We'll look at the science supporting the effects of exercise on a healthy brain and on brains afflicted with disease. We'll also explore several types of exercise.
Let's first start by defining various levels of physical activity. Daily activity is defined in terms of total daily energy expenditure or the amount of energy in the form of calories burned each day. This is usually calculated from the average number of steps a person takes in a given day. A sedentary life style is defined as less than 5,000 steps a day. But lets say a person goes for a short run or a brisk walk and then spends the rest of the day sitting, therefore totaling under 5,000 steps. This is still considered sedentary. An active lifestyle is generally defined as equal to or more than 7,500 steps per day.
The World Health Organization defines physical inactivity as being less than 150 minutes of moderate intensity physical activity per week. Moderate intensity physical activity is defined as requiring a moderate amount of effort causing a moderate increase in heart rate. Examples are dancing, brisk walking, gardening, playing with kids, or walking domestic animals.
An epidemic of sedentary lifestyle is occurring across the globe and this affects adults and children. In the US, the amount of sedentary time has increased and is projected to continue increasing through 2030. This trend is not limited to the US, however. In China, a health and nutrition survey evaluated adults' physical activity and sedentary behavior, the results were similar. There has been a sharp drop in physical activities since the 1990s and it's projected to continue through to 2030. What happens in the body and particularly in the brain when you exercise?
One of the main functions performed by the body is homeostasis, meaning maintaining basic levels of physiological functions necessary for life. Examples of this are body temperature, pH in the body, and glucose amounts.
At any given time, the body must ensure sufficient fuel levels, usually in the form of glucose. Glucose is maintained by a highly sensitive control system. It's used by a variety of tissues to sustain metabolism and a big portion of it goes to the brain. In a normal resting state, glucose is released by the liver from breakdown of glycogen at rates equivalent to the rate of glucose uptake. Of course, during exercise, your muscles are working more, so they need more fuel in the form of glucose.
Glucose shifts from the blood into the muscle at a rate that helps satisfy the metabolic requirement of the working muscle. The liver is stimulated to produce more glucose, maintaining a stable blood glucose. If glucose production by the liver did not increase and the muscle was consuming more, you would end up hypoglycemic or with low blood sugar.
"As relative exercise intensity is increased, there is a decrease in the proportion of the energy requirement derived from fat oxidation and an increase in that provided by carbohydrate oxidation. During moderately strenuous exercise of an intensity that can be maintained for 90 minutes or longer ( approximately 55-75% of VO2max), there is a progressive decline in the proportion of energy derived from muscle glycogen and a progressive increase in plasma fatty acid oxidation. The adaptations induced by endurance exercise training result in a marked sparing of carbohydrate during exercise, with an increased proportion of the energy being provided by fat oxidation. The mechanisms by which training decreases utilization of blood glucose are not well understood. However, the slower rate of glycogenolysis can be explained on the basis of lower concentrations of inorganic phosphate (Pi) in trained, as compared to untrained, muscles during exercise of the same intensity."
The liver must also have efficient means of replenishing glucose following exercise. This is achieved by the effect of the brain on the liver. During exercise, your heart rate goes up as soar your levels of epinephrine and adrenaline and norepinephrine.
The brain acts through the autonomic nervous system to increase your levels of cortisol and epinephrine.
This influences the pancreas, which stimulates the liver to make more glucagon, which is the storage form of glucose. That way the liver stores are not depleted. So, during exercise, glucagon levels go up, insulin goes down. Let's focus on what happens at the level of the brain. During exercise, the muscles secrete various chemicals that act as messengers to the brain. These include cyclic AMP and irisin that swim up to the brain and induce the brain to start pumping out more serotonin and or epinephrine which act as neurotransmitters. In addition to neurotransmitters, the brain also starts making what are called growth factors. These include insulin-like growth factor or IGF, vascular endothelial growth factor or VEGF, and brain derived neurotrophic factor or BDMF. BDNF is a very important protein, it was relatively unknown until the 1990s but there are now thousands of studies on the subject.
We now know that BDNF causes growth and the formation of new neurons, or neurogenesis. It favors long-term potentiation in the hippocampus, which is the molecular mechanism behind learning. And it even acts on the DNA of neurons to cause more transcription of genes that favor brain growth.
If you were to sprinkle BDNF on neurons in a petri dish, it would cause growth of those neurons. Doctor John Ratey, a psychiatrist at Harvard and the Massachusetts General Hospital, became interested in the effects of exercise on mental illness. And calls BDNF Miracle-Gro for the brain. I couldn't think of a more fitting term. BDNF has actually been one of the most important molecules looked at when observing the effects of exercise on the brain. In some studies, impaired BDNF signalling has been shown to be a factor in Alzheimer's disease, depression, and even in some eating disorders.