By Dan Gwartney, M.D.
Everyone has his/her own ways to “chill out.” Some may enjoy a long drive to nowhere on a sunny day; others may choose to soak in a hot tub, feeling jets massage tense back muscles. My personal choice is sitting on a white sand beach, enjoying the Caribbean sun and surf with my wife— a good book and a cold Corona are just icing on the cake.
Most people don’t associate chilling out with losing fat. In fact, it is often the opposite as the frustration of losing fat is anything but relaxing, and isn’t that the purpose of chilling?1 Step back a few decades before the slang definition of chilling overshadowed the literal definition— to cool by exposing to a lower temperature. Through their years, many have lived through cold winters, and seeking a way to chill (literally) is the last thing on their minds. Few wish to actively seek out frigid climates. Yet, it appears that the pleasure and luxury of an artificially temperate indoor environment may be a factor in the epidemic rise of obesity in the U.S. and other developed countries.
To Chill……Or Not
People naturally seek out a temperature that allows for the least metabolic demand; this is referred to as the “thermal neutral zone” or TNZ.2 For humans, the lower limit of the TNZ in the unclothed (naked) state is 25ºC-27ºC (~77ºF-81ºF). Appropriate clothing allows people to tolerate lower temperatures, but in cooler temperatures, the body is forced to raise its metabolism to generate heat, even when wearing jackets or parkas. Obviously, better apparel provides better insulation, but at low temperatures, the body still reacts. This may be due to the colder air entering the lungs, conductive loss through shoes or other body surface areas in direct contact with the ground (e.g., sitting on a metal bench), or convective loss due to wind or failing to wear a cap.
Exposure to the cold is generally experienced only in the outdoors in the U.S. Most homes and businesses are climate-controlled, maintaining a state called “thermal monotony.”3 This was not always the case; drafty homes, inefficient furnaces, lack of central air conditioning, and fiscal conservation combined to keep homes much cooler in the winters and warmer in the summers. Now, most people expect to stay warm in the winter and cool in the summers. The trend for warmer winter living conditions mirrors the trend for obesity, causing one to question if that is due to coincidence or is there something to the association?
An excellent review on the association between chronic living conditions and energy expenditure (calorie burning) was published in the journal Obesity Reviews.4 The authors reviewed reports of the increasing average indoor temperatures, especially in the bedrooms as opposed to living areas. Over the last 30+ years, the period during which the obesity rate in the U.S. has tripled, indoor temperatures are being kept approximately 3ºC (nearly 6ºF) warmer during the winter, and bedrooms are 5ºC (9ºF) warmer. These numbers seem inconsequentially small, no more convincing than Al Gore’s global warming arguments.
However, as these numbers range from a point below the TNZ, the impact on the body is significant. People are aware of extreme changes in temperature, such as being caught unaware by a hard winter’s wind or spring shower. When responding to a sudden and drastic hypothermia, the body reacts in a variety of ways to insulate itself and reduce heat loss.5
Everyone has experienced “goose pimples” which are smooth muscle contraction about the hair follicle. This thickens the skin a bit and raises the hair follicles of the skin, increasing the insulating factor by keeping heat close to the skin. Blood is shuttled away from the body’s surface and diverted to the core. The heart is forced to beat more rapidly and forcefully to account for the sudden reduction in vascular volume. A classic experiment in physiology is to measure the increase in blood pressure and heart rate when hands or feet are submerged in ice water.
More relevant to this discussion is the response to longer-term exposure of less extreme cold. When a man is tolerating a chill below the TNZ, certain responses occur that increase the production of body heat (thermogenesis). Again, people are familiar with the thermogenic response to extreme cold— shivering.6 However, a more important, and long-ignored process might hold the key to weight/fat loss.
Fat-burning occurs in tiny compartments of cells called mitochondria. Mitochondria are dense in skeletal muscle, which is why exercise is such an effective means of burning calories and losing body fat. There is another tissue that is even more dense with mitochondria, and ironically, it is a form of fat. White adipose tissue (WAT) is the familiar form of fat, basically a storage site for excess fat consumed in the diet. WAT does have certain metabolic roles, but is relatively inert in regard to burning calories.
Another form of fat, once felt to be irrelevant to adult humans, is brown adipose tissue (BAT). BAT doesn’t store fat, but serves a different function— it burns fat.7 BAT is called brown because it is so tightly packed with mitochondria that it is dark. It is also very vascular, an important fact as it allows for the dissipation of heat, rather than trapping it in the local area where the fat lies.
BAT is normally inactive, as long as a person is in his TNZ or present in an even warmer area. This is part of the reason it is rarely detected, as it is an internal fat that is scanned for by an imaging technique called positron-emission tomography (PET). PET lights up tissue that is actively burning glucose. BAT does not burn glucose when the environment is warm and comfortable, thus it is invisible to PET scans, unless the patient is maintained in a cold room.8
When activated, BAT activity increases 10-15 times above baseline.9 These properties should make it no surprise that BAT is actually more closely related to skeletal muscle than WAT. WAT, BAT, and skeletal muscle share a common progenitor; they arise from the same early stem cell.10 People who exist in a state of thermal monotony tend to divert maturation of this stem cell to WAT rather than to BAT or muscle. This native inactivity and stealth presence is why BAT was felt to be irrelevant in adults.
However, when a person is in a cool to cold place, BAT activates and burns fatty acids like an acetylene torch to generate heat. If BAT were allowed to burn at maximal capacity, there is hope that fat would drip off a body like wax from a candle. This hypothesis is being actively pursued by pharmaceutical companies, primarily through activation of beta3-adrenoreceptors.11
As evidence of the fat loss potential of BAT, one can look at the studies reviewed in the Obesity Reviews article. The authors found a common metabolic response to changes in the environment. Calorie burning can be measured in sealed chambers, which can be controlled for room temperature. Compared to being in the TNZ, dropping the room temperature by a few degrees (Celsius) can raise the basal metabolic rate (BMR) 10 percent or more.4 Sitting in a room that is warmer than normal living conditions can lower BMR by 5 percent. Over the course of a year, “chilling out” could lead to the loss of nearly nine pounds of fat.4
Many people have no control over the temperature of their workplace or school; spouses or roommates may have no tolerance for an interior climate that is best suited for penguins. The challenge then becomes how to introduce enough chilling to activate the BAT as often as possible.
BAT responds quickly to changes in body temperature or sensed changes in the environment.10 For PET scans, a relatively brief exposure to a cold room and table surface activates the thermogenic tissue. Throughout the day, many opportunities arise for boosting the BAT for short periods. To offer a few examples: stepping outside upon awakening and enjoying a few minutes of the morning’s chill; turning the shower to cool/cold at the end for as long as possible; after showering, air-drying while finishing other morning grooming (e.g., brushing teeth or hair); dressing lightly; drinking iced beverages; sitting on conductive surfaces (e.g., tile, concrete); sitting in a bath and letting the water cool; sleeping with very light covers and a fan circulating the air.
All BAT-centric techniques will be much more effective in the winter months, as BAT is more easily recruited and plays a greater role in thermogenesis in cold-acclimated conditions.7
A question that might arise is whether there is any additive benefit to training in colder places, compared to warm. In general, physical performance is degraded by thermal stress, i.e., heat or cold, particularly cold below 50ºF.12 However, there may be some hormonal advantage to training in mild cold. A study published in 1984 reported that a 45-minute treadmill session increased testosterone 33 percent, but subjects training in 44ºF conditions experienced a 46 percent increase.13 A more recent study showed ice pack application directly to exercised muscles reduced pro- and anti-inflammatory cytokines, as well as testosterone.14 While conclusions are varied, it appears that a cool environment may be best, whereas a cold one negatively affects training and response.
Most people look to escape the cold, leading to a migration of snowbirds to Florida and Arizona, and sunbathers to the Caribbean and Cancun. But to get that beach body, it may be wise to spend a little time in chilling. Activating BAT can be achieved just sitting in a room that is just a bit cooler than comfortable; sleep can be a more fat-burning experience if the bedroom is kept cold (around 60ºF); and it is possible that working out in a 44ºF gym may result in greater gains. Just be sure to avoid the extremes; frigid cold can be a detriment to training and lead to coordination difficulties that might increase the risk of injury.
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