How cold weather might actually help you burn more fat
Feeling chilly? Good. Shivering? Even better. Scientific research says cold weather might trigger a way to burn more fat – and that could lead to a new kind of weight loss strategy.
Study results from 2011 and 2014 are likely to further fuel the quest to identify new ways to pharmaceutically tame obesity by targeting how much energy we burn, not just how many calories we consume.
Brown fat and white fat
Throughout the interior spaces of humans and other warm-blooded creatures is a special type of tissue known as brown fat, which may hold the secret to diets and weight-loss programs of the future.
Unlike ordinary “white” fat, in which the body stores excess calories, brown fat can burn calories to heat up the body. It’s one of the things that helps keep wild critters warm on cold nights.
Investigating how brown fat works in mice, a team of researchers at the University of California, San Francisco uncovered what may be a holdover from our evolutionary past: in response to cold, tiny immune cells known as macrophages can switch on the brown fat, inducing it to burn energy to make heat.
Prior to this research, published in 2011 in the journal Nature, scientists had assumed that brown fat metabolism was completely controlled by the brain. But the UCSF research suggests that the immune system plays a backup role in this process—a legacy, perhaps, of some ancient ancestral creature whose metabolic and immune systems were much more intertwined.
“This is a very important secondary system that the body uses to provide a backup for the thermal stress response,” said Ajay Chawla, MD, PhD, an associate professor at UCSF’s Cardiovascular Research Institute who led the research. “It raises the possibility that we can perhaps modulate this program and enhance it in humans to rev up metabolism.”
The modern human immune system relies on these macrophages to gobble up bacteria, helping protect us against infection. Macrophages were never known to play a role in metabolism, but the evidence Chawla and his colleagues gathered suggests otherwise.
How our bodies maintain our temperature
Using brown fat to burn calories and produce heat is one of the ways that mammals maintain thermoregulation — an essential adaptation that defines warm blooded creature and enables them to thrive in the face of challenging environmental extremes. Not all animals share this ability.
Many animals, like lizards, are “cold blooded” or exothermic. They maintain their body temperature through completely external means, sunbathing at certain times of the day and huddling in warm, protective places at night. This naturally limits their range and explains why lizards, so abundant in tropical climates, are far rarer in cold climates.
Mammals, on the other hand, are “warm-blooded” or endothermic. They produce heat internally by a variety of means: shivering, sweating, regulating the size of their blood vessels and burning off excess calories in brown fat.
Scientists have known for years that brown fat burns calories in response to signals from the brain. These signals cause break down of molecules known as triglycerides in white fat, which are then released into the bloodstream as fatty acids. These circulating fatty acids are taken up by brown fat and burned to generate heat. Brown fat is full of blood vessels, and the heat warms the blood, which in turn circulates and warms the body.
The brain controls this process by monitoring the body’s temperature and, in face of extreme cold, releasing a hormone called norepinephrine, which kick-starts the brown fat.
The work of the UCSF team showed that macrophage cells within the brown fat can also do this directly. Macrophages residing in brown and white fat produce an enzyme that makes norepinephrine when mice are exposed to the cold. This leads to the production, breakdown and mobilization of stored fat, which is then burned in brown fat to produce heat.
What these results suggest, Chawla said, is that immune cells help facilitate the function of brown fat.
Updated research about how cold can help burn more fat
The kind of calorie-burning triggered by cold temperatures can be achieved biochemically — without the chill — raising hopes for a weight-loss strategy focused on the immune system rather than the brain, according to a 2014 study by the same UC San Francisco researchers.
The team determined that two signaling molecules secreted by cells of the immune system trigger the conversion of fat-storing white fat cells to fat-burning beige fat cells. Ajay Chawla, MD, PhD, an associate professor of medicine at the UCSF Cardiovascular Research Institute, led the study, published online June 5, 2014 in the journal Cell.
Working with mice, Chawla’s team discovered that the signaling molecules, called interleukin 4 and interleukin 13, activate cells known as macrophages, which in turn drive the fat conversion. In one experiment, the researchers gave interleukin 4 to fat mice, which increased beige fat mass, leading to weight loss.
The finding builds on previous work by Chawla’s team, which reported in 2011 in Nature that cold activates part of the immune system, and specifically activates interleukin 4 in fat. In the new study, Chawla’s team determined that both interleukin 4 and interleukin 13 recruit macrophages to fat and that the production of molecules called catecholamines by the macrophages causes the browning of white fat.
When the researchers inhibited interleukin 4 signaling in white fat, they found that the mice made less beige fat, burned less energy and could no longer maintain normal body temperature in the cold.
The study results are likely to further fuel the quest to identify new ways to pharmaceutically tame obesity by targeting how much energy we burn, not just how many calories we ingest, according to Chawla.
“If you could increase energy expenditure by even a few percent, over a period of a year or two years, you would make a big difference,” he said.
Shivering leads to weight loss
The new discovery is surprising, Chawla said, because it makes it clear that this control mechanism for fat burning bypasses components of the autonomic nervous system that govern many physiological adaptations.
“Nutrient and energy metabolism has largely been thought to be under the control of the brain and endocrine system,” he said.
In comparison to the nervous system, the immune pathway might be more easily manipulated to increase energy expenditure, Chawla said. In fact, another study published simultaneously in Cell by researchers from the Dana-Farber Cancer Institute and Harvard Medical School reports the identification of a hormone, produced in fat tissue after cold exposure, that activates interleukin 4 and interleukin 13 to drive fat burning.
Humans and other mammals shiver to keep warm, but cold also triggers the growth of fat cells that burn fuel, instead of the fat cells that store it.
Keep humans indoors at 61 degrees to 63 degrees Fahrenheit without allowing them to bundle up, and they lose weight, research shows. That’s because they adapt by generating more fat-burning cells to help them keep warm.
In contrast to the power-converting mechanisms in white fat cells, the gears in the power plants within fat-burning fat cells spin inefficiently, which causes them to burn more energy and generate heat. The trigger for this accelerated fat burning is the activation within the cell’s power plants – called mitochondria – of a protein called uncoupling protein 1 (UCP1). Cells with UCP1 are capable of heat generation and fat burning, and are known as brown fat or beige fat, depending on the tissue from which they originate. They have more mitochondria than white cells, and therefore have a darker tinge.
In comparison to other mammals, ranging in size from mice to bears, until a few years ago it was widely thought that humans had little brown or beige fat and little potential to generate it.
Although Chawla and many other researchers now believe that the potential to exploit brown fat for weight loss is significant, the amount of individual variation when it comes to brown fat reserves and the potential to generate more brown fat is unclear. “We don’t know what the dynamic range is,” Chawla said. “It appears that women have more, that we have less as we age, and that obesity is associated with having less brown fat.”