When you think about fat, you might picture it as your body’s insulation layer or storage for extra calories. But fat plays a much bigger role in the body. And if you assume that all fat is unhealthy, it's time to update your thinking.
Fat is found throughout the body: inside your nerves and bones, around your heart and blood vessels, and even behind your eyeballs. We need it to function and survive. “Think of fat as a mastermind in our bodies,” says Silvia Corvera, MD, a professor at the UMass Chan Medical School. “When things affect fat, it affects our entire body.”
The Function of Fat
Body fat, or adipose tissue, is a complex organ. It contains fat cells, nerves, immune cells, and connective tissue. Its main job is to store and release energy, depending on the body’s needs, says Susan K. Fried, PhD, a professor at the Icahn School of Medicine at Mount Sinai.
Fat responds to signals like the hormone insulin, which tells adipose tissue to store fat. “There are also nerves that go from your brain to your fat and say, ‘Hey, we don’t have enough energy here. Can you liberate some fat because other cells in the body need it?’ ” Fried says.
Fat tissue is also a major maker of hormones, chemical messengers that communicate with tissues and organs throughout the body. “It’s a classic endocrine organ, the biggest one in the body,” Fried says.
Hormones produced by adipose tissue regulate metabolism and insulin sensitivity. They help the body use nutrients efficiently. For example, it’s the primary producer of adiponectin. This hormone increases insulin sensitivity – a good thing for keeping blood glucose levels in check – and decreases inflammation. Too little adiponectin can lead to type 2 diabetes and other metabolic diseases.
Another hormone is leptin, which controls appetite. “If you have no adipocytes [fat cells], you have no leptin. You feel like you have no energy stores and get ravenously hungry,” Corvera says. Fat tissue also releases other substances that influence inflammation and immune function.
Different Types of Fat Cells
While all fat cells might look alike from the outside, they can have different functions, depending on their type. There are three main types of fat cells.
- White fat: These are the body’s main type of fat cells. They store energy and produce hormones like leptin and adiponectin. They are largely found in the chest, belly, and legs.
- Brown fat: What’s unique about brown, or thermogenic, fat is that it burns energy and produces heat in certain conditions, like cold weather. People with more brown fat tend to be leaner and healthier, compared to those with less brown fat. Studies show that brown fat improves metabolism and reduces the risk of diseases like type 2 diabetes, coronary artery disease, and high blood pressure. It’s found in the neck, upper chest, shoulder, and stomach.
- Beige fat: In some cases, white fat transforms into “beige” or “brite” fat cells. Like brown fat, it burns energy to produce heat.
Researchers are studying whether they can harness the positive traits of brown and beige fat and use them to treat obesity.
Fat also behaves differently depending on where it’s located: the belly, thigh, or near your organs. When it comes to health, location matters.
- Visceral fat: Fat stored deep in the belly and around organs has a major impact on the liver, an organ critical to metabolism. It’s also linked to diseases like cardiovascular disease, cancer, asthma, and dementia. Blood that leaves visceral fat goes directly to the liver and brings with it anything made by the fat tissue, including fatty acids, hormones, and pro-inflammatory chemicals. We build up more visceral fat with age. Fat storage shifts from the lower body to the belly, especially in women.
- Subcutaneous fat: The fat just under the skin is the most plentiful in the body. This type of fat acts differently depending on where it's located, according to Fried. Subcutaneous belly fat makes more fatty acids, which can increase insulin resistance and the risk of metabolic disease. Subcutaneous fat in the lower body, on the other hand, takes up and stores fat efficiently. It’s considered protective against disease.
How Much Is Too Much?
Fat is an essential part of our bodies. Too little or too much fat is unhealthy. “You have to have the right amount,” says Fried, but the right amount varies from person to person.
An important consideration is how much tissue you have available to store fat, which can depend on your genes. “If you don’t have the capacity to make a lot of fat, you won’t have enough space to store extra calories,” Corvera says. “It will spill over into your liver, muscles, and heart. That’s what gives rise to metabolic disease.”
Rather than thinking about weight or percentage of body fat, consider your waist-to-hip ratio. Research suggests that the way fat is distributed across the body matters more than the amount of body fat when it comes to overall health.
To get your waist-to-hip ratio, use a tape measure to measure your waist and hips in centimeters. Divide your waist measurement by your hip measurement. The World Health Organization says that the risk of health issues is greater for men who have a waist-to-hip ratio of 0.90 or higher, and for women with a waist-to-hip ratio of 0.85 centimeters or higher.
You can also just check your waist circumference. The chances of health problems related to obesity are higher for men with a waist of more than 40 inches, and for a woman (who’s not pregnant) with a waist circumference of more than 35 inches, according to the CDC.
“People realized that waist size is a risk factor for type 2 diabetes, other chronic disease, and even premature death,” Fried says. Conversely, having more body fat in your lower body may help protect your health.
Research also suggests that fat behaves differently in women and men. In one study, higher muscle mass seemed to protect women and men from cardiovascular disease. But women with higher fat, regardless of muscle mass, were less likely to die of heart disease – but only if their blood pressure, blood sugar, and cholesterol were under control. (Whether they were taking hormone replacement therapy for menopausal symptoms also mattered.).
Body fat isn’t just about your size or how you look. “What matters is how it affects your health,” Corvera says. “You have to have healthy adipose tissue to be healthy in all other aspects of your physical and mental health.”