Negative calorie foods don’t exist, and these biologists have the flaming lizard poop to prove it

Bearded dragons are much more cooperative than humans.

It’s hard to convince humans to eat precisely measured portions of celery and let you collect the resulting feces and urine, though admittedly not impossible. So if you want to calculate exactly how much energy a creature might gain or lose by eating celery, you might want to pick a bearded dragon instead.

That’s what researchers at the University of Alabama, Tuscaloosa, did in their recent pursuit of a pervasive diet myth: negative-calorie foods. They recently put their results on the preprint server biorxiv. In our endless pursuit to be physically smaller, we’ve all heard about fruits and veggies that supposedly require more energy to digest than they provide in calorie form. In theory, eating such a snack would not only fill your stomach without adding to your caloric intake for the day; it would also burn off some of the calories contained in other (hopefully tastier) food you ate. Celery is oft-cited as such because it’s basically just fiber and water—it doesn’t sound totally outlandish for your body to expend extra energy breaking down all that green roughage. Nutritionists, biologists, and physicians alike have told us for years now that this is simply not true—negative calorie foods don’t exist—but that’s not how science works. To really prove it, you have to test it.

Enter the bearded dragons.

The authors note that “Bearded dragons were selected for study because they are naturally omnivorous, possess a GI tract similar to that of omnivorous mammals (including humans), and can easily be studied in the lab due to their docile temperament and willingness to consume celery.” They also have very little objection to the very strict procedure required to thoroughly measure how much net energy a piece of celery gives you.

It begins with an enema. The lizards must fast for 10 days in order to ensure that they’re not producing feces associated with some other meal, then researchers have to rinse their intestines out to remove any residual feces or urate (the lizard version of pee). Once they’re all cleaned out the lizards go into a tiny version of the metabolic chambers used in human studies. The airtight rooms have controlled intake and outtake flows for air so that researchers can measure the exact balance of gases, which (via some complex calculations) tells them how many joules of energy a body is using up during any given period of time.

These lizard-sized experimental chambers gave the biologists a measure of their standard metabolic rate. After that, the reptiles received precisely 7.83 kilojoules of celery (plus or minus 0.23) and were placed in their normal aquariums for monitoring. Over the next days, the valiant researchers collected every tiny bit of urate or feces the lizards produced, carefully dried each sample out, then weighed them and put samples into a bomb calorimeter to determine precisely how much energy each nugget of poop or pee contained. A final enema ensured they really got every last ounce of digested celery out of the lizards.

This lengthy process enabled the biologists to come up with this precise tallying of where exactly the energy from celery goes:

  • 2.29 kilojoules lost as feces
  • 1.06 kilojoules excreted as urate
  • 2.53 kilojoules expended on digesting

Net gain: an average of 1.89 kilojoules, or 23.4 percent of the energy ingested.

Although humans are not bearded dragons, the researchers note that our species would be expected to have an even smaller metabolic increase following food digestion, which is to say that we’d likely absorb more of the calories from celery than would a lizard. On average, they estimate that for most foods purported to be calorie-negative, a person would actually retain roughly 64 percent of the calories therein.

Of course, none of this means that fiber-dense, calorie-light foods aren’t part of a negative calorie diet, more commonly referred to as a caloric deficit. As the biologists point out in the paper, even though the lizards gained energy from each bite of celery, their overall balance of calories over the three days was in the red. That’s because your basal metabolic rate quickly overtakes any small gain in calories from these kinds of food.

Most of the energy you burn on a daily basis comes not from walking or even exercising, but from maintaining your bodily functions. Your basal metabolic rate accounts for 60 to 80 percent of the total energy you expend, while physical activity only accounts for 10 to 30 percent (digestion takes up the remaining 10 percent). That means taking on fewer calories can quickly result in a net energy loss over the course of days or weeks. You cannot only survive on fibrous fruits and veggies alone, but they do help fill you up (plus they provide important vitamins and nutrients) without adding many calories to your diet, helping you achieve a lower calorie count.

So though celery and other fruits and veggies don’t burn more calories than they contain, they are part of a healthy diet, and can even help with weight loss if that’s your goal. Just remember to keep that diet balanced. Even bearded dragons normally eat kale, mustard greens, carrots, squash, and vitamin-dusted crickets. Don’t let a quest to lower your calorie count deprive your body of delicious and nutritious meals.

Sara Chodosh

Sara Chodoshis an associate editor at PopSci where she writes about everything from vaccine hesitancy to extreme animal sex. She got her master's degree in science journalism at NYU's Science Health and Environmental Reporting Program, and is getting a second master's in data visualization from the University of Girona. Contact the author here.