There has been a flurry of articles recently about the rise in research on brown adipose tissue (BAT), endogenous body fat that contains higher levels of mitochondria and is used to help keep the body warm. Until only three years ago, this holy grail of body tissues ("good" fat that burns significantly more calories and can help rid the body of "bad" fat) was thought to exist only in rodents, where it was more commonly seen in the young and in thinner animals. However, BAT has also been seen in human infants, important in helping to keep newborns warm as they can not shiver to create their own body heat. BAT was thought to gradually disappear as individuals aged, but it is now believed that adults can retain small levels of their brown fat from childhood, with thinner individuals maintaining more, and that exercise can aid in this retention. How BAT promoted weight loss was not understood until recently. Researchers from Canada shed light on this process using PET-CT scans to identify the metabolic processes involved in BAT. Published this month in the Journal of Clinical Investigation, researchers subjected participants to acute cold exposure by placing them in a special liquid thermo-controlled suit at a temperature of 18 degrees Celsius (64 degrees Fahrenheit) for 90 minutes. During the course of this exposure, brown fat in the upper back experienced a significant increase in cell metabolism while working to keep the body warm, a process dubbed "cold-induced nonshivering thermogenesis". Researchers hypothesized that this increase in BAT metabolism was initially fueled by elevations in extracellular glucose and fatty acid uptake, but that when these levels were depleted the tissue began drawing on stores of intracellular triglycerides, meaning that BAT was burning off lipid reserves during cold exposure.
As such, total energy expenditure of participants increased during the study by a whopping 80%, resulting in an average burn of an additional 250 calories. Curiously, there was no significant interaction between the amount of brown fat an individual had and caloric expenditure despite a wide variability in BAT levels, though this could be due to the small study sample size, involving only six participants. There was an interaction between BAT and thermogenesis though, with the added increase in metabolism helping to keep an individual warmer longer, and the more brown fat a participant had the longer and colder conditions he or she could stand before starting to shiver.
A second study on brown fat looked at a more elusive type whose production is promoted through exercise. Published this month in the journal Nature, researchers at the Dana-Farber Cancer Institute discovered a new hormone, christened irisin, that seems to help transform white fat into brown fat. This production is dependent upon the transcriptional co-activator PGC1-α, a protein found in muscle tissue that is generated during exercise and involved in metabolism, cell genesis, and protection against muscle atrophy.
Knowing its vast beneficial effects, scientists bred mice to have elevated levels of PGC1-α to determine its influence on BAT and energy expenditure. Although the increase in protein had no effect on either brown or typical white adipose tissue, there were effects in a special type of subcutaneous white fat that is more susceptible to "browning". This process involves increases in levels of the protein UCP1, which is highly active in brown fat cells and is involved in thermogenesis. These same effects also seem to occur following a regular exercise program in mice, facilitated by changes in mRNA expression induced by increases in PGC1-α and subsequent protein production.
Through several elaborate experiments, researchers were able to narrow down the proteins to those affected by the gene expression of FNDC5, including the newly discovered irisin. Irisin is significantly elevated in both mice and humans after exercise, and appears to be the key ingredient in the expression of UCP1 in the transition from white to brown fat. Direct injections of the protein resulted in increased levels of UCP1 in subcutaneous white fat, as well as subsequent increases in metabolism and small decreases in body weight in obese mice 10 days after exposure.
While both of these studies are still in their infancy, their potential implications for future research are very exciting. In the mean time, if you want to lose weight try going for a run, the benefits may be twofold. Alternatively, if you're too lazy to work out you could try sitting outside in the cold for a while.*
*Please note, I do not actually recommend this as a safe or valid weight loss plan.