Over the past few years, a new class of medications has transformed the treatment of obesity. Drugs like Ozempic, Wegovy and Mounjaro work primarily by reducing appetite, helping people eat less and feel full sooner. Their success has demonstrated something important: Body weight is biologically regulated, and targeting the right biological pathways can lead to meaningful weight loss that can help transform lives.
But appetite is only half of the equation. Your weight reflects a balance between the calories you consume through your diet and the energy you expend through movement, exercise and maintaining basic cellular function. While recent therapies have focused on controlling energy intake, scientists are increasingly turning their attention to the other side of the ledger: the tissues that burn energy.
At the center of this conversation is an organ most people misunderstand: fat. For decades, fat – also known as adipose tissue – was thought of as passive storage: a biological pantry for excess calories. Scientists now know this view is incomplete.
Fat is not just storage
White adipose tissue, the most abundant type of fat in adults, does store energy in the form of triglycerides. But it also has several other functions.
For one, white fat is a powerful endocrine organ, releasing hormones like leptin that reduce appetite, as well as adiponectin, which regulates insulin and blood sugar levels. It also cushions organs, insulates against heat loss and acts as a metabolic buffer, safely storing excess lipids that would otherwise accumulate in the liver or muscle.
White adipose cells provide several essential bodily functions.
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When white adipose cells expand in a healthy, flexible way, they protect the body. When they become inflamed or dysfunctional, they contribute to insulin resistance, fatty liver disease and cardiovascular risk. Obesity arises from both the expansion of white adipose cells and an increase in their number.
In other words, fat is not inherently harmful. Its health impact depends on the size of adipose cells, and when they become too large, they are unable to function optimally. Increasing the number of new fat cells can sometimes improve metabolic function.
Moreover, there are additional types of fat, and they behave in different ways.
Brown fat: The cellular furnace
Unlike white fat, brown fat is specialized to burn energy. Brown adipose cells are packed with mitochondria – the tiny power plants inside cells – and contain a protein called UCP1 that allows them to convert chemical energy directly into heat. Instead of storing calories, brown fat dissipates them.
In infants, brown fat helps maintain body temperature. For years, scientists believed it largely disappeared in adulthood. But imaging studies in the late 2000s revealed that many adults retain metabolically active brown fat, particularly in the neck and upper chest.
