From the “low-fat” food craze in the 1990s to the “eat more healthy fats” wave which swept over the country in the 2000s, dietary fads have fluctuated back and forth in the U.S. between eating fat and ditching it altogether. It’s hard to keep up, especially when you don’t have all the information about what fat is exactly and what it does in your body. Don’t miss this expert guide to body fat for answers to all your questions:

There are two main types of fat founds in humans and other mammals – white fat and brown fat. Each plays its own role and gets added and lost in its own way.

White Fat

The excess weight you are carrying around your midsection, hips, and thighs is composed in part by white fat, the most dominant fatty tissue in the body. White fat cells contain a single large lipid droplet and much fewer mitochondria and blood vessels than brown fat. Its appearance is white or yellowish in color. You may think fat just sits around doing nothing, but on the contrary, white fat plays many roles in your body. In addition to storing the bulk of your reserve energy, white fat cues hormones to generate metabolic processes, and it helps to insulate the body.

As a major organ of the endocrine system, white fat has receptors for hormones like insulin, norepinephrine, and cortisol, as well as manufactures some hormones like leptin, which helps control your appetite. Your body fat percentage is largely based on the amount of white fat you carry around and will vary by sex and age. A healthy adult male may have anywhere from 8% to 26% body fat depending on their age, and women 20% to 34%. White fat also helps cushion organs during strenuous activities like exercise.

Brown Fat

Brown fat has thermogenic properties in that it helps your body heat up when you are cold. In fact, as your body temperature drops in colder weather, the mitochondria in brown fat are activated to start burning calories for heat. Interestingly, brown fat contains more mitochondria than white fat. Mitochondria are the powerhouse organelles inside cells which take absorbed nutrients, break them down, and generate the molecules which fuel a cell’s energy. Brown fat has a darker color than white fat in part because the extra mitochondria in it contain higher amounts of iron.

Scientists used to believe only infants had brown fat, however, recent research has shown that brown fat accumulates around the neck and shoulders in adults, and sometimes randomly in other locations as well. Touted for its potential ability to harness greater weight loss results than other supplements and diet plans, the calorie-burning powers of muscle-like brown fat continue to be studied as a tool to combat the worldwide obesity epidemic. A 2012 study in the Journal of Clinical Investigation revealed that as a non-shivering thermogenesis effector, brown fat (brown adipose tissue or BAT) burns more calories than white fat when activated.

How do you measure fat?

There are a handful of ways to measure the amount of fat on your body to help you monitor whether you have too much or too little. Older tools like skinfold calipers pinch the fatty areas of your body to measure them in inches and then apply those numbers to a body mass index formula which generates an estimated body fat percentage. More expensive digital tools like body scanners are high dollar items which actually scan your body to reveal your fat percentage, waist girth, and lean muscle mass amongst other key data sets.

A good in-between is a body fat scale which you can use at home. You stand on it like any other scale, except it uses Bioelectric Impedance Analysis (BIA) to measure so much more than your weight. By sending low electrical currents up through your body, a body fat scale can accurately gauge your body fat percentage, bone density, water content, muscle mass, and BMI.

What does it mean to “lose fat”?

If you store fat when you eat and drink, how does it then magically go away when you burn calories? Well, physical exercise and activity taps stored fat reserves and converts those cells into energy you can expend. The metabolic processes involved fuel your muscles and other tissues, and thusly, shrink the fat cells from which they pulled.

Unfortunately, 1 pound of fat is equal to roughly 3,500 calories so losing fat isn’t super easy. Even if you restricted your caloric intake vastly, much of what you lose in addition to fat is lean muscle and water. Losing fat takes healthy diet modifications and lifestyle changes like exercising more and getting quality sleep.