People of South Asian origin are at markedly higher risk for developing type 2 diabetes and do so at a younger age and lower body mass index compared with whites. Now researchers have linked less brown adipose tissue (BAT) stores, used to generate heat in colder climates, and activity in South Asian people to this increased risk, as reported online ahead of print in Lancet Diabetes & Endocrinology.
“Based on the current research it appears that BAT may now be considered one of several contributors to the higher risk that South Asians have for diabetes,” commented Aaron M. Cypess, MD, PhD, MMSc, Assistant Professor at Harvard Medical School and Assistant Investigator and Staff Physician at Joslin Diabetes Center, Boston, MA.
“Other variables that may also play a role include the relative amounts of different kinds of fat in the body of South Asians. They may have more visceral fat, which is prone to causing metabolic disease, and they may have proportionally less skeletal muscle than other populations,” said Dr. Cypess whose early research using PET/CT imaging described the metabolic importance of BAT.
“These results encourage us to do more studies to see just how BAT fits into the body’s overall regulation of energy balance and determine if there are other populations in whom BAT may not work as well as it could,” Dr. Cypess said.
Metabolic Effects of BAT
BAT is associated with beneficial effects on glucose tolerance, lipid metabolism, and body weight in preclinical studies. Unlike white adipose tissue, which stores excess calories, BAT can convert some of this energy (lipids and glucose) into large amounts of heat when the body is exposed to cold, thus burning the energy surplus rather than storing it. Earlier studies have shown that obese people have less BAT stores and activity than non-obese people.
In this study, researchers from the Netherlands compared resting energy expenditure and BAT volume and activity in 12 healthy lean South Asian men (age 18-28 years) and 12 matched white men after exposure to cold using 18F-fluorodeoxyglucose positron emission tomography and computed tomography (18F-FDG-PET-CT) scans. The effect of cold exposure on non-shivering thermogenesis and plasma lipid levels also was measured.
South Asian participants had a significantly lower resting energy expenditure (-32%; P=0.0008), volume of metabolically active BAT (-34%; P=0.04). Similarly, the South Asian men had a significantly higher shiver temperature (10.9°C vs 8.9°C) upon cold exposure, despite their higher total fat mass and a lower cold-induced non-shivering thermogenesis compared with white men.
“The chief strengths of the study are that it is a controlled, prospective clinical trial, and it is the first of this type to look at ethnic differences in BAT function,” Dr. Cypess commented. “The principal limitation is the small numbers of subjects evaluated. It will take additional studies in far larger and more diverse groups of subjects to confirm the differences between Europeans and South Asians,” he said.
Clinical Implications of the Findings
“Our findings suggest that making more BAT or increasing its activity could have great therapeutic potential in South Asians, helping them increase the clearance of glucose and fatty acids and converting surplus white fat into heat, potentially lessening the risk of diabetes,” according to lead authors Mariëtte Boon, MSc, and Patrick Rensen, PhD, from Leiden University Medical Center in the Netherlands.
“It has recently been shown that BAT can be recruited in humans following 10 days of cold intervention. Future studies should be directed towards the efficacy of this strategy, as well as other options, such as medication, to increase BAT activity. These strategies might finally be used to improve the metabolic phenotype in South Asians,” Drs. Boon and Rensen said.
Dr. Cypess believes it is important to understand that BAT is a focus of treatment for three reasons: “One, it certainly is an appealing target for increasing the number of calories burned in the body as a way of treating obesity; 2), BAT also consumes large amounts of glucose and triglycerides, so we are also interested in seeing if it can be used as a more direct treatment for diabetes and insulin resistance; and 3) information from recent research studies suggests that BAT may be an endocrine organ, capable of regulating metabolism in many organs of the body.”