Calories ingested

July 9, 2012 2:24 AM

Do I get all the calories from everything I eat?

Say I eat a single thing, let's say a chocolate. The wrapper says it contains 100 calories. Do I get all 100 calories, or does my body only store some of that?

What happens if I eat ten chocolates, or one fifty, or one hundred?

posted by devnull to health & fitness (8 answers total)

I'm basically certain the answer is no: this is why you can burn dung. I have no idea about percentages though; I suspect they would be so small as to render any discrepancy between what's on the label largely moot - especially when you're talking something with virtually no fibre like chocolate (as opposed to say, a carrot).
posted by smoke at 2:31 AM on July 9, 2012

The calorie count on food packaging represents a rough estimate of how much energy is made available to your body by eating it. By "made available", I mean the body may use it in the short term, or store it chemically in the long term. I'm sure the proportion of energy stored long term vs used short term varies considerably depending on type of food, and the state of the body.

So the short answer is of the 100 calories in your chocolate, your body stores some, and uses some to power its activities.
posted by Salvor Hardin at 2:32 AM on July 9, 2012 [3 favorites]

So the short answer is of the 100 calories in your chocolate, your body stores some, and uses some to power its activities.

But how does what my body does change as I eat more of something?
posted by devnull at 3:15 AM on July 9, 2012

There was a question about that on here a year or two ago. Someone was asking what happens if you eat like 20,000 kilocalories in a day - whether your body can store all of it. The answer was no, and that this is a problem for people like tour de france cyclists who are basically limited by their bodies ability to turn intake into energy per day.

I think there's quite a high amount that the average person can use before they reach that limit, though. I seem to recall a figure of about 9000 kcal mentioned in that thread that I now can't find.
posted by lollusc at 3:26 AM on July 9, 2012

Check out this article entitled "Is a calorie a calorie?" from the American Journal of Clinical Nutrition. The section "Metabolizable Energy" addresses how the numbers on nutrition labels came about. In summary, the numbers are supposed to represent the amount of energy in the food that your body can actually use, but they are just averages, and the energy you actually get will vary slightly depending on the context.

As for what your body will do with the energy: it might use it immediately, or if your energy needs are already met it might store it as glycogen, fat, or lean tissue, and it might raise your body temperature and increase your metabolic rate or involuntary activity level in order to compensate for the surplus, depending on what you're doing at the time, the composition of what you ate, and genetic factors. I read an interesting study on these effects recently but I can't find it at the moment, but if you search for "overfeeding" on pubmed you can find lots of similar stuff.
posted by ludwig_van at 5:50 AM on July 9, 2012

But how does what my body does change as I eat more of something?

Just as there is a limit to how much you could physically eat in one day, there is a limit to how much food your gut can process.
posted by gjc at 5:59 AM on July 9, 2012

devnull: "But how does what my body does change as I eat more of something?"

I am not a dietician, I am not your dietician, and this does not constitute dietetic advice. I suppose I could call myself a nutritionist, but then so can everyone from your dog to the craziest quack imaginable to Dick Cheney. I am however a biochem heavy microbiologist who spent way to long in college studying nutrition.

This is actually a really good question that nutritional science has spent a lot of effort trying to answer. We have a lot of good and empirically verified ideas about how this works but understanding them will require a bit of background.

The central way in which our bodies decide what chemical energy gets stored or used is through the insulin/glucagon pathway. What is does is continuously adjust the amount of glucose in your blood to keep it at exactly where it should be. You may recognize insulin as being related to diabetes, which is a disease caused by an inability to properly regulate blood sugar. In your pancreas there are these very specialized cells called beta cells, and these cells are found in little groups called islets of Langerhans. Your beta cells are constantly exposed to your blood as it flows past them and they are constantly sampling it. Thus, when they detect to much glucose, they excrete insulin. To much glucose is a really bad thing, it doesn't supercharge you or anything but it does cause damage to your blood vessels, particularly your more fragile capillaries. The insulin then has a number of important effects like getting your adipocytes (fat cells) to produce fat which is energy dense but not very accessible from the glucose, as well as getting you muscles to absorb the glucose and chain it together into glycogen (lots of glucoses tied together into one big molecule), which is easily accessible but not very energy dense. It has a lot of other effects that change you from being optimized for hungry to being optimized for full, but these are the important ones for now.

Here is an amazingly pretty video that explains this in relation to Type I diabetes

Glucagon works in an almost opposite way, it is secreted by the alpha cells of the pancreas when blood glucose is low. Blood glucose is low between meals and during exercise. When blood glucose is high, no glucagon is secreted from the alpha cells. Glucagon has the greatest effect on the liver although it affects many different cells in the body. Glucagon's primary function in the liver is to release stored glucose, in the form of glycogen, from its cells into the blood. Glucagon also the starts systems that produce glucose in the liver, which insulin also shuts down, out of building blocks obtained from other nutrients found in the body, for example, protein.

Now that we've gotten the background out of the was lets get to the part you're interested in,

As you eat more of something your blood glucose will continue to rise, and what your body does with it in response to the insulin your pancreas secretes will depend on a number of factors. The more you exercise the more you muscles will take up and turn into glycogen for temporary storage until you need it for more exercise. Also, different kinds of sugars will affect this pathway in different ways by being detected in different ways, here is a really good, if long, video explaining the current consensus on how this works in ways that affect us (1.5 hours). Additionally depending on how much of it you eat and how quickly you do it your blood sugar might rise a lot requiring a larger response to sequester more blood glucose more quickly, kind of like with this graph. In theory, during a blood sugar spike you will put more energy into fat metabolism and less into other pathways. Incidentally, one of the more solid models for where Type II (Acquired) diabetes comes from is to many large spikes like this will make the insulin receptors in your muscles less effective at detecting insulin causing them to stop responding to it.

Also check out this article, it isn't to math heavy or technical but gives a good history of what calories are and how calories from different kinds of sources interact with you.
posted by Blasdelb at 6:09 AM on July 9, 2012 [20 favorites]

Oh! I only addressed your followup question, for your original question, which is also a really good one.

[[I'll get there I promise]]

A calorie is a unit of measurement, kind of like how a yard measures distance, it measures energy. Specifically it is precisely how much energy it takes to heat 1 kilogram, or liter, of water from 15°C to 16°C. If it helps for reference, think heating a half a 2 liter bottle of soda by 1.8°F. We know that this amount of energy is roughly equal to 4.184 J and we can inter-convert with the rest of physics from there, but what is important for us here is food. When measuring the amount of energy in food we generally use kilocalories or Kcals, which are equal to 1000 calories, as this gives us much more useful numbers to work with.

So far we are talking about straight up simple chemistry and physics, with few of the complications or inefficiencies of biological systems, but calories are generally only used to measure gross energy exchange in biological systems. Antoine Lavoisier, after demonstrating the (very counter-intuitive for the time) law of conservation of mass by showing that no matter what you did to a substance its mass never disappears, made another discovery that would lead to the development of the calorie. Lavoisier designed a primitive but ingenious calorimeter that would measure heat production as a result of respiration in a hapless guinea pig. The outer shell of the device was packed with snow, which melted to maintain a constant temperature of 0°C around an inner shell filled with ice. The guinea pig in the center of the chamber produced heat, which melted the ice. The water that flowed out of the calorimeter from the ice chamber was then collected and weighed. Lavoisier used the amount of water that flowed out to estimate the heat produced by the critter's metabolism. He concluded, "la respiration est donc une combustion," that is, respiration is therefore a combustion, like that of a candle burning. Later experiments confirmed this by showing that the amount of oxygen consumed per amount of heat energy released in animals was indistinguishable to them from the ratio in a candle.

Once we knew that the source of the 'vital force' was like a burning candle, we could measure it like one. By measuring the amount of energy released in combustion, we could also measure the amount of energy that would be transformed into what they knew of as vital force in the body. Generally, the best way to do this is with a bomb calorimeter. Essentially, a food item, like a big mac, is dehydrated and placed in a sealed chamber before being zapped with a precisely measured amount of electricity that will ignite the sample and keep it burning until nothing is left to burn. By measuring the heat released from the bomb you can calculate how many calories were originally in the big mac, and thus in theory how much it would impart into you.

Now to your question,

devnull: " Say I eat a single thing, let's say a chocolate. The wrapper says it contains 100 calories. Do I get all 100 calories, or does my body only store some of that?"

The most correct answer is either 'absolutely', 'sort of', 'no', or maybe all three depending on how you look at it. The number of calories in your food is just the total amount of chemical energy present in it. As you chew that chocolate some of it will get stuck to your teeth where most of those calories will be consumed by your oral microbes, incidentally most of the sugars will be fermented into acidic byproducts that are really terrible for you acid labile teeth. Depending on your brushing habits, you will end up getting many of these calories back as dead and planktonic bacteria when they get swallowed with your spit. Then in your stomach there is a small population of bacteria that will take their tiny cut and get consumed in the end themselves. Once your food makes its way to the small and large intestines the two to five pounds of bacteria that live there get to do their thing as well, and again much of the energy that is lost to them gets clawed back as they die. Also, for reference, your poop is 60% bacteria by dry mass and does contain quite a few calories, mostly in those bacteria as well as in waste products not worth reprocessing for various reasons. The whole process of digestion also requires quite a bit of energy input, all of those gastric juices and enzymes have to be produced somehow. This is how very fiber (the stuff we can't digest) rich foods like celery can be, sort of, said to have negative calories. Celery has around 6 calories per 8 inch stalk and one can reasonably estimate that the amount of energy required for digesting 8 inches of wet cellulose is greater than those 6 calories. The net loss would not be in any way significant, though this is still a decent strategy for calorie restriction if the celery replaces a 576 calorie big mac, which is not to endorse calorie restriction for anyone not advised to do so by their physician.

So I suppose the best answer to your question depends on what exactly you mean by getting 100 calories.
posted by Blasdelb at 8:10 AM on July 9, 2012 [2 favorites]