When we exercise, our bodies become hot and we start to feel flushed. But something else happens: our appetites decrease after the workout. Researchers set out to explore exactly why and how this happens
For a long time, I lived with the conviction that the more physically active I was, the more my appetite would increase. Makes sense, right?
Surely, I would think, the body will call for a replacement of all the calories burnt while jogging or dancing.
But my appetite never increased, and all of my expectations had been entirely wrong.
Studies have now shown that aerobic exercise — such as running, cycling, and swimming — actually
However, the underlying biological mechanisms that are therefore set in motion, and which tell our bodies to secrete fewer of the hormones that drive hunger, have remained uncertain.
But recently, one researcher decided to take steps toward understanding what goes on in the body after a decent workout.
Young-Hwan Jo, of the Albert Einstein College of Medicine in the Bronx, NY, was intrigued by how his regular 45-minute runs always left him craving less food than usual.
He believed that the fact that body heat goes up during exercise may play a role in signaling to the brain that appetite needs to go down. He thought the process might be similar to what happens in the body when we eat very spicy foods.
When we eat foods that contain hot chili peppers, our body temperature seems to go up, and our appetite decreases. That is because chili peppers contain a compound called “capsaicin,” which interacts with sensory receptors (TRPV1 receptors) in the body, bringing about the
Capsaicin has also been shown to create a
Following this train of thought, Jo wondered whether the increased body heat felt after exercising might not stimulate neurons in brain areas responsible with homeostasis, the regulation of basic bodily processes, including eating.
“I’m a neuroscientist,” says Jo, “who studies the hypothalamus — the portion of the brain that plays the central role in regulating metabolism and weight,” adding:
And, sure enough, the results of the research that followed — which have now been published in the journal PLOS Biology – indicate that he was on the right track.
Jo and team decided to zoom in on a set of neurons that coordinate appetite suppression, called “proopiomelanocortin” (POMC) neurons. These cells are found in a region of the hypothalamus known as the “arcuate nucleus,” and some are not screened by the
This is a membrane that prevents most of the cells in the brain from being exposed to serious fluctuations in blood plasma composition, thus protecting neural function.
But since some POMC brain cells have more direct communication with the rest of the system and interact with hormones released into the blood, Jo thought they may also be able to respond to fluctuations in body temperature.
In order to test this hypothesis, the researchers first experimented with mouse hypothalamus tissue that contained POMC brain cells. They exposed this tissue first to capsaicin, and then to heat, to see whether these cells would be able to respond to both stimuli.
The researchers were not disappointed — both the presence of heat and that of the chili pepper compound activated POMC neurons, meaning that they had TRPV1 receptors. Two thirds of these cells responded to these two different heat stimuli, the researchers noted.
At the next stage of their research, the scientists conducted various tests using mice, in order to understand how POMC neurons reduced appetite after their TRPV1 receptors were activated.
So, Jo and colleagues saw that when they exposed the arcuate nuclei of mice to capsaicin, the animals tended to eat less food over the following 12 hours.
The scientists, however, were able to block the loss of appetite associated with capsaicin exposure either by blocking the TRPV1 receptors of POMC neurons before administering the compound, or by turning off the gene that encodes such receptors in mice.
Jo and team also put some mice on treadmills, leaving them to run for a period for 40 minutes. In this way, they created the conditions that are typical of a regular workout session.
As a result of this exercise, the animals’ body temperatures initially shot up, and then they reached a plateau after 20 minutes. Body heat remained high for over an hour, and the mice’s appetite visibly decreased.
The exercising rodents had an approximately 50 percent lower food intake after the treadmill session than their counterparts that had not taken part in the exercise.
And, finally, exposure to treadmill exercise had no effect on the appetite of the mice whose TRPV1 receptors had been suppressed. This suggests that heightened body heat due to physical activity stimulates relevant receptors in the brain to decrease the desire for food.
“Our study provides evidence,” Jo concludes, “that body temperature can act as a biological signal that regulates feeding behavior, just like hormones and nutrients do.”
He adds that this knowledge may eventually lead to improved strategies for weight loss.
Sitting harms brain health, regardless of exercise
adults aged 45–75 without dementia, those who spent more time sitting in the day had greater thinning of the medial temporal lobe.
This is an area of the brain that is important for making new memories.
Even high levels of physical activity did not make a difference, the authors conclude, in a report on their findings that is now published in the journal PLOS ONE.
The study adds to a growing body of evidence that suggests that too much sitting can increase the risk of heart disease, diabetes, and other diseases, even among those who are physically active.
Senior study author David Merrill, an assistant clinical professor of psychiatry and biobehavioral sciences at UCLA, and colleagues propose that further research should now be done to see whether reducing sedentary behavior reverses the effect that they found.
In their study background, the authors refer to the increasing amount of literature that suggests that physical exercise might delay the development of Alzheimer’s disease and other dementias and can benefit brain structure.
One explanation that has been suggested for this effect is that physical activity raises blood flow in the brain, which, in turn, helps the growth of new nerve cells and offsets decline in structure and function.
But in comparison with the volume of literature on the impact of exercise, “there is a paucity of research on the relationship between sedentary behavior and dementia risk,” and only few studies have examined the “mechanistic” effect on the brain, note the authors.
This should be a cause for concern, they argue — especially as it has been suggested that around 13 percent of the global burden of Alzheimer’s disease may be due to spending too much time sitting.
Based on such an estimate, they calculate that reducing sedentary behavior by 25 percent “could potentially prevent more than 1 million” cases of Alzheimer’s disease worldwide.
The team decided to focus on the medial temporal lobe because it is known that this area of the brain declines with age and that this leads to memory impairment.
Also, they note, greater “aerobic fitness” has been tied to greater volume of the hippocampus, an area of the medial temporal lobe that has been “heavily studied” and is important for memory.
For their study, the researchers explored links between medial temporal lobe thickness, exercise, and sitting time in 25 women and 10 men aged 45–75 who did not have symptoms of dementia.
The data on average hours spent sitting every day and physical activity levels came from detailed questionnaires that the men and women filled in. Medial temporal lobe thickness was measured from MRI scans of their brains.
When they analyzed the data, the researchers found “[n]o significant correlations” between levels of physical activity and medial temporal lobe thickness.
However, they did find that the more sedentary people had less medial temporal lobe thickness.
While they did not investigate the mechanisms through which prolonged sitting might be bad for the brain, the authors refer to a suggestion that “sedentary behavior may have deleterious effects on glycemic control.”
They speculate that this could result in increased variability of blood sugar and lead to reduced blood flow in the brain, which, in turn, impairs brain health.
They also point out that their findings are “preliminary” and do not prove that prolonged sitting actually causes the medial temporal lobe to become thinner. They propose that:
“Future studies should include longitudinal analyses and explore mechanisms, as well as the efficacy of decreasing sedentary behaviors to reverse this association.”
10 minutes of exercise enough to boost brain
Over recent years, there has been great interest in the relationship between physical activity and mental acuity. It is becoming ever clearer that there is a positive link.
For instance, being physically active has been shown to
Exercise may also protect against Alzheimer’s disease. And, at the opposite end of the age spectrum, physical activity appears to boost children’s academic performance over time.
However, the effects of a single, short burst of activity on cognitive performance are not as well-established.
A review, published earlier this year, concluded that bouts of exercise lasting around 1 hour have significant benefits on executive function, which includes parameters such as attentional control, cognitive inhibition, inhibitory control, working memory, and cognitive flexibility.
Another study found that a 20-minute, one-off bout of exercise can benefit mental performance. But what is the smallest amount of exercise that can impart a measurable benefit to the brain?
Recently, researchers from Western University in Ontario, Canada, set out to understand whether there is a minimum amount of activity that can give significant benefits to one’s brain. The study group was led by Prof. Matthew Heath, supervisor in the Graduate Program in Neuroscience.
He explains the importance of the study, saying, “Some people can’t commit to a long-term exercise regime because of time or physical capacity.”
Gaining an understanding of the minimum requirements could be very useful for these people. For someone who has trouble moving, knowing that even a small amount of exercise will be beneficial could offer valuable motivation.
For the current study, participants either sat and read a magazine for 10 minutes or carried out a 10-minute burst of moderate to vigorous activity on an exercise bike.
Following the activities, the participants’ brain power was rated. Using sophisticated eye-tracking equipment, they measured their reaction times to a cognitively challenging eye-movement task called the antisaccade task.
For this task, people are asked to fixate on a stationary target, often a small dot. A second dot is then presented to one side of the stationary target. The individual is asked to move their eyes in the opposite direction from the stimulus. So, if a stimulus appears to the left of the initial target, participants must look to the right.
This task has been widely used to assess executive performance. The results of the new study were recently published in the journal Neuropsychologia.
“Those who had exercised showed immediate improvement. Their responses were more accurate and their reaction times were up to 50 milliseconds shorter than their pre-exercise values. That may seem minuscule, but it represented a 14 percent gain in cognitive performance in some instances.”
Prof. Matthew Heath
The researchers believe that the burst of physical activity fires up the frontoparietal network, which is a part of the brain that has previously been implicated in exercise-based brain boosts.
For instance, a 2014 study revealed that an exercise program improved frontoparietal integrity in obese children. However, further studies will need to be done to accurately pinpoint the parts of the brain involved.
These findings could be important for a wide variety of people — for instance, anyone who has early stage dementia and is perhaps not able to move around for long periods of time.
As Prof. Heath says, “This shows that people can cycle or walk briskly for a short duration, even once, and find immediate benefits.”
However, the findings are applicable for all of us. Prof. Heath gives further advice: “I always tell my students before they write a test or an exam or go into an interview — or do anything that is cognitively demanding — they should get some exercise first. Our study shows the brain’s networks like it. They perform better.”
The take-home message is that our brains thank us when we move about, even if it’s just for a short time.
Source - Medical News Today
No comments:
Post a Comment