Low metabolism can have many causes, but one of them deals with a certain type of fat.
People have two types of fat: white and brown fat.
White fat is what people typically refer to when they mention losing body fat. This fat is not very metabolic and is usually the culprit of health consequences. However, brown fat is quite the opposite.
It is very metabolic and when it’s activated, it tends to even burn white fat!
Typically babies have the highest amounts of brown fat and have less as they reach adulthood. Most people have pounds of white fat but ounces to grams of brown fat. And while white fat is stored primarily around the stomach, hips and thighs, brown fat does not have a predictable pattern (although it’s more often around neck and shoulder area).
Plus, research has emerged that shows brown fat may help fight off chronic diseases.
In a 2021 study published by Nature Medicine, 52,000 subjects were studied. Only 10% of subjects had detectable brown fat (seen via PET scans). And those subjects were less likely to have heart and metabolic conditions such as type II diabetes.
In fact, out of those with detectable brown fat, 4.6% had type II diabetes (compared to 9.5%) and 18.9% had abnormal cholesterol levels (compared to 22.2%). And they had lower levels of congestive heart failure, hypertension.
So what controls brown fat?
Researchers used to think that brown fat just disappears as we age. However, some research has shown that this is not the case. Exposure to colder temperatures can actually increase brown fat – and with that an increase in metabolism! In one study, researchers found that they could turn stem cells into brown fat instead of white fat with temperature change.
In a small 2014 study, published in Diabetes, five subjects stayed in rooms set for different temperatures for four months. They did their normal everyday activities and then stayed and slept in their rooms for 10 hours each evening. They wore the same outfits in their rooms. They used the same type of bedding. They all ate the same amount of calories that had the same nutrient density. Each month they had fat and muscle biopsies, PET/CT scans of the upper back and neck, and measurements on energy expenditure.
After a month of exposure to the cold rooms, subjects increased their brown fat by 42%! Plus, they increased their fat metabolism by 10%, had significant changes in the hormones adiponectin and leptin (metabolism hormones), and improved their insulin sensitivity after meals. And remember this was just after one month of exposure to colder temperatures.
So why does the cold cause an increase in brown fat?
A 2014 study published in Cell Metabolism opened up this topic even further.
Scientists have known that during contraction of the muscles, the body releases a hormone called irisin. Irisin can literally take white fat cells and convert them into brown fat cells. But they haven’t known exactly how this happens. In this study, they had 10 female subjects exercise on a stationary bike. Researchers found that during brief intense bouts of cycling, irisin levels rose. And during an hour of low to moderate intensity, it rose 3x. This demonstrated exercise’s power to boost irisin levels.
However, they took this study one step further.
This time they wrapped the subjects in water-infused thermo-blankets that progressively went from 80 degrees to 53. When this happened, subjects burned 48% more energy. And when they measured their muscular activity, they found that it increased by 88% for those who shivered (13% for those who didn’t shiver). Plus, the more shivering, the more irisin was released.
“Cold-induced shivering, which is an energy-inefficient mechanism, stimulates the highly efficient brown adipose tissue to maintain the core temperature of the organism”. - Dr. Francesco S. Celi
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