The human body temperature more or less hovers around 98.6 °F (37.0 °C), although this varies throughout your day according to how active you are, what you eat, and so forth. The weight loss we experience when we exercise is related to, among other things, our increased body temperature. A higher core body temperature increases the rate of moisture evaporation and burns more calories. This is why our appetites increase following prolonged exercise or physical activity—for example, a burly lumberjack working outside all day may burn twice as many daily calories as an accountant, and that lumberjack will need to replace those calories just to maintain his original weight.
We have all lost weight while lying flat on our backs after catching some nasty bug. True, some of this has to do with reduced appetite and loss of fluids, but much of it has to do with our increased core body temperature, which rapidly burns through our stored calories: fat deposits in adipose tissue. Depending on the severity and length of your illness, body temperatures in the range of 99 to 101 degrees can increase the number of daily calories burned by 5 to 20%. Staggering when you think about it.
This “infection” burn rate for calories, when viewed through the lens of our evolutionary past, becomes interesting—or at least it should—for those of us fighting the bulge at home or in guiding public health policy.
Before the age of infection-fighting drugs and antibiotics—and you can include antimicrobial soaps as well—our ancestors battled infection and its temperature-raising, calorie-burning effects on a daily basis. When I say ancestors, I’m talking about our pre-agricultural predecessors, not the old world folks who created a freakish relationship with the microbial world through crowded cities and tainted water and food supplies—all of which resulted in average life expectancy of a little more than 20 years.
If modern medicine has given us anything, it has been lower core body temperatures through reduced infection rates. While high fever and infection get all the attention, it’s the “low grade” infections and “slightly” high temperatures that should interest obesity researchers and public health officials. We have all heard that 3,500 or so calories equal a pound of body weight. That is, burn 3,500 calories on the treadmill or running around the park and you will lose one pound. Now think for a minute. The effects of a low-grade infection that raises your core body temperature from our current average of 98.6 degrees to, say, between 99.0 and 99.6 degrees on a more or less permanent basis. Without boring you with the math, this modest increase in body core temperature will increase our resting metabolic rate (the amount of energy one burns just sitting on the couch and not eating, as digestion burns calories, too) by 2 to 7 percent, depending on a dizzying number of variables. Imagine for a moment that your body automatically burned (needed) 2 to 7 percent more calories on a daily basis, and that your increased body temperature was not noticeable and did not affect your daily routine. For someone on a 2,500 kcal daily diet, an increased burn rate from a slightly elevated core body temperature (from a constant low grade infection) might result in an additional 100 or more calories burned in a day. Using the less than perfect assumption that 3,500 calories burned equals one pound lost, you would carve almost a pound from your frame every month if you just went about your daily routine. And in a year, well, you get the idea.
So what was the low-grade infection that our ancestors experienced? While there were a number, the most likely characters were various parasitic worms (e.g., helminths) that lived deep in our ancestral gastrointestinal tract. The presence of these parasites sends our immune system into action causing body temperatures to rise. Because the parasites compete with “us” for nutrients in our intestinal tract and “can” cause a great deal of problems if they get out of hand, the World Health Organization and modern medicine spend a lot of time trying to eradicate them from our species. These efforts are especially intense in developing countries, where dirty water and minimally processed meats easily transmit these parasitic infections. An estimated 40 million Americans have some form of parasitic infection. That said, much of the world’s population lives in a symbiotic relationship with our evolutionary hitchhikers. In fact, intestinal parasites are used to combat some autoimmune diseases.
Anyone in the livestock industry will tell you that administering antibiotics to your herd or flock will result in weight gain. In other words, reducing low-grade inflammation and general infection reduces calories burned and thus increases weight. Might our overuse of antibiotics and antimicrobials have reduced our “natural” low-grade inflammation just enough to tip the scales against us? Makes you wonder.