chills
Loss of appetite
Can Temperature Variation Predict Mortality?
Another research team designed a six-year study observational to determine if variations in individual body temperature were correlated with measurements of health.12 From 2009 to 2014 they enrolled 35,488 patients from outpatient clinics at a large teaching hospital who presented without infection, fever or having been prescribed antibiotics at the visit.
The group consisted of 64% women and 41% who were of a nonwhite race. The study was designed to examine any correlations among medical health conditions, demographics and one-year mortality rate.
The researchers measured each individual’s temperature in a room where the ambient temperature was the same for all participants. A total of 243,506 temperatures were taken. Interestingly, the mean (average) temperature of the large patient cohort was 97.88 F, similar to the decline in average temperatures found in the Stanford study.
The researchers also found that demographically, those with the lowest temperatures were older, while those with the highest were black women. When physical conditions were considered, participants with lower temperatures were more likely to have hypothyroidism, while those with higher temperatures were more likely to have a higher body mass index or a diagnosis of cancer.
After controlling for variables, the researchers determined that an unexplained temperature fluctuation should be taken seriously:
“Baseline temperatures correlated with demographics, comorbid conditions, and physiology, but these factors explained only a small part of individual temperature variation. Unexplained variation in baseline temperature, however, strongly predicted mortality.”
How to Take an Accurate Temperature
You have several different options for taking a temperature at home.13 A digital thermometer can be used in the mouth or rectum or under the arm. Purchase disposable protective sleeves to help keep things clean and to make sure you’re not passing germs from person to person.
If you plan to take both oral and rectal temperatures, have a separate thermometer for each use and make sure they’re labeled accordingly. While rectal thermometer readings are most accurate for infants younger than three months, talk with your pediatrician before doing this because it can perforate (poke a hole in) the rectum.14 When taking an oral temperature, wait at least 15 minutes after consuming anything to avoid an inaccurate reading.
A tympanic thermometer takes a digital reading from the inside of the ear canal. It must be positioned correctly to get an accurate and quick reading. You’ll find earwax and small canals interfere with accuracy, as will age. That’s why this type of thermometer isn’t recommended for use with newborns.
A temporal artery thermometer may be labeled as a forehead thermometer; this type uses an infrared scanner to test the temporal artery temperature. While this is more accurate for children three months and older, it is also more expensive and must be placed over the artery to get an accurate reading.
One type that may be useful in children is built into a pacifier. The time it takes to get an accurate reading from a digital pacifier thermometer is variable, from two to four minutes. Data show the temperature taken with a digital pacifier thermometer should be adjusted upward by 0.5 F to approximate a reading from a rectal thermometer.15,16 While glass mercury thermometers were once a staple, they are no longer recommended because mercury is toxic.
The type of thermometer you choose may not be as important as following the manufacturer’s directions to achieve an accurate reading, ensuring you don’t pass germs with use and you don’t rely on a temperature alone to determine how sick you may be. Consider additional symptoms, especially dehydration, lethargy and confusion.
Should You Feed a Fever, Starve a Cold or Vice Versa?
The question of whether to feed a fever and starve a cold (or the reverse) has been the subject of much debate.17 Since you may experience a fever anytime during the year, not just during cold and flu season, be on the lookout for symptoms of illness.
You can tell the difference between a cold and flu by the severity of your symptoms and how high your fever gets. The rule of thumb is that if you feel hungry, then eat, regardless of your temperature. The caveat to consider is whether you are vomiting; see my tips on what to do when this happens.
As it turns out, what you have been eating may make a difference in how sick you get — or not. In one animal study,18 mice fed a high-fat, low-carbohydrate diet had a lower inflammatory response and an improved ability to withstand flu. Those fed a standard diet were all infected with the virus after exposure, as compared to 50% of those eating a ketogenic diet.19
The researchers discovered the immune system in the group eating a keto diet promoted gamma-delta T cells in the lungs. These cells increase mucus production to protect from viral infection by trapping the virus and keeping it from spreading.
To eat a ketogenic diet, aim for 50% to 85% of your daily calories from healthy fats. In addition, seek to limit net carbohydrates to 20 to 50 grams each day. Your net carbohydrates are measured by subtracting grams of fiber from your total carbohydrates. You may improve this limitation by cutting out carbohydrates from grains and all forms of sugar, including fruit high in fructose.
Add healthy sources of fat to your daily intake, such as avocados, coconut oil, butter, seeds, olives, olive oil and fatty fish. Macadamia nuts and pecans are high in healthy fat and low in protein, making them ideal to add into your meals or as snacks.
Seek to include organic, pastured egg yolks, grass fed animal products, MCT oil and raw cacao to raise your level of healthy fats. Avoid all trans fats and vegetable oils as they trigger more cellular damage than excess carbohydrates. For more benefits, seek to incorporate a cyclical ketogenic diet with intermittent fasting to support your immune system and overall health.