Tag Archives: Heat Index

Food and Thermogenesis: How what you eat affects your body temperature

Energy produces heat. If you didn’t sleep through science class, they probably taught you this.

There are all sorts of circumstances behind what we now call climate change, the steadily rising temperature of the planet. But one key element is the fundamental existence of more human beings than have ever been on the planet in recorded history.

All humans produce heat. Every mechanical, electrical, chemical anything we have ever done produces heat. Vehicles and other machines produce heat when they operate. Anything we built that moves produces heat. Even the coldest fridges, freezers and air conditioners produce heat to cool what’s inside: The heat is just emitted out of the back or top of the device into the surrounding atmosphere.

And our bodies produce heat. The bigger we are, the more active we are, the more heat we produce. This is a key reason why your perceived temperature is hotter when you’re running than it is when you’re walking or still. You produce a lot more heat when you exercise.

Even the energy required to digest food produces heat. The act of digestion producing this energy is a little something scientists call thermogenesis.

Some foods require more energy from thermogenesis than others. This is one of the keys behind why it’s generally healthier to eat unprocessed meat and vegetables than processed sugar.

Insoluble fiber and most proteins require a lot of digestive energy for the body to digest its nutrients. These foods are highly thermogenic.

Meanwhile, chemically refined sugar is by design quickly digested, as these foods are chemically engineered to not satisfy you hunger and make you crave more of them. These foods are lightly thermogenic.

You can eat 500 calories of sugar cookies, and still be very hungry immediately after eating them. Meanwhile, you can eat 500 calories of steak, and be so full you won’t want another bite of anything for several hours. And woe is the poor soul who tries to eat 500 calories of broccoli… if he even manages to get it all down (1 cup of cooked broccoli is about 60 calories). He will end up spending a regretful amount of time on a toilet at some future point.

Broccoli and other vegetables are among the most thermogenic of foods. Many require more caloric energy to burn them than the calories the vegetables themselves contain!

Now, why bring up global warming when bringing up the thermic effect of food? Is Steven saying that broccoli causes climate change?

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Heat Acclimation and Blood Volume In Running

One of the subjects Jonathan Savage (the FellRNR running guy) discusses at length is heat acclimation training, where you train specifically in hot conditions to either prepare yourself to race in hot conditions or to successfully race in less extreme, even more normal temperatures.

This is of course amusing to me as we in Las Vegas (and most of the United States, admittedly) are currently suffering through a bout of extreme heat. And in Vegas, we’re used to high heat, with summer days topping 100° Fahrenheit (38° Celsius), but the 110-120°F heat we have now is even above our typical pay grade.

Thus, those of us who run in Vegas get to practice heat acclimation training whether we want to or not!

Of course, Savage refers mainly to winter training for a spring marathon. In the winter temperatures fall below freezing in most parts of the country. If eventually you have to run a marathon in 60-80°F weather (15-25°C), you’re going to get hit hard by relatively warm conditions, even though you’d love to have those conditions in the middle of August. Your body will have acclimated to the other extreme of those cold conditions.

On top of that, Savage typically runs ultra-marathons in more extreme conditions like the hot and dry Badwater 135 race. So he’s not just dealing with a slightly warm marathon in London or Boston. He’s dealing with potentially 100°F heat with doubly dehydrating dry conditions. So even if it’s negative celsius or fahrenheit outside he needs to bundle up to prepare for running in 100°F weather.

Now, all that said, just because you or I have no intention of attempting such a race coming out of winter doesn’t mean that heat acclimation isn’t valuable.

Even with no races on the horizon, running in summer heat and the resulting heat acclimation (within healthy reason: Don’t go taking extended runs once the temperatures are over 100° without an abundance of cooling resources and support)… has one additional key benefit.

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The ideal running temperature vs most people’s ideal temperature

One thing I find annoying is right before a relatively warm race when the obnoxious race emcee says, “It’s a perfect day outside for a race!” You can clearly tell from such a statement that the guy never runs, ever.

Because while 60-70°F (15-20°C) feels amazing if you’re out for an easy walk, or laying out in the sun… that temperature enters the somewhat-warm zone for distance runners in a race, who are moving a lot faster than a walk and producing a lot of body heat throughout their run. Add in substantial humidity, which interferes with the evaporation of sweat, and now it begins to feel really hot.

Jonathan Savage created a ‘perceived heat index for runners’ calculator to accurately show what a runner’s “heat index” is for a given pace at their height and weight, depending on the temperature and humidity.

Below is a chart from the calculator. All temperatures are shown in both Fahrenheit and Celsius, and assumes a person of my size (5’10”, 160 lbs as of this writing) running a 9:05 per mile pace.

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As you can see, 65°F (18°C) at even a mild 40% humidity can feel like 101°F (38°C) for a runner my size (5’10”, 160 lbs) running at a 9:05/mile pace.

To be fair, that’s mild compared to how most of summer feels for me and others, running in 80-95°F (25-35°C) afternoons with 60-75% humidity. Even running at slower paces, my heat indices during summer reach the 130-145°F (55-60°C) range.

And, of course, you can train to handle this, and training in hot conditions has all sorts of fundamental long term benefits. But once you get to race day, all the heat is doing is making your race tougher. And the threshold where it begins to impair your performance isn’t particularly high.

In any case, I’m not big on clapbacks or “educating” people, but the next time someone attempts to point out that 60+°F weather (15+°C) is perfect running weather, you may want to throw some knowledge at them about how running changes everything.

P.S. As for the ideal temperature for running, your best conditions are 40-60°F, or 5-15°C.

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