Your body uses a complex thermoregulatory system to maintain a homeostatic body temperature, protecting your heart and other organs as well as your circulatory and immune systems.¹ For more than 100 years, a normal internal body temperature has been defined as 98.6 Fahrenheit (37 Celsius). 

However, this “normal” temperature is affected by the time of day, your environment, your hydration status and whether you’ve been exercising, to name a few variables. Your temperature is regulated by the hypothalamus, a small portion of the brain located between the pituitary gland and the thalamus.² 

Your temperature plays a significant role in keeping the body in homeostasis (in balance) affecting temperature regulation, thirst, sleep cycles, blood pressure and heart rate by secreting hormones telling your nervous system what to do.

Body temperature increases may be caused by environmental factors, an infection, a reaction to a vaccination or medication or even an allergy.³ Also called a fever, when high body temperature is not caused by a viral or bacterial infection it doesn’t usually trigger the same body aches or headaches you experience with an infection.

Conversely, hypothermia, or a low body temperature, is a medical emergency occurring when your body is losing heat faster than it can generate it.⁴ As your body temperature drops, your organs, respiratory system and nervous system are unable to work appropriately. Left untreated it leads to death.

To regulate temperature your hypothalamus communicates with your skin, body fluids, salt concentrations, blood vessels and sweat glands⁵ in a process called thermoregulation.

Upper Limit of Normal Temperature Lower Than You Thought

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.⁶ Carl Reinhold August Wunderlich is credited with the clinical application of a thermometer and the determination of a normal temperature.

Most medical professionals classify a temperature greater than 100.4 F as a fever.⁷However, you may have felt like you had a fever, but the thermometer didn’t break 100 F. Analysis of daily average temperatures in research participants show what you’re feeling is likely a fever.⁸

Researchers evaluated the current temperature axiom and designed a study using 148 healthy men and women ages 18 to 40 years. Temperatures were taken throughout the day for three consecutive days.

The researchers found the results conflicted with the historical normal temperature, finding 98.6 F (37 C), instead of 98.2 F (36.8 C), was the mean temperature in all subjects. The upper limit was not 100.4 F (38 C), but 99.9 F (37.7 C) instead.

The participants’ maximum temperature and mean temperatures were different during different times of the day. The data did corroborate that mean temperature was lowest at 6 a.m. and highest from 4 p.m. to 6 p.m. They recommended abandoning the concept of 98.6 F (37 C) as relevant and that the upper limit of those younger than 40 years should also be revised.

Diurnal Temperature Variability Factors Into Fever

Similar findings were published in research from 2018.⁹ Scientists wrote that despite earlier data describing lower temperature averages and fluctuations based on demographics and circadian rhythms, the most widely used definition of fever, a temperature greater than 100.4 F (38 C), continues to drive medical decisions, including those related to invasive procedures, hospitalizations and the use of antibiotics.

Dr. Jonathan Hausmann, rheumatologist at Boston Children's Hospital gathered temperature data using an iPhone app the team developed called Feverprints. Participants downloaded the app, provided consent and manually recorded temperatures along with information on symptoms and antipyretics use, if any.

The researchers used data to calculate normal temperature from those who presented without symptoms or medication. Fever was defined as those in the 99th percentile for normal temperature; the analysis was restricted to oral temperatures to limit variability.

The study was conducted from March 2016 to June 2017 and included 329 participants. Researchers found the mean normal temperature was 97.7 F (36.5 C). Fever was defined as a temperature of 99.5 F (37.5 C) or higher; both temperatures are slightly lower than norms from the previous study.

Hausmann stressed that a “normal temperature” is a flexible concept, saying,¹⁰"A temperature of 99 at 6 o’clock in the morning is very abnormal, whereas that same temperature at 4 o’clock in the afternoon can be totally normal."

Female Hormones Affect Temperature and Sleep

In both studies, data showed women had slightly higher normal temperatures than men. This higher core temperature may mean cool air “feels” even colder, which may be one explanation why many women feel colder at the same time men are peeling off layers of clothing.

Reproductive hormones, including estrogen and progesterone, affect a woman's core temperature. These affect thermoregulation and sleep. In one study,¹¹researchers investigated body temperature and sleep in young women taking oral contraceptives throughout the active and placebo phases of the steroid pill pack.

This was compared against a similar group of women during their ovulatory cycles. In those taking the oral contraceptives, body temperature was higher in the 24 hours of the active phase in a similar way for those women who were not taking oral steroids.

However, during the placebo phase, the women on oral contraceptives continued to have higher body temperatures indicating there was a prolonged action from the synthetic steroids. The data also showed sleep was influenced independent of body temperature in those taking oral contraceptives as measured by stage-2 non-rapid eye movement sleep.

Men Have Higher Resting Metabolic Rate Than Women

As with most processes, the human body is a complex organism depending on several systems to maintain the balance of others. While a woman's core body temperature is higher, potentially because of hormones, a man's resting metabolic rate is faster, which produces heat.

In one study¹² researchers examined gender differences across a broad age range for resting metabolic rate. They included 328 healthy men and 194 healthy women from 17 to 81 years.

The participants had their resting metabolic rate, body composition, peak oxygen consumption, energy intake and other factors measured. Gross measurements demonstrated that the males’ resting metabolic rate was 23% higher than that of the females’. Analysis showed that 84% of the variation in individuals was explained by fat mass, peak oxygen consumption and gender.

Even after controlling for these differences women continued to have a lower resting metabolic rate compared to men. The researchers concluded the lower rate was not dependent on body composition or fitness.

Should You Feed a Cold and Starve a Fever?

Moving into cold and flu season, it is helpful to know what temperature may or may not be within normal range. Colds and flu are more commonly diagnosed during the fall and winter months, although you may get them at any time during the year.

In 2018, the CDC found on average 8% of the U.S. became sick with flu. A recent animal study showed eating a high-fat, low-carbohydrate diet could reduce the inflammatory response and improve the ability of the animal to withstand the flu virus.

Mice eating a standard diet were all infected, compared to 50% of those eating a ketogenic diet. Those on a keto diet that did get infected did not lose did not lose as much weight — a sign of the severity of flu infection in animals.

The researchers evaluating the animals eating a keto diet found they had an immune response that promoted gamma-delta T cells in the lungs. These cells line the lungs, increasing mucus production and protecting the animal from viral infection by trapping the virus and keeping it from spreading.

To follow a ketogenic diet, aim for eating 50% to 85% of your daily calories from healthy fat and limiting your net carbohydrates to 20 to 50 grams per day. Your net carbohydrates are measured by subtracting grams in fiber from your total carbohydrates. Aim to cut out carbohydrates from grains and all forms of sugar, including high fructose fruit.

Add healthy sources of fat, including avocados, coconut oil, animal-based omega-3 from fatty fish, butter, seeds, olives and olive oil. 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.

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