How do they measure the calorie content of food? When you lose weight, where does it go? Matt Soniak has the answers to these questions and more.

How does the treadmill know how many calories I've burned?

If a piece of exercise equipment has a screen that tells you how many calories you've burned, then inside there's a computer using standard mathematical formulas to calculate that number.

Most of these formulas, which differ among equipment manufacturers, revolve around distance covered and body weight. That makes the short answer "they don't really know." Just like there is a wide range of factors to consider when calculating how many calories you need, there's a number of factors that determine how many calories you burn during exercise that the machine don't take into account, like muscle mass, basal metabolic rate and efficiency of stride.Given these variables, exercise equipment isn't 100% accurate in calculating caloric expenditure and can only give you a rough estimate.

Back up. What is a calorie, anyway?

Calories are units of energy that we often use to measure the amount of energy in food that is available through digestion, but can also apply to just about anything containing energy (1,000 tons of TNT is roughly equal to 1012 calories).

The common parlance "calorie" "“ those found in PB&J and not TNT "“ is actually a "kilocalorie" (1,000 calories = 1 kilocalorie), also called "food calories." One of these bad boys is equal to 4,184 joules, and is the amount of heat energy it takes to raise the temperature of one kilogram of water (about 4.4 cups) by one degree Celsius (1.8 degrees Fahrenheit).

Depending on how you make your peanut butter and jelly sandwich, the final product should contain about 300 calories. If you could rig up a PB&J-powered water heater, burning that sandwich completely would produce enough energy to raise the temperature of 300 kilograms (about 82.67 gallons) of water 1 degree Celsius.

Our bodies need a certain amount of energy to function well, and therefore a certain number of calories. The body gets energy from food through metabolic processes that break down the food's nutrients into simpler molecules, which are then absorbed by cells for immediate use or reacted with oxygen later to release their stored energy. The "percent daily value" you see on nutritional labels is based on is a rough average of the number of calories a person needs to consume in a day to function— nutritional labels assume the number to be 2000. However, people need more or less depending on their height, weight, age, gender, level of physical activity, basal metabolic rate (the amount of energy the body needs to function at rest), and the thermic effect of food (the amount of energy the body uses to digest food) are all factors a person needs to consider when figuring out how many calories they need in a day.

And this should be obvious: when you take in more calories than you need, you gain weight "“ 3,500 extra calories get stored by the body as a pound of fat. Burning more calories than you consume, either through exercise or eating less, results in weight loss. If the body needs energy and is facing a caloric deficit it will convert stored fat into energy.

How do they measure the calorie content of food?

Remember that sandwich-powered water heater we had rigged up in the basement? Scientists actually used to use something along those lines called a "bomb calorimeter," a device invented by Wilbur Olin Atwater (whose work helped put the calorie in the spotlight, and also proved that alcohol is somewhat nutritious) to literally burn calories.

To measure caloric content, a food sample was dried and ground into a powder so all water content was eliminated. The powder was placed into the bomb calorimeter, which consisted of a strong metal container in a water bath. The container was filled with pure, high-pressure oxygen to promote combustion and the food was ignited.

The result was a fast and violent energy release as the stored energy in the food was turned into heat. The heat raised the temperature of the metal container and the surrounding water, and the temperature increase revealed how many calories the food contained (remember, 1 calorie increases the temperature of 1 kilogram of water by 1 degree Celsius). The number of calories was then multiplied, usually by 89%, to account for the energy used during digestion.

These days, divining caloric content is lighter on pyrotechnics. The Nutrition Labeling and Education Act of 1990 requires the calorie count on food packaging to be calculated from food components, so food labs use the Atwater system, a set of conversion factors derived by, yup, Wilbur Atwater.

Using the Atwater system, scientists calculate caloric value by adding up the calories in a food's energy-containing nutrients. The average values for these nutrients, originally determined by burning and then averaging samples, are 4 calories per gram of protein, 4 cal/g of carbohydrate, 9 cal/g of fat and 7 cal/g of alcohol.

Of course the amounts of these nutrients in a given food need to be figured out before their calories can be added up. Again, all moisture is removed from a food sample and it is ground into a fine powder. Gas chromatography is used to separate fat from the rest of the sample so it can be measured. The amount of protein is determined by the Kjeldahl method. Carbohydrates are determined by process of elimination, the assumption being that once fat and protein are removed, whatever's left is carbs.

How can some foods have 0 calories?*

Water is the only naturally occurring calorie-free food, but you've no doubt seen some diet sodas advertised as having zero calories. Soda usually doesn't have any fat or protein, so the caloric stumbling block is a carbohydrate, namely, sugar.

Sweetening something with sucrose, the sugar we know from the sugar bowl and the little packets, gives it caloric content because our bodies metabolize sucrose. Some artificial sweeteners, though, like saccharin (Sweet 'n Low) and sucralose (Splenda), pass through the body without being metabolized and therefore have no caloric value.

When you lose weight, where does it go?

We learned before that the body, when expending more energy than it's receiving, converts its stored fat into usable energy. Most of that stored fat exists in chemical form as triglycerides (a glycerol molecule and three fatty acid chains) and is tucked away as oil droplets within the fat cells that make up the fat tissue in our beer bellies.

When you're cutting calories in your diet or working out, lipase, a hormone-sensitive enzyme located in fat cells, responds to hormonal messages and breaks down triglycerides into their component parts. The glycerol and fatty acids then exit the fat cells and enter the bloodstream, where they're absorbed by the liver and muscles.

After absorption, the triglyceride components are further broken down and modified by chemical reactions to create usable energy. The results of these reactions are carbon dioxide, water, heat and an energy-carrying molecule called adenosine triphosphate (ATP). We exhale the carbon dioxide, get rid of the water as urine and sweat, use the heat to maintain body temperature and the ATP goes off to power cellular activities and provide the energy it takes to put in one more mile on the treadmill or walk away from a plate of doughnuts.

* No doubt someone will ask about celery containing fewer calories than it takes to digest. Discussing that could take a whole post, so I'll turn you Anahad O'Connor's Never Shower in a Thunderstorm for his take on it.