12 Enlightening Facts About Body Fat


The human body is an amazing thing. For each one of us, it’s the most intimate object we know. And yet most of us don’t know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

Let’s face it: Fat gets a bad rap. Entire industries have been built upon the criticism and attempted reduction of body fat. But fat, formally known as adipose tissue, is a crucial part of your hormonal and metabolic processes. Adipose tissue is a major site of energy storage, and has a key role in the regulation of metabolism and insulin production in your body—not to mention, it helps keep you warm. Having too much fat can be a bad thing, but having too little can pose problems as well.

Fat is stored in the body in the form of triglycerides, free fatty acid (FFA) molecules that are held together by a molecule called glycerol, a type of alcohol. Most of our body fat is stored in fat cells called adipocytes, but fat can also be stored as droplets within skeletal muscle cells. In addition, some triglycerides even roam freely in your blood stream. (These are the ones most easily broken down through exercise.)

Before you demonize fat, take a look at these 12 facts about your adipose tissue.


Your fat is not just a layer of padding—it’s actually an organ of the endocrine system. “Fat secretes a lot of hormones, while activating or deactivating many more,” Indraneil Mukherjee, a doctor at The Southeastern Center for Digestive Disorders and Pancreatic Cancer, Florida, tells Mental Floss. It’s comprised of adipocytes and fat cell types called the stroma-vascular fraction, which are made up of growth factors—messengers the body uses to signal cells—as well as stem cells, blood cells, and a host of other cell types.


Adipose tissue is “a metabolically dynamic organ,” according to a study in Archives of Medical Science, whose primary job is to store excess energy. It also synthesizes “a number of biologically active compounds that regulate metabolic homeostasis.” In other words, it controls your body’s energy balance by regulating appetite signals from the central nervous system and metabolic activity in peripheral tissues. Chronic over-nutrition—eating too much on a regular basis—can cause inflammatory responses and metabolic disorders that can lead to disease: most obviously, obesity.


White adipose tissue stores your body’s reserves of energy, and the endocrine cells mentioned above, which secrete crucial hormones and molecules. There are even “adipose depots” where white adipose tissue tends to gather more easily, located around organs such as the heart, lung, and kidney.


Brown adipose tissue is typically found in newborn human babies and hibernating mammals, according to a study in Frontiers in Endocrinology. Its main function is to generate heat—keeping you warm—and to do so, it contains more mitochondria and capillaries than white adipose tissue.

Up until recently, researchers weren’t sure brown fat existed in adults. “There's a lot of excitement around the discovery,” Yi Sherry Zhang, an assistant professor at the TOPS Obesity and Metabolic Research Center at the Medical College of Wisconsin, tells Mental Floss. “It helps to regulate energy expenditure. This is important because drugs that target this type of fat may provide a new way to treat obesity.”


While it is now commonly known that obesity—when a person carries more weight than is considered healthy for their height—can predispose a person to type 2 diabetes, too little fat has a similar effect, according to the American Diabetes Association. Type 2 diabetes is a group of diseases in which the body has an impaired ability to produce or respond to the hormone insulin. And it turns out having too little fat is due in part to a lack of a lipid-storing “compartments,” which leads to an imbalance of triglyceride and free fatty acid levels, leading to insulin resistance.


Researchers at McMaster University have begun studying a new realm of therapies known as postbiotics, the by-products that bacteria leave behind, which help the body synthesize insulin more effectively. In a new study, scientists discovered that administering postbiotics to mice with obesity reduced their insulin sensitivity—without any need for weight loss—heralding promising potential treatments for obesity with type 2 diabetes.


Adipose tissue also secrets “hormones that make cancer cells grow quicker,” says Mukherjee. In fact, when adipose tissue expands, it also allows more immune cells to enter the tissue. These B and T immune cells secrete pro-inflammatory molecules such as adipokines [PDF]—peptides that signal other organs—and cytokines, which create the perfect microenvironment for tumor growth, according to a study in Frontiers in Physiology.


If you are so inclined, Mukherjee points out that “fat transplant is legal”—so you can technically surgically move it from one body part to another without any harm done, “for vanity,” he says. These so-called fat transfers can augment a formerly flat part of your body, but buyer beware—not only can you experience the side effects of surgery such as swelling, bruising, several weeks of recovery time, you can develop lumps.


The number of your fat cells can increase, but once the cellular structures have developed, they never go away. “With dieting, they just get smaller,” Mukherjee says. Zhang adds, “Each of us has 10 billion to 30 billion fat cells in our body.” Obese people can eventually have up to 100 billion fat cells.


It does so by sending out small molecules called microRNAs (miRNAs) that control gene activity, according to a study in Nature. After injecting genetically modified mice with fluorescent liver cell miRnas, researchers saw a significant drop in liver cell fluorescence, which suggested that the fat tissue was communicating with the liver to regulate gene expression. They hope to further study this process to discover new treatment methods for obesity and type 2 diabetes.


“We are beginning to understand the genetic basis for fat distribution and obesity,” says Zhang. “We have recently published genes that play a role in determining how body fat is distributed,” she continues. She hopes that these discoveries will help researchers understand the genetic component of common disorders like metabolic syndrome, type 2 diabetes, and obesity.


Researchers studying fat to better understand metabolic disorders recently focused on the field of epigenetics, which is the study of “the various elements that regulate which genes are active in particular cells and how they are regulated,” Zhang says. She believes that epigenetic changes are likely to play a critical role in the development of chronic disorders like metabolic syndrome and type 2 diabetes. “Unlike the genetic code, it is possible to reverse and alter these elements, which means we can potentially develop new ways to prevent and treat these common disorders.”

Why Can You Sometimes See Your Breath?

Chalabala/iStock via Getty Images
Chalabala/iStock via Getty Images

The human body is made up of about 60 percent water, meaning that when we breathe, we don’t just exhale carbon dioxide—we also exhale a certain amount of water vapor.

Water molecules need a lot of energy in order to remain in a gaseous form. When the warm water vapor molecules from your lungs reach colder air, they condense into “tiny droplets of liquid water and ice,” according to Wonderopolis. In fact, this process of condensation is also how clouds are formed.

But it’s actually relative humidity, not just temperature, that determines whether you can see your breath. The water vapor in your breath condenses into a liquid when it hits dew point—the temperature at which the air is saturated and can’t hold any more water in gas form. Since cold air can’t hold as much water vapor as warm air, you're much more likely to see your breath on a chilly day, but that's not always the case.

On more humid days, you may be able to see your breath even when it’s relatively warm outside. That’s because the air is already more saturated, making the dew point higher. And on especially dry days, even if it's cold outside, you may not be able to see your breath at all. Dry, unsaturated air can hold more water vapor, so you can huff and puff without seeing any evidence of it at all.

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7 Facts About Blood

Moussa81/iStock via Getty Images
Moussa81/iStock via Getty Images

Everyone knows that when you get cut, you bleed—a result of the constant movement of blood through our bodies. But do you know all of the functions the circulatory system actually performs? Here are some surprising facts about human blood—and a few cringe-worthy theories that preceded the modern scientific understanding of this vital fluid.

1. Doctors still use bloodletting and leeches to treat diseases.

Ancient peoples knew the circulatory system was important to overall health. That may be one reason for bloodletting, the practice of cutting people to “cure” everything from cancer to infections to mental illness. For the better part of two millennia, it persisted as one of the most common medical procedures.

Hippocrates believed that illness was caused by an imbalance of four “humors”—blood, phlegm, black bile, and yellow bile. For centuries, doctors believed balance could be restored by removing excess blood, often by bloodletting or leeches. It didn’t always go so well. George Washington, for example, died soon after his physician treated a sore throat with bloodletting and a series of other agonizing procedures.

By the mid-19th century, bloodletting was on its way out, but it hasn’t completely disappeared. Bloodletting is an effective treatment for some rare conditions like hemochromatosis, a hereditary condition causing your body to absorb too much iron.

Leeches have also made a comeback in medicine. We now know that leech saliva contains substances with anti-inflammatory, antibiotic, and anesthetic properties. It also contains hirudin, an enzyme that prevents clotting. It lets more oxygenated blood into the wound, reducing swelling and helping to rebuild tiny blood vessels so that it can heal faster. That’s why leeches are still sometimes used in treating certain circulatory diseases, arthritis, and skin grafting, and helps reattach fingers and toes. (Contrary to popular belief, even the blood-sucking variety of leech is not all that interested in human blood.)

2. Scientists didn't understand how blood circulation worked until the 17th century.

William Harvey, an English physician, is generally credited with discovering and demonstrating the mechanics of circulation, though his work developed out of the cumulative body of research on the subject over centuries.

The prevailing theory in Harvey’s time was that the lungs, not the heart, moved blood through the body. In part by dissecting living animals and studying their still-beating hearts, Harvey was able to describe how the heart pumped blood through the body and how blood returned to the heart. He also showed how valves in veins helped control the flow of blood through the body. Harvey was ridiculed by many of his contemporaries, but his theories were ultimately vindicated.

3. Blood types were discovered in the early 20th century.

Austrian physician Karl Landsteiner discovered different blood groups in 1901, after he noticed that blood mixed from people with different types would clot. His subsequent research classified types A, B and O. (Later research identified an additional type, AB). Blood types are differentiated by the kinds of antigens—molecules that provoke an immune system reaction—that attach to red blood cells.

People with Type A blood have only A antigens attached to their red cells but have B antigens in their plasma. In those with Type B blood, the location of the antigens is reversed. Type O blood has neither A nor B antigens on red cells, but both are present in the plasma. And finally, Type AB has both A and B antigens on red cells but neither in plasma. But wait, there’s more! When a third antigen, called the Rh factor, is present, the blood type is classified as positive. When Rh factor is absent, the blood type is negative.

Scientists still don’t understand why humans have different blood types, but knowing yours is important: Some people have life-threatening reactions if they receive a blood type during a transfusion that doesn’t “mix” with their own. Before researchers developed reliable ways to detect blood types, that tended to turn out badly for people receiving an incompatible human (or animal!) blood transfusion.

4. Blood makes up about 8 percent of our total body weight.

Adult bodies contain about 5 liters (5.3 quarts) of blood. An exception is pregnant women, whose bodies can produce about 50 percent more blood to nourish a fetus.)

Plasma, the liquid portion of blood, accounts for about 3 liters. It carries red and white blood cells and platelets, which deliver oxygen to our cells, fight disease, and repair damaged vessels. These cells are joined by electrolytes, antibodies, vitamins, proteins, and other nutrients required to maintain all the other cells in the body.

5. A healthy red blood cell lasts for roughly 120 days.

Red blood cells contain an important protein called hemoglobin that delivers oxygen to all the other cells in our bodies. It also carries carbon dioxide from those cells back to the lungs.

Red blood cells are produced in bone marrow, but not everyone produces healthy ones. People with sickle cell anemia, a hereditary condition, develop malformed red blood cells that get stuck in blood vessels. These blood cells last about 10 to 20 days, which leads to a chronic shortage of red blood cells, often causing to pain, infection, and organ damage.

6. Blood might play a role in treating Alzheimer's disease.

In 2014, research led by Stanford University scientists found that injecting the plasma of young mice into older mice improved memory and learning. Their findings follow years of experiments in which scientists surgically joined the circulatory systems of old and young mice to test whether young blood could reverse signs of aging. Those results showed rejuvenating effects of a particular blood protein on the organs of older mice.

The Stanford team’s findings that young blood had positive effects on mouse memory and learning sparked intense interest in whether it could eventually lead to new treatments for Alzheimer’s disease and other age-related conditions.

7. The sight of blood can make people faint.

For 3 to 4 percent of people, squeamishness associated with blood, injury, or invasive medical procedures like injections rises to the level of a true phobia called blood injury injection phobia (BII). And most sufferers share a common reaction: fainting.

Most phobias cause an increase in heart rate and blood pressure, and often muscle tension, shakes, and sweating: part of the body’s sympathetic nervous system’s “fight or flight” response. But sufferers of BII experience an added symptom. After initially increasing, their blood pressure and heart rate will abruptly drop.

This reaction is caused by the vagus nerve, which works to keep a steady heart rate, among other things. But the vagus nerve sometimes overdoes it, pushing blood pressure and heart rate too low. (You may have experienced this phenomenon if you’ve ever felt faint while hungry, dehydrated, startled, or standing up too fast.) For people with BII, the vasovagal response can happen at the mere sight or suggestion of blood, needles, or bodily injury, making even a routine medical or dental checkup cause for dread and embarrassment.