Aerobic exercise can be defined as physical activity that increases aerobic metabolism, meaning energy-generating processes that require oxygen. Common examples of aerobic exercises are walking, running, swimming, dancing, and other activities that increase heart rate and breathing. Exercise elicits a wide range of physiological changes in the body that improve cardiovascular, neurological, and metabolic health.
Current physical activity guidelines from the United States Department of Health and Human Services recommend the following.
In 2008, just 18 percent of adults in the United States met the combined aerobic and muscle-strengthening physical activity guidelines. In 2017, this rate increased to over 24 percent. While this improvement is encouraging, increasing exercise participation is still a top priority for reducing the disease burden for the individual and the population.
A wealth of research is available to support a strong relationship between physical activity and disease prevention. A large-scale prospective study found that among adults 50 years of age and older, achieving 150 minutes of moderate-intensity exercise per week reduced the risk of death from all causes by 27 percent. Older adults who achieved 60 minutes of vigorous exercise per week had a 32 percent reduction in risk of death. Among adults 40 years and older, more calories burned with daily physical activity reduced the risk of death from all causes.
While aerobic exercise is often viewed preferentially through the lens of cardiovascular health, the diverse cascade of biological effects from exercise as a whole — and aerobic exercise, in particular — culminate to make it one of the most formidable and immediately practical real-world interventions for human aging and the brain.
In the context of the brain, exercise is a potent stimulant of brain-derived neurotrophic factor (BDNF), a cell signaling protein that plays a key role in numerous signaling pathways associated with a variety of disorders ranging from depression and dementia to obesity and diabetes. The primary role of BDNF is to stimulate the formation, growth, survival, and development of neurons. It is responsible for mediating the beneficial cognitive effects associated with exercise. The decline of BDNF function during aging may contribute to deficits in learning and memory and the progression of dementia and Alzheimer's disease. Some have estimated that one-third of dementia cases are attributable to modifiable risk factors, including aerobic exercise.
Exercise that requires output at 80 percent of a person's maximum heart rate for at least 40 minutes creates the greatest probability of significant BDNF elevation. See below for common methods for calculating an age-based estimated maximal heart rate.
Thinking about exercise from the standpoint of the physiological demand it creates as a function of percent maximal heart rate allows clinical studies to compare training that may have similar perceptual effort even among participants with varying degrees of fitness.
There are a variety of formulas that estimate maximal heart rate (HRmax), each with varying degrees of underestimation or overestimation, depending on the aerobic fitness, gender, and age of the population. These formulas overestimate maximum heart rate in trained populations and underestimate them in older and untrained populations. Nevertheless, estimates are a valuable starting point for heart rate-based zone training:
Aerobic exercise is associated with improvements in whole-body health; however, the cellular mechanisms that support this relationship require further research. Findings from one study suggest that an acute bout of aerobic exercise alters more than 9,000 distinct molecules in the human body to influence health positively.
The study involved 36 adults between the ages of 40 and 75 years with insulin resistance and overweight or obesity (BMI greater than 25). Half of the participants engaged in an acute bout of aerobic exercise on a treadmill for 8 to 12 minutes at their maximum capacity, while the other group of participants did not perform any exercise. The authors collected blood before and after the participants exercised and when they were fasted.
One acute bout of aerobic exercise elicited extensive changes in 9,815 molecules involved in energy metabolism, oxidative stress, inflammation, tissue repair, and growth factor response. Acute exercise was powerful enough to reduce insulin resistance, amplifying the molecular response to exercise. This finding suggests insulin participates in many molecular pathways modulated by exercise. The study's authors suggested that their findings could lay the groundwork for developing a simple blood test that measures fitness in the future. Learn more about exercise's molecular metabolic and immune benefits with Dr. Michael Synder in this clip.
During aerobic exercise, the body increases the delivery of fats to the muscle mitochondria and increases mitochondrial capacity to metabolize fats, a process called beta-oxidation. The inner membrane of mitochondria possesses a series of enzymes called the electron transport chain. These enzymes transfer electrons from carbohydrates and fats to the final enzyme in the chain that produces ATP. Electron transfer flavoprotein is an enzyme in this chain that transfers electrons from fats, specifically.
The investigators recruited fifteen healthy sedentary adults (average age, 28 years) with a normal body mass index. Participants completed one hour of cycling at 65 percent of their maximum aerobic capacity on one day and rested the next day. The researchers collected biopsies from the participants' thigh muscles after they had rested and 15 minutes after they exercised. They analyzed the muscle mitochondria for the abundance of electron transfer flavoprotein activity and the metabolism of fats and nonfat fuel sources.
Following exercise training, mitochondrial metabolism of fats and nonfat sources increased, although this relationship was not statistically significant. Also noted was a six percent increase in hydrogen peroxide, a byproduct of fat metabolism that damages cells. Although fat metabolism increased, the authors reported no increase in the abundance of electron transfer flavoprotein activity.
The authors concluded that just one session of moderate-intensity aerobic exercise in sedentary adults increases the metabolism of both fat and nonfat fuel sources.
"Incorporating aerobic exercise programs into treatment protocols for people with depression might be beneficial in reducing depressive symptoms, especially in cases of more severe symptoms." Click To Tweet
Major depressive disorder is characterized by altered sleep and appetite, fatigue, sadness, and feelings of guilt or low self-worth. Although pharmaceutical treatments for depression are available, many people with depression do not respond to the medications. The findings of one study demonstrate that aerobic exercise benefits people with depression, especially those with more severe symptoms.
The intervention study involved 66 young adults (average age, 20 years) who had major depressive disorder. About half of the participants completed an eight-week moderate-intensity aerobic exercise program, while the remainder completed an eight-week light stretching program. The study's authors tracked the participants' depressive symptoms throughout the study and assessed their reward processing and cognitive control before and after the intervention.
They found that aerobic exercise reduced the participants' depressive symptoms better than light stretching. Neither form of activity affected reward processing or cognitive control. However, participants whose depressive symptoms were more severe and scored high in reward processing were more likely to respond to the aerobic program than the stretching program.
These findings demonstrate that incorporating aerobic exercise programs into treatment protocols for people with depression might be beneficial in reducing depressive symptoms. Learn more about exercise as a treatment for depression in this video featuring Dr. Rhonda Patrick.
Mild cognitive impairment and memory loss often precede the development of Alzheimer's disease, a neurodegenerative disorder characterized by progressive memory loss, spatial disorientation, cognitive dysfunction, and behavioral changes. Aerobic exercise may improve memory function in older adults. Findings from one study suggest that exercise improves cerebral blood flow in brain regions involved in memory.
The prospective clinical trial involved 30 adults (average age, 66 years) with mild cognitive impairment. Half of the participants engaged in a supervised aerobic exercise program of 25 to 30 minutes per session three times per week, gradually increasing to 30 to 40 minutes per session three or four times per week. The other half of the participants engaged in stretching only. At the end of the 12-month intervention, the study's authors assessed the participants' memory function, cardiorespiratory fitness, and cerebral blood flow.
Participants enrolled in the one-year exercise program exhibited marked improvements in cardiorespiratory fitness and memory performance. Compared to stretching, aerobic exercise increased blood flow to the anterior cingulate cortex region of the brain, an area associated with empathy, impulse control, decision-making, and memory.
Exercise also reduced blood flow to the posterior cingulate cortex, a component of the default mode network associated with internally directed thought. The posterior cingulate cortex is highly sensitive to dementia and is resistant to the benefits of exercise. In cases of cognitive impairment, the brain shifts blood flow away from the posterior cingulate cortex toward healthier brain areas. These findings suggest that exercise can forestall the effects of age-related cognitive decline.
Sauna use exposes the body to extreme temperatures that stress the body, eliciting physiological responses that are remarkably similar to those experienced during moderate- to vigorous-intensity exercise. Findings from a study in Finland indicate that good cardiorespiratory fitness combined with frequent sauna use may work synergistically to reduce cardiovascular and all-cause mortality.
Data from a Finnish cohort demonstrated that high cardiorespiratory fitness or frequent sauna use was independently associated with reduced cardiovascular-related mortality and all-cause mortality. In addition, cardiorespiratory fitness in combination with frequent sauna bathing (three to seven sessions per week) had a synergistic effect on lowering cardiovascular and all-cause mortality. The most robust reductions in mortality were found in people with high cardiorespiratory fitness and high frequent sauna bathing, followed by high cardiorespiratory fitness and low frequent sauna bathing, and then low cardiorespiratory fitness and high frequent sauna bathing.
Sauna use is an exercise mimetic that induces the activity of heat shock proteins, a class of proteins that protect against neurodegenerative conditions like Alzheimer's disease, providing an alternative option for people who cannot exercise. Learn more about sauna use in our overview article.
"People who engaged in recommended amounts of aerobic or muscle-strengthening activity had a lower risk of death from all causes, and these benefits were even greater if they engaged in both types of activities." Click To Tweet
Findings from one population-based cohort study demonstrate that combining aerobic exercise and strength training is best. The study, which involved nearly 480,000 adults, drew on data from the National Health Interview Survey, an ongoing, cross-sectional survey of people living in the United States. The study participants reported how much leisure time they engaged in aerobic and strength physical activity each week. Then, the study's authors categorized them as having insufficient activity, aerobic activity only, strength activity only, and both aerobic and strengthening activities, based on recommended guidelines.
The authors found that the participants who engaged in recommended amounts of aerobic or muscle-strengthening activity had a lower risk of death from all causes, and these benefits were even greater if they engaged in both types of activities. They noted similar reductions in the risk of death from cardiovascular disease, cancer, and chronic lower respiratory tract diseases.
These findings suggest that adherence to public health guidelines for exercise reduces the risk of disease and death and supports interventions to improve compliance.
Some of these benefits of aerobic exercise are due to adaptations in skeletal muscle that arise from the repair of cellular damage produced by the demand of exercise. Histamine is an organic molecule that acts as a neurotransmitter in the brain and spinal cord while regulating gut and immune function. Much of the research surrounding histamine relates to its effects on allergies and inflammation; however, recent research has demonstrated the role of histamine as a mediator of exercise-induced physiological responses. Exercise stimulates histamine release and promotes increased blood circulation, which may be important for facilitating muscle adaptation.
The investigators conducted two phases of research, including a randomized cross-over study of antihistamines and their effect in acute exercise and a double-blind, placebo-controlled, randomized study of chronic exercise training and antihistamine use. Participants in the acute exercise trial completed two exercise sessions on separate days. One day, they took an antihistamine one hour before aerobic exercise; on the alternate day, they took a placebo treatment. The researchers measured heart rate during exercise and blood pressure, heart rate, and arterial blood flow in the thigh before and after exercise.
In the placebo group, blood flow in the thigh was increased by approximately threefold 15 minutes following exercise and remained 50 percent higher two hours following exercise. However, post-exercise blood flow reduced markedly in participants who took an antihistamine (about 35 percent). Histamine treatment did not alter blood pressure or heart rate before or after exercise compared to the placebo group.
For the study on chronic exercise, participants completed six weeks of high-intensity interval training consisting of three weekly cycling sessions. Half of the participants took an antihistamine one hour before exercise, and the other half took a placebo. The researchers measured maximum exercise capacity by having participants exercise to exhaustion and measuring their oxygen output. They measured metabolic health by having participants complete an oral glucose tolerance test in which they consumed 75 grams of sugar (the amount of sugar in about two cans of non-diet soda) and measured their blood glucose over two hours. They assessed vascular function by using ultrasound to measure the rate of blood flow in the thigh. Finally, the researchers took muscle biopsies from participants' thighs to measure muscle adaptation.
Over the six weeks of training, resting heart rate tended to decrease in the placebo group but not in the antihistamine group, indicating a blunting of exercise-induced adaptation. Peak power output during exercise also increased in both groups; however, this increase was significantly greater in the placebo group (12 percent increase) compared to the antihistamine group (7 percent increase). The time to exhaustion also improved significantly with training in the placebo group (81 percent increase) compared to the antihistamine group (31 percent increase). Training improved glucose tolerance significantly in the placebo group (26 percent increase), but this effect was blocked in the antihistamine group (1 percent increase). Finally, participants in the placebo group experienced a 37 percent improvement in vascular function, while those in the antihistamine group experienced a 14 percent decrease in vascular function. Maximal blood flow during exercise increased due to training in the placebo group only.
In conclusion, antihistamine use blunted the functional adaptations in exercise capacity, whole-body glycemic control, and vascular function produced by exercise.