It's True, Hot Baths Burn Calories

Why calorie counting is almost useless and often misleading

Petros Giannakouris / AP

“A new study says taking a hot bath burns as many calories as a 30-minute walk.”

That popped up in a tweet from Time on Tuesday night. It referred to a small investigation into the physiologic effects of heat exposure.

The responses on social media were an inevitable, exultant mix of self-identification and self-deprecation. Things like, This is so meeeeee, and Guess I’m right for not going to the gym, suckers, and Fill ’er up (the tub) I’m taking a bath forever and going to eat the whole time [gluttonous emoji].

And, of course, This is a distraction from the Russia scandal.

We are all constantly projecting meaning onto the world to suit the templates we’ve committed to, so I saw the study’s finding as an indictment of calories. My own reply was that the study serves to prove that calories are “an almost useless and often misleading metric.”

My tweet was popular and beloved by almost everyone. I considered retiring on the royalties. But some took issue. One began with a timeless rhetorical device: “B.S. Eat more calories than you burn and you gain weight. Eat less [sic] calories = weight loss. That’s the truth.”

Others had more specific questions: “What do you mean, ‘useless’?! I just learned that I should take more hot baths!”

And even more specific: “How hot does this bath have to be?”

But what stuck with me were these: “Any suggestions as to what I should be paying attention to? … Every damn thing I read says something different.” And “Dummy here. Say more! I’d read.”

So I will say more, though I’ve already said a lot about why calorie-counting is an ineffective approach to eating. Calories are a crude metric that takes into account nothing about the properties of foods other than the total energy they contain. The value of activities can’t be reduced to a number, and nor can foods. Still calories are listed everywhere, enumerated in enormous fonts on food packaging and across menus and ads for packaged products with nothing to recommend them but a lack of calories.

A calorie is the amount of energy required to raise the temperature of a gram of water by one degree Celsius. The “calories” we talk about in food are the amount of energy released when that food is burned. Of course the first law of thermodynamics applies to humans, so if you take in less energy than you use, it’s impossible to store that energy (as body fat). But the factors that go into energy balance are many. The body burns and stores energy from different foods in different forms at different times in different people in different ways.

That crudity leads to mistakes, like the idea that 200 calories of Skittles are in any way equivalent to 200 calories of salad. In that way, calories have been weaponized by marketers to claim their ingestible products are innocuous. As Coca-Cola has advertised, for one, drinking soda is fine as long as you exercise enough to burn off those calories. That’s reasonable if it weren’t also true that constant exposure to high-sugar foods changes the way our bodies store energy. It’s like saying it’s fine to insult someone as long as you follow it with a compliment.

Worse still are the loudly advertised “100-calorie” packs of sugar-based edible products. They cause insulin levels to surge, affecting nutrient absorption and subsequent hunger in ways fundamentally different from eating 100 calories of almonds or spinach. That’s so much spinach. It would fill your stomach and please the microbes of your bowel.

I could go on, but suffice to say that evaluating food by its calorie count is like evaluating literature by the number of pages in a book. It’s usually worth knowing whether you’re committing yourself to something like The Stranger or something more like Infinite Jest, but the number alone is a poor measure of what that book will do for your health. A day is not measured in number of pages read, nor a person by the number of books on their shelf.

Of course, calories are indeed a valuable tool for researchers parsing the intricacies of human physiology. The bath study was a novel investigation of the immune system. It appears in the March issue of the journal Temperature, under the title, “The effect of passive heating on heat shock protein 70 and interleukin-6: A possible treatment tool for metabolic diseases?” (Not the clickiest headline.)

The researchers set out to see how exposure to heat can alter the molecules in our bodies. There were only 14 people (all men) in the study. They took hour-long baths at 104-degrees Fahrenheit and did burn calories, which were also measured, since energy is required to keep our cores around 98.6-degrees. But the men only burned an average of about 61 calories more than if they had been sitting at room temperature. When they exercised on a bike for the same amount of time, they burned between 515 and 597 calories.

I’ve also written about cold exposure—including a stint wearing an ice vest and trying “cryotherapy”—which has been shown to burn calories. But the researchers who study cold exposure aren’t as interested in burning calories to maintain homeostasis as they are in the longer-term, systemic changes that happen when we live in dynamic temperature environments as opposed to year-round climate-controlled homes and cars and offices, where a thermostat that’s set just a few degrees above or below someone’s ideal can lead an entire office to descend into mob violence.

The same was true of the hot-bath study. Though the researchers found a big difference in calories burned when comparing exercising to bathing, the two activities showed similar changes in the headline molecules, heat shock protein 70 and interleukin-6, as well as blood sugar. And it was because of those changes that the researchers concluded that hot baths could potentially confer metabolic benefits similar to exercise for people “who are too physically impaired to undertake prolonged aerobic activity to improve their cardio-metabolic health.”

Interesting as the physiological effects of passive heating are, they may be less promising for human health than the effects of taking up a calming ritual like sitting alone in water. Baths tend to be a rare time for reflection and disconnectedness, which are incomparably beneficial. Unless you have your phone in the bath, and you are reading Twitter at night and getting all riled up when people say your tweet is B.S., even though it wasn’t.

The only problem with the serenity argument for bathing is that it doesn’t satisfy a hunger for quantification—to know that what we are doing is not only good, but good to a specific degree that undoes or counterbalances a specific degree of bad. Calories lull us into this sort of complacent bartering. They give us a false sense of security that we can do damaging things so long as we zero out the balance.

Though maybe that sort of belief system, however flawed, has a calming effect. Like a bath.