How Does the Treadmill Know How Many Calories I've Burned?

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).

pbjDepending 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.

Nutrition-LabelThese 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?

pantsWe 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.

Live Smarter
A Simple Trick for Keeping Lemons Fresher for Longer

Lemons don't get much respect in the average refrigerator. After taking a slice or two to punch up drinks or add to a recipe, the remaining wedges can often be pushed out of view by incoming groceries and left to go to waste.

But the folks at Food52 have come up with a solution to get more use out of those lemons by keeping them fresher longer. Because citrus needs moisture in order to remain fresh, all you need to do is place your lemon in a bowl of water before putting it in the fridge.

Another idea: Put them in a sealed plastic bag and make sure you remove all the air to prevent mold growth. You'll get up to three months of freshness with this method. If your lemons are already cut into wedges, you can expect they'll last three to four days.

The "hack" also works for oranges and grapefruits. As for freezing, you can do that, too, but the resulting mushy fruit is probably best left for making juices.

[h/t Food52]

The Body
13 Facts About Skin

Skin isn't just the outermost layer of our bodies. Without it, we couldn't do most of the things we take for granted, like breathing, moving, and keeping the body's inner workings where they belong. And while skin also evolved to keep pathogens and other bad stuff out of our bodies, consumers spend millions of dollars on products to penetrate that defense (with mixed results). Read on for more fascinating facts about the skin.


Skin is considered an organ in its own right. It's comprised of three layers: the waterproof top layer, the epidermis; a middle layer of tougher connective tissue, hair follicles, and glands called the dermis; and the inner layer, the hypodermis, which is mostly fat and connective tissue that supports the skin's structure and attaches it to muscles.


Those cells are known as melanocytes, which secrete a pigmented substance called melanin; the more melanin in the cells, the darker the skin. Having too little or too much melanin can lead to some skin color disorders: On one end of the spectrum are conditions like vitiligo—which occurs when some melanocytes lose the ability to produce melanin, resulting in whitish patches on the skin—and albinism, a condition in which melanocytes don't produce any melanin. On the other end is hyperpigmentation—the presence of excess melanin, which can cause darker patches of skin.


"Your skin accounts for 15 percent of your body weight," says Toral Patel, M.D., a board-certified dermatologist and supervising physician at D&A Dermatology in Chicago and a clinical instructor of medicine at Northwestern University. This makes it your body's largest organ.

According to that calculation and data from the Centers for Disease Control and Prevention, an average American woman weighs 168.5 pounds and carries more than 25 pounds of skin. An average man weighing 195.7 pounds will have nearly 30 pounds of skin.


New cells are created in that deep layer of the skin and take about four weeks to rise to the surface. There, they grow hard and then shed. This process, in which old skin is sloughed off and replaced by newer skin, might occur more than 1000 times over the average American's lifespan. But all skin is not created equal: Its thickness varies naturally among all areas of the body. Thickness can also be affected by age, gender, and habits (like smoking) that can change the cells' elasticity and other traits. According to Patel, the skin on the soles of your feet is up to seven times thicker than the skin of your eyelids.


If your skin cells shed every month, how do tattoos stick around? It turns out to be a function of your immune system. The puncture of the tattoo needle causes inflammation in the dermis, the skin's middle layer. In response, white blood cells known as macrophages are sent in to help heal the damage. These macrophages "eat" the dye and can pass it to newer macrophages when they die off, so the pigment is essentially transferred from one cell to another. Any leftover pigment is soaked up by fibroblasts, which are longer-lasting skin cells that don't regenerate as often. Only lasers designed for tattoo removal are strong enough to kill off the macrophages and fibroblasts that hold the dye.


Your skin hosts a microbiome that can contain more than 1000 types of bacteria (along with other microbes, viruses, and pathogens). These "tiny ecosystems," as Patel describes them, are mostly friendly bacteria that work in concert with our bodies for many beneficial purposes, including wound healing, reducing skin inflammation, and assisting the immune system to help fight infection. These bacteria were once thought to outnumber your own cells 10 to one, but more recent research has found the ratio is closer to 1:1.


Injuring or breaking the skin's dermis, the layer below the epidermis, can expose the inner tissues to pathogens. To prevent infections from reaching any further into the skin, body fat, or muscle, ancient Egyptians cared for topical wounds with salt (yes, really!), fresh meat, moldy bread, and onions.

While these may seem like unsanitary things to put on a cut, modern research has found that there was actually merit in their methods. With its high iron content, meat was a good blood coagulant and recommended for the first day of a wound, according to a 2016 paper in the Journal of the German Society of Dermatology. Salt and onions are both astringent, which can stop blood flow. Moldy bread likely had antibacterial properties—a very early form of penicillin, you might say. Skin wounds would then be sealed with a combination of oils, fats, honey, and plant fibers.


Your skin is a significant shield against billions of tiny microbes and pathogens. But just as importantly, skin keeps fluids in. Another way to think of this, Patel says, is that your skin resembles a brick and mortar pattern. The bricks are the cells. The mortar is made up of lipids, fatty acids, and other sticky proteins that form the watertight layer. "If you have any ‘holes' in skin where moisture can escape, which are more susceptible to damage, that leads to dryness, cracking, and inflammation," Patel says.

People who have suffered burns often have fluid-balance problems, says Robert T. Brodell, M.D., professor of dermatology at University of Mississippi Medical Center in Jackson, Mississippi. "Fluids are seeping out, and they can't keep them balanced internally," he tells Mental Floss. This can be incredibly dangerous, because fluid loss can cause the heart to stop pumping blood to the rest of the body. Dehydration, hypertension, and other problems may also occur when skin is injured.


Psoriasis is an autoimmune condition in which the skin cells in an affected area grow rapidly, leading to excess skin buildup, inflammation, and a red and scaly rash. While it can be uncomfortable to live with the condition on its own, studies [PDF] have shown that inflammation of the skin can lead to inflammation of other tissues and internal organs, and eventually certain diseases. For example, psoriasis has been linked to a greater risk for heart disease, as well as diabetes, Crohn's disease, metabolic syndrome, and other conditions thought to be correlated with inflammation.

Patel says that association makes treatment even more important: "If one organ is inflamed, you have to make sure another isn't."


Unless you live in the tropics, you've probably noticed that the skin of your lower legs becomes drier in winter—and there's a biological reason for that. "You have fewer oil glands on your legs than any other area of your body," Brodell tells Mental Floss. Oil (or sebaceous) glands, found near the dermis's border with the epidermis, secrete an oily substance called sebum that lubricates skin and hair. As people age, the glands secrete less oil, and that means drier skin. Winter's low humidity and our tendency to spend more time around heat sources dries out skin even more.

The solution is to install a humidifier or apply some moisturizer. Certain skincare products, such as those with emulsifiers like sodium laureth sulfate, can also dry out or irritate your skin, so read your labels carefully.


Both types of sweat glands are also located in the dermis. Eccrine glands, found all over the body, emit sweat directly through pores in the epidermis. Apocrine glands release sweat along hair follicles, so it's no surprise that these glands are concentrated in the hairiest parts of the body—head, armpit, and groin. Both types help regulate body temperature: In hot conditions, the glands release water and fatty liquids to cool the skin.

A lack of sweat glands puts people in danger of overheating. Those with a condition known as anhidrotic ectodermal dysplasia have few to no sweat glands, so they can't properly cool off when the body overheats. "They get heatstroke easily," Brodell says. A subset of people with this disorder suffer from immunodeficiency. They produce low levels of antibodies and infection-fighting immune T- and B-cells, so they are more prone to skin and lung infections.


The gut and the skin never come into direct contact with one another, yet research shows that the gut has a profound impact on the skin.

"The skin becomes very unhealthy when the microbiome of the gut goes into a state of dysbiosis," meaning when something attacks the gut's good bacteria, says Gregory Maguire, Ph.D., a former professor of neuroscience at UC San Diego and the founder and chief scientific officer of BioRegenerative Sciences, a stem-cell technology company.

Dysbiosis can lead to inflammation, irritation, rashes, and pain. "There's good evidence that eczema [or] atopic dermatitis is partially due to dysbiosis of the gut and skin," he says.

In a 2017 paper published in the Archives of Dermatological Research, Maguire writes that normal gut bacteria can actually calm the body's response to stress. A reduction in the release of the stress hormone cortisol, which is thought to cause breakouts, also reduces the chance of skin irritation—all thanks to microbes in your intestine.


When the skin's pores get clogged with sebum from the sebaceous glands and dead cells, a condition usually associated with hormonal changes, you've got acne. Clogged pores that stay closed are called whiteheads; if the pore opens and reveals the gunk inside, it's a blackhead. (The medical term for a blackhead, an "open comedo," stems from a Latin phrase alluding to "worms which devour the body." But don't worry, blackheads are not actual worms living in your face.)

While acne may seem like a rite of passage associated with puberty, researchers are experimenting with fighting "bad" bacteria (in this case, Propiobacterium acnes, which is linked to acne breakouts) with "good" bacteria, also known as probiotics. "One of the things [probiotics] do is ferment things on the skin like ammonia and nitrites, and metabolize it and turn it into other chemicals that are beneficial to the stem cells in your skin," Maguire explains. A 2015 study in the Journal of Women's Dermatology and other research has found that applying topical probiotics like Streptococcus salivarius and Streptococcus thermophiles inhibits P. acnes and may make skin more resilient against it in the long run.


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