How to burn calories

Now that the holiday feasting is over, you might want to burn some calories. The fastest way is to place the food in a fireproof container and ignite it. No kidding. That’s how scientists first determined calories in food. The heat released by combustion is a measure of the energy in the food.


From those initial experiments, tables were made from which manufacturers use for food labeling. If only determining the metabolism of food was as simple as reading the label. Biologist Rob Dunn lists the problems.

Digestive enzymes in the mouth stomach and intestines break down complex molecules into simpler ones which are circulated through our bloodstream. Even that process is not predictable.

First, genetic factors can determine actual food absorption. For example, the length of your colon can determine the energy extracted from the food before it exits. Certain Russian populations have colons that are 57 centimetres longer than Poles, on average. Based on colon length alone, more calories could be extracted by Russians.

Then there are the individual bacteria in our guts. Not only do the bacteria extract some calories for their own use, a kind of processing fee, but variation from person to person plays a role. Two types of bacteria dominate our guts: Bacteroidetes and Firmicutes.

Firmicutes are more efficient in metabolizing food. Researchers suggest that people with more of these bacteria could be susceptible to obesity since the extra calories could pile on the pounds.

Other bacteria and pathogens can hitchhike down the alimentary cannel. Some Japanese have a bacteria that is good at breaking down seaweed. The likely sources are genes stolen by gut bacteria from marine bacteria found on raw seafood salads.

Pathogens are seen by our bodies as foreign invaders, so undercooked foods that carry them invoke an immune response. Since immune responses themselves consume calories, these calories are not added to our daily intake.

Worse still, pathogens can lead to diarrhea and more calorie loss. And to life-threatening disease — not exactly the best way to lose weight.

In addition, the calories we extract from food depends on how much it’s been processed by grinding, cooking and fermenting.

In one experiment, mice lost weight when fed raw sweet potatoes but gained when fed them cooked. Processing food was a big evolutionary advantage for humans who have large, energy hungry brains relative to body size. Dunn explains:

“When humans learned to cook food —particularly, meat— they would have dramatically increased the number of calories they extracted from that food. Biologist Richard Wrangham proposes that getting more energy from food allowed humans to develop and nourish exceptionally large brains relative to body size.”

The type of food matters, too. People who ate almonds received only 129 calories from a package marked as 170 calories. This seems to be true for other nuts as well, perhaps because nuts are seeds and the plants that produce them intend seeds to be distributed through excretion, not consumed as food.

With so many factors determining the way we extract calories from food, labels should be regarded as a rough guide.


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