1. Brown Fat: Body Fat That Makes Heat Instead of Storing Energy
When you hear the word "fat," you might imagine a warehouse that stores energy from food. But the body has another type of fat tissue called brown fat, whose main job is to produce heat [1]. This tissue is found in large amounts in human babies [2].
In newborns, reports say it is found under the skin at the back of the neck, between the shoulder blades, in the armpits, in the center of the chest, at the base of the legs, and around the spine and kidneys [1]. But what does it actually mean for fat to turn into heat?
2. How Cold Signals Make Mitochondria in Brown Fat Cells Produce Heat
Inside the cells that produce heat, mitochondria perform a special task. Normally, mitochondria use the flow of small particles called protons moving back across a membrane to create ATP, a form of energy. In brown fat mitochondria, there is a protein called UCP1. This protein lets protons flow through a different path. As a result, the efficiency of making ATP drops, and much of the energy is released as heat [1][2].
You can imagine a dam that uses a water wheel to generate power. Now imagine building a channel next to the wheel that lets water bypass the wheel entirely. In the body, the "water" is protons, and they leak back across the membrane instead of turning the energy wheel.
The trigger for this process is cold. When the body feels cold, the sympathetic nervous system releases a substance called norepinephrine, which starts the brown fat working [1]. Because this creates heat without making muscles shake, it is called non-shivering thermogenesis. How much does a newborn body rely on this system?
3. Why Newborns Cannot Rely on Shivering Alone to Stay Warm
There is a report on heat production in newborn lambs. It found that shivering made up about 60% of heat production right after birth, but this rose to about 95% by 30 days old [1].
These numbers show that in lambs, heat production methods other than shivering play a large role right after birth. In animals like sheep, it is said that heat production via UCP1 in brown fat is essential for adapting to cold after birth [1]. However, these are numbers from lambs, so they cannot be applied directly to human babies.
Heat production without shivering is used in other situations too.
4. How Brown Fat Helps Hibernating Animals Warm Up and Wake
Brown adipose tissue was first identified in 1551. Swiss scientist Gesner found it between the shoulder blades of a marmot. At that time, it was linked to hibernation [2].
In small hibernating mammals, it is explained that heat from brown fat is essential to start waking up [3]. In a study of the 13-lined ground squirrel, a type of squirrel, it is said that when they wake up periodically during hibernation, their body temperature returns to normal within 3 hours. At that moment, stored fat is converted into heat in brown fat [3].
These numbers come from a study of this specific squirrel species and do not apply to all hibernating animals. So, is this important tissue also present in adults?
5. Do Adults Have Brown Fat? What Was Believed Until 2009
Brown fat is abundant in human babies, but until 2009, the widely held view was that adults did not have it [2].
This view changed with three papers published simultaneously in 2009. Using a scan called PET-CT, it was shown that adults also have brown fat. One of these papers examined 3,640 PET scans and reported that brown fat images were found in 7.5% of women and 3.1% of men [4]. However, these are numbers for patients scanned at normal room temperature, not the percentage for all adults.
In adults, brown fat is often found above the collarbone, in the neck, and near the center of the chest [4]. The amount per body weight is said to be much less than in mice [5]. The role of this small amount of brown fat in adults is still a question under study.
6. How to Observe Your Own Body's Reactions on a Cold Day
If you want to explore this, go outside on a cold day and write down what your body does in a notebook. Visible reactions like "shrugging shoulders," "getting goosebumps," or "shivering" are enough. You cannot see brown fat working, but you can see shivering. By listing the visible reactions alongside the concept of "heat production without shivering" from this article, you can find clues about which mechanisms are separate.
SAFETY NOTE: Do not stay in places that are too cold for too long.
If you want to know more about body temperature, looking for books on the human body or physiology at the library is one option.