Why Some People Are Naturally Thinner

Did you know that not all fat in the body is the same? And that some fat actually helps burn more calories than other types? It’s true, and this discovery may help explain why some people seem effortlessly thin while others struggle with their weight.

The Two Types of Body Fat

There are two main types of fat in the human body: white fat and brown fat. These tissues serve very different purposes and have dramatically different effects on metabolism.

Brown Fat: The Calorie Burner

Brown fat breaks down blood sugar (glucose) and fat molecules to create heat and help maintain body temperature. Babies are born with abundant brown fat—in fact, brown fat is what helps babies regulate their body temperature during those early critical days. Unfortunately, as we age, our brown fat begins to disappear.

Recent NIH research has revealed fascinating details about how brown fat works. Scientists now believe that people who are “naturally thin” retain more brown fat than those who are more prone to weight gain. The thought is that brown fat helps people stay thin because it burns significantly more calories than white fat.

The Energy Efficiency Factor

Another possible reason some lucky individuals stay thin despite their calorie intake has to do with how efficiently our bodies use energy. Surprisingly, when it comes to metabolism, it’s actually better to be wasteful than to conserve!

Bodies that “waste” a lot of energy need to burn more calories to perform the same tasks than bodies that are super-efficient. Studies show that energy efficiency can vary from person to person by two or more percentage points, which can lead to a big difference in body weight over time.

In fact, an efficient body can “save” enough calories to store up to 6 additional pounds each year! While being efficient might sound like a good thing, in terms of weight management, having a slightly “wasteful” metabolism that burns more calories can be advantageous.

How Brown Fat Affects Metabolism

A 2019 study funded by NIH’s NIDDK carefully analyzed blood levels of glucose, fats, and amino acids before and after activation of brown fat in 33 healthy young men. The research revealed that men with high brown fat activity had reduced levels of branched chain amino acids (BCAAs) in their bloodstreams, suggesting that brown fat was actively metabolizing these nutrients.

When brown fat is activated—typically by cold temperatures—it leads to various beneficial metabolic changes in the body. Research published in Nature identified a separate heat-production pathway in brown fat that can reduce weight gain by increasing energy use to compensate for increased calorie intake.

Current Research and Future Possibilities

Scientists continue to investigate ways to activate or increase brown fat in adults. One study showed that after four weeks of treatment with mirabegron, a drug that activates brown fat, women’s metabolism at rest was almost 6% higher, and brown fat activity increased during the study. At the end of the study, the women had higher levels of HDL cholesterol and other markers of reduced risk for heart disease.

However, it’s important to note that current evidence suggests brown fat contributes a relatively small amount to overall energy metabolism—unlikely to cause dramatic weight loss on its own, though improvements in glucose metabolism have been demonstrated in humans.

What This Means for You

While we can’t yet dramatically increase our brown fat stores or significantly alter our metabolic efficiency, understanding these factors helps explain why weight management isn’t simply a matter of willpower. Some people genuinely do have metabolic advantages that make staying thin easier.

This knowledge shouldn’t discourage those who struggle with weight—instead, it should inspire compassion and understanding. It also highlights the importance of developing healthy eating habits and regular physical activity, which benefit everyone regardless of their brown fat levels or metabolic efficiency.

The Bottom Line

The discovery of brown fat’s role in metabolism and the understanding of individual variations in energy efficiency provide fascinating insights into why some people are naturally thinner than others. While we may not be able to dramatically change our brown fat levels yet, ongoing research continues to reveal new possibilities for understanding and managing weight.

I’ll keep you updated as new studies are reported. In the meantime, focus on what you can control: eating nutritious foods, staying physically active, and developing healthy lifestyle habits that will serve you well regardless of your metabolic type.