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Some studies show that when you are cold, you burn more calories to keep warm. A special kind of fat — called brown fat — may use more energy (and calories) to keep your body warm. For many years, scientists have known that small animals and infants have this kind of fat. Recently, research shows that adult humans may also have brown fat.

If brown fat becomes active and burns calories in humans when exposed to cold, then these people would tend to burn off more calories and might not gain weight easily. Learning more about the relationship between energy expenditure, brown fat, age, environmental temperature and body temperature may help explain why some people become obese and other people do not.

A study at NIH is seeking to get more answers about how the body burns energy at different temperatures. It is currently seeking healthy men from ages 55 to 75. The study is also recruiting both men and women ages 18 to 35.

Inpatient study

Participants will stay in the Metabolic Unit of the National Institutes of Health Clinical Center in Bethesda, Md., as inpatients for up to two weeks. The length of the hospital stay will depend on how participants respond to the different study temperatures. All meals will be provided, as well as compensation for participating in the study.

Those in the study will first be screened with a physical exam and medical history. Blood and urine samples will be collected.

Participants will stay up to five hours per day in a specialized room with different temperature settings. Temperatures will range from about 61 degrees to 88 degrees Fahrenheit. Body temperature, activity, calorie burning, and cold/hot sensations will be monitored. On the study day of the coldest temperature, participants will have an imaging study to look for brown fat activity.

Every afternoon, participants will walk for 30 minutes on a treadmill.

Who can participate?

To control for ethnicity, participants must be non-Hispanic whites or African Americans. They must be very thin (BMI less than 18.5), slightly overweight (BMI 25 to 29) or very obese (BMI over 40). They cannot have gained more than 5 percent of their body weight in the last six months or be trained athletes.

Participants cannot have an over- or under-active thyroid, clinical depression, bipolar disorder or claustrophobia. Those with high blood pressure, diabetes, history of cardiovascular disease, liver disease, iron deficiency and abnormal kidney function cannot take part in the study. Participants also cannot have a history of drug or alcohol abuse in the last five years, or currently smoke.

To learn more, including compensation for accepted participants, contact the NIH Patient Recruitment and Public Liaison Office at 1-800-411-1222 or email prpl@mail.cc.nih.gov. Refer to study NCT01568671.Z