--- title: 'Why a Calorie Isn''t a Calorie: The Science of Food Energy' source: 'https://youtube.com/watch?v=LTDVLg6whsM' video_id: 'LTDVLg6whsM' date: 2026-09-09 duration_sec: 338 channel: 'AsapSCIENCE' --- # Why a Calorie Isn't a Calorie: The Science of Food Energy > Source: [Why a Calorie Isn't a Calorie: The Science of Food Energy](https://youtube.com/watch?v=LTDVLg6whsM) ## Summary This video explores the science behind calories, explaining that a calorie is a unit of energy, but the number on food labels is based on an outdated system. It reveals how food processing, genetics, and gut bacteria all influence the calories we actually absorb, making the concept of a calorie far more complex than we think. ### Key Points - **Calorie Definition and Atwater System** [00:29] — A calorie is the energy to heat 1 kg of water by 1°C; food calories are measured using the Atwater system (4-9-4 rule) from over 100 years ago. - **Empty Calories vs. Nutrient-Dense Foods** [01:09] — Empty calories (e.g., a 100-calorie amendment) lack nutrients, while 100 calories of hazelnuts provide vitamin E and magnesium. - **Protein vs. Carbohydrate Digestion** [02:36] — Digesting protein requires more energy than carbs, so 100 calories of turkey yields less net energy than 100 calories of cake. - **Impact of Food Processing** [03:08] — Processing food makes it easier to digest, so raw grains pass through mostly undigested, while processed bread yields more calories. - **Genetics and Microbiome Influence** [03:36] — Genetics (e.g., celiac disease) and microbiome composition (e.g., Firmicutes vs. Bacteroides) affect calorie absorption; people with more Firmicutes tend to be more obese. ## Transcript Three cucumbers, two heads of lettuce, and three carrots equals 100 calories. But a quarter table food of ranch dressing easily adds another 100 calories. Half a cup of chickpeas is 100 calories, but only four table foods of hummus. This is because hummus contains olive oil. It's high in fat, bringing up the calorie count, but what is a calorie? A calorie is simply a measurement of energy. When it comes to food, a calorie with an uppercase C, now this is important, weirdly, is the amount of energy it takes to heat up one kilogram of water by one degree Celsius. In other words, it's basically how much energy is available in the foods that we eat. We've become so obsessed with counting calories because if we eat more energy than we can use, that extra energy actually can get stored in our bodies as fat or muscles, and our society seems to be obsessed with fat and muscles. But the way we actually calculate calories in foods is based on the Atwater system, which is derived from experiments done over 100 years ago. Basically, Mr. Atwater said, four calories per gram of protein, nine calories per gram in fat, and four calories per gram in carbohydrates. But we now know not all calories are the same. Where you get the calories from matters. Most of us may have heard the term empty calories. It refers to foods that have lots of calories, but little of the nutrients our bodies need to literally survive This amendment of M contains 100 calories but no vitamins and only 1 of the calcium and iron you need 100 calories of hazelnuts, on the other hand, will give you a good chunk of your daily vitamin E and magnesium. This is one example of why counting calories can be unhealthy, but also be confusing. 100 calories is a third of a chicken breast, or a quarter cup of macaroni with nothing you know, hashtag white people food, or a third of an impossible burger patty, or one third of a scotch egg, half a veggie sushi roll, or three ounces of palace panini. You could make a smoothie with half a banana, 12.5 strawberries, 65 blueberries, which is 100 calories, or just make a smoothie with water and protein powder, as most protein powders tend to be 100 calories for sure. And long after Mr. Atwater died, we learned a lot about our metabolism. It's not just calories in, there's a wide range of factors that affect how our calories in the food we eat affects our bodies. It all starts with physically breaking apart the food in our mouth, chewing. These smaller chewed up bites go down like a ball to our stomach where acid breaks down cell walls. We get into the small intestine where lots of enzymes and bacteria get to work breaking down molecules into glucose, which we can use in our cells for energy. In this process, calories aren't treated all the same in our body. It takes more energy to break down proteins than carbohydrates so the net gain of calories is higher with carbs For example 100 calories of turkey slices takes more energy to digest than 100 calories of vanilla buttercream cake. Processing food helps humans extract lots of calories. Even simple masking and eating starts the process of breaking down food so there's less our bodies have to do. Heavy-duty processed food yields more calories for the same reason. So a handful of raw grains would be hard for our body to digest and mostly pass through a fiber, so we wouldn't absorb many calories, but a yummy slice of mossy grain bread would give us 109 calories. But even then, once the food is ground or cooked, our bodies absorb, again, these calories differently. That's why some bread might be a good source of nutrients for one person, but a trip to the bathroom for someone with celiac disease. Celiac is a genetic disease that's essentially an allergen to gluten. That's just one example of how genetics affects our digestion, but it's not just genetics. It's also our microbiome, too. All those tens of trillions of bacteria, archaea, and other microbes mostly in your gut make up the microbiome. Without them, we couldn't break down basic foods. For instance, the microbiome helps babies break down sugars from the mother's milk, which is important. But everyone's microbiome is slightly different, so people can access different nutrients based on their microbiome. For example about 50 of those living in Asia can break down certain compounds in soy while only 25 of us people in North America can The delicate balance of microbes might be set early in childhood We get our first dose of bacteria from our mama, but then it changes based on what we eat and what's in our environment. These shifts could alter the way calories are absorbed. The ability to break down soy is actually decreasing in younger generations in Japan, may be due to the rise of a more Western diet. Most people have a combination of two types of bacteria in their gut, bacteroides and firmicutes. Studies show that people with a greater proportion of firmicutes tend to be more obese. Mice given a microbiome transplant from obese sources tend to gain weight when fed the same diet. This microbe combo could break down sugars more easily and be so efficient at attracting calories from food that rather than just passing through, more calories get absorbed and stored at a time. This could explain why two people could eat lots of under-calorie servings of peanut butter candy and one person gaining weight while their other friend doesn't at all. So the caloric value of anything, meat, fruit, bread, veggies, will change based on your microbiome, the way it's processed, your genetics. So it feels like maybe it's time for us to change the concept of what a calorie is. Go forth and feast with this new knowledge. Thank you for watching and make sure you are subscribed for more new science videos. F.A. Peace.