A new study found exercise changes how our brain reacts to images of food and internal stimuli such as hunger.
Exercise helps us lose weight by altering our energy balance; weight is lost when the energy balance is negative. Thus, the term "burning calories" refers to a negative shift in the body's energy balance. Research by a team in Colorado suggests that exercise may also affect weight profile by changing how the brain responds to food and appetite. The team first published work on the subject two years ago (The Effects of Exercise on the Neuronal Response to Food Cues, 2012. Cornier M, et al.). The Colorado-based research team then published a more in-depth study in the Summer of 2013 (Effects of exercise on resting-state default mode and salience network activity in overweight/obese adults, 2013. McFadden KL, et al.).
The study participants performed a six month exercise program consisting of treadmill-walking. Subjects underwent a behavioral assessment and cerebral MRI analysis at baseline and then following the six-month exercise intervention. Behavioral measures were taken using a variety of questionnaires and assessments. For example, participants assessed appetite using a visual analog scale to rate prospective food consumption. All measurements were taken after a overnight fast and during rest.
The MRI scans focused on two areas of the brain: the default mode network and the salience network. Studies have found that obese people have altered activity in these cerebral regions. The deault mode network is dedicated to self-relevant mentalizing and interoception. Activity in the default mode network is increased in obese individuals. The salience network is responsible for assessing relevance of external and internal stimuli and reward-driven behavior. The salience network also has increased activity in obese individuals.
The researchers found no changes in behavior, but significant changes in brain activity in the default mode network. As the figure above demonstrates, activity in the default mode network was reduced from baseline following the six-month exercise intervention. However, no changes in the salience network were observed. No differences in appetite or anticipated food consumption were measured following the six-month exercise intervention. The authors suggest that default mode network activity may serve as a more sensitive measure of changes in appetite. Furthermore, leptin concentration was found to be reduced following the exercise intervention. Leptin is an important energy-balance regulating hormone as it plays a key role in regulating appetite and metabolism.
Further analysis of the results reveals that a greater reduction in default mode network activity was associated with a greater drop in perceived hunger and bodyweight. Interestingly, although the salience network activity was unchanged at rest, the 2012 study by this group found salience network activity to be reduced in response to food stimuli in the exercised participants.
Exercise reduces resting-state default mode network activity and salience network activity in response to food stimuli. Thus, this presents a model by which exercise exerts control over excess weight gain. By reducing salience network activity in response to food stimuli, exercised persons have a tampered response to food causing them to plausibly eat less at a time. Reducing resting-state default mode network activity causes people who exercise to have less focus on their hunger causing them to eat less often.
The authors suggest that exercise may have a normalizing effect on obese individuals' default mode network and salience network activity.
Kelli Emmett (right), the 2007 Single Speed Mountain Biking World Champion, precooling in an ice vest before a bike race. Precooling is the removal of heat from the body prior to an athletic performance. Several precooling strategies have been scientifically found to help maintain core body temperature and increase athletic performance.
Precooling before a workout or athletic competition is fueled by the theory that lowering the body's core temperature before exercise aids the body in maintaining homeostasis, specifically thermoregulation, during exercise. In hot or humid environments, the body sends up to 20% of the cardiac output to the skin for thermoregulation. This blood is being diverted away from active muscles. Theoretically, having a lowered core temperature before and during exercise will lower the amount of blood being shunted to the skin, increase muscular blood flow and, thus, have a beneficial effect on athletic performance. The idea of precooling has been around for a while, but only recently began gaining traction in elite athlete circles.
Meb Keflezighi won the silver medal at the 2004 Olympics in Athens. He wore a Nike ice vest before the race.
Nike sent ice vests for precooling to select olympic athletes competing in Athens in 2004 and Beijing in 2008. One of those athletes was Meb Keflezighi. Meb won the silver medal at Athens, surprising many in the running community. A study done in 2006 found that runners wearing Nike Ice Vests before a cross country race had a lower core temperature after the race (Warming Up With an Ice Vest: Core Body Temperature Before and After Cross-Country Racing, 2006. Ian Hunter, et al.).
Two great reviews on precooling and athletic performance were published this year. One of the papers provides a comprehensive review of different precooling strategies (Precooling Methods and Their Effects on Athletic Performance, 2013. Megan Ross, et al.). The other review focuses primarily on external cooling (The effect of cooling prior to and during exercise on exercise performance and capacity in the heat: a meta-analysis, 2013. Christopher James Tyler, et al.).
In hot or humid environments, core body temperature is a limiting factor on athletic performance. Human and animal studies have found the body ceases to perform when the core temperature reaches about 40°C (104°F). The athletic activity determines whether or not precooling will be beneficial to the performance of the athlete. For instance, precooling actually hampers the performance of a sprinter. However, in sports with intermittent sprints, precooling is beneficial to performance. In endurance events, such as triathlons, cycling races or marathons, the largest performance increases are observed from precooling. A number of studies have found that cooling during a performance has a dampened positive effect on performance compared to precooling beforehand.
One method of precooling is cold air exposure. Cold air exposure employs intermittent exposure to cold (0-5°C) air. This strategy has been found to boost running and cycling performance. The athlete is removed from the cold air exposure prior to the athletic performance creating an "after drop". This is when the peripheral blood vessels dilate causing blood from the core to return to the periphery. When the chilled peripheral blood goes back to the core, it further lowers the core body temperature. Galen Rupp, the silver medalist in the 10,000 meter run at the London 2012 Olympics, used a buffed-up concept for cryotherapy. He immersed himself in a nitrogen vapor shower, which has a temperature of -245°C! Galen Rupp is a fan of precooling; at the 2013 Track and Field World Championship in Moscow he was spotted wearing ice packs on his head and chest before the 10,000 meter final.
Immersing the body in cold water (17-30°C) for 30 minutes has been shown to be effective in lowering the body temperature. Cold defense mechanisms, however, can impair the desired heat loss. Warming up somewhat before cold water immersion helps prevent the body's cold defense mechanisms from being activated. In high power output exercise, whole-body cold water immersion can result in vasoconstriction of blood vessels in muscle tissue and inhibit metabolic enzyme activity. Because of this, athletes sometimes report feeling heavy or sluggish following whole-body cold water immersion. An alternative to whole-body, cold water immersion is exposing just part of the body to the cold water. This can be done in the form of water-soaked garments or submerging active or inactive parts of the body directly in the water. An additional benefit of submerging inactive parts of the body in cold water is that vasoconstriction shunts blood from the unnecessary body parts (for example, the hands of a runner) to active parts (the legs of a runner). Besides increasing performance, cold water immersion can prevent heat related illnesses such as heat exhaustion and heat stroke.
Applying ice to the skin can provide another means of precooling. To be effective, ice must be applied to areas with high blood flow such as the neck or between the thighs. Because thermoregulating the brain is essential, ice on the neck significantly relieves perceived heat stress. One study found a 20% increase in cycling power output done during an intermittent sprint protocol when ice was placed between the thighs. Aforementioned ice vests have also been found to be an effective heat-fighting strategy.
The aforementioned precooling strategies have been external in nature; that is, heat is lost externally. Internal cooling strategies have been found to be effective at aiding in thermoregulation and enhancing performance as well. Internal cooling methods come in a variety of forms: cold beverages, ice slurries, and ice bars. One company that has popped up in response to precooling exercise science is PowerIce. PowerIce makes a "frozen ice bar" with electrolytes. Internal cooling strategies cool the body as the ingested substance comes to equilibrium with the body. It is also believed that ingesting cold substances relieve perceived heat stress via oral temperature sensors. Internal cooling strategies not only aid in thermoregulation, but can provide hydration and nutritional supplementation.
A combination of the discussed precooling strategies can be used for athletes competing in hot or humid environments. Researchers still do not understand the mechanisms by which heat limits performance. For example, how much of heat stress is mental and how much is physical? What anatomical regions act to limit exercise capacity when overheated? While physiologists continue to garner the mechanisms, remember to use these precooling strategies to extend your athletic capacity the next time you compete in the heat.