What happens to your engine at the end of a long car trip? It doesn’t require a degree in automotive engineering to know that once you’ve reached your destination, your car’s engine stays warm as it gradually cools to a resting temperature.
Here's a cool fact: The same thing happens to your body after exercise. Similar to how a car’s engine remains warm after being turned off, once a workout is over and you’re back in your daily routine, your body’s metabolism can continue to burn more calories then when at complete rest. This physiological effect is called excess post-exercise oxygen consumption, or EPOC. Also known as oxygen debt, EPOC is the amount of oxygen required to restore your body to its normal, resting level of metabolic function. It also explains how your body can continue to burn calories long after you’ve finished your workout.
Your metabolism is how your body converts the nutrients you consume in your diet to adenosine triphosphate (ATP), the fuel your body uses for muscular activity. ATP is produced either with oxygen using the aerobic pathways or without oxygen relying on the anaerobic pathways. When you first start to exercise, your body uses the anaerobic energy pathways and stored ATP to fuel that activity. A proper warm-up is important because it can take about five to eight minutes to be able to efficiently use aerobic metabolism to produce the ATP necessary to sustain physical activity. Once a steady-state of oxygen consumption is achieved, the aerobic energy pathways are able to provide most of the ATP needed for the workout. Exercise that places a greater demand on the anaerobic energy pathways during the workout can increase the need for oxygen after the workout, thereby enhancing the EPOC effect.
Here are seven things you should know about EPOC and how it can help you achieve optimal levels of calorie burning from your workouts:
1. During the immediate post-exercise recovery period, oxygen is used for the following functions:
2. Exercise that consumes more oxygen burns more calories.
The body expends approximately 5 calories of energy (a calorie is the amount of energy required to heat 1 liter of water 1 degree centigrade) to consume 1 liter of oxygen. Therefore, increasing the amount of oxygen consumed both during and after a workout, can increase the amount of net calories burned.
3. Circuit training and heavy resistance training with short rest intervals require ATP from the anaerobic pathways, leading to a significant EPOC effect.
Strength training with compound, multijoint weightlifting exercises or doing a weightlifting circuit that alternates between upper- and lower-body movements places a greater demand on the involved muscles for ATP from the anaerobic pathways. Increased need for anaerobic ATP also creates a greater demand on the aerobic system to replenish that ATP during the rest intervals and the post-exercise recovery process. Heavy training loads or shorter recovery intervals increase the demand on the anaerobic energy pathways during exercise, which yields a greater EPOC effect during the post-exercise recovery period.
4. High-intensity interval training (HIIT) is the most effective way to stimulate the EPOC effect.
The body is most efficient at producing ATP through aerobic metabolism; however, at higher intensities when energy is needed immediately, the anaerobic pathways can provide the necessary ATP much more quickly. This is why we can only sustain high-intensity activity for a brief period of time—we simply run out of energy. HIIT works because during high-intensity exercise ATP is produced by the anaerobic pathways; once that ATP exhausted, it is necessary to allow ATP to be replenished. The rest interval or active-recovery period during an anaerobic workout allows aerobic metabolism to produce and replace ATP in the involved muscles. The oxygen deficit is the difference between the volume of O2 consumed during exercise and the amount that would be consumed if energy demands were met through only the aerobic energy pathway.
5. EPOC is influenced by the intensity, not the duration of exercise.
Higher intensities require ATP from anaerobic pathways. If the ATP required to exercise at a particular intensity was not obtained aerobically, it must come from the anaerobic pathways. During EPOC, the body uses oxygen to restore muscle glycogen and rebuild muscle proteins damaged during exercise. Even after a HIIT workout is over, the body will continue to use the aerobic energy pathway to replace the ATP consumed during the workout, thus enhancing the EPOC effect.
6. Research has shown that resistance training can provide a greater EPOC effect than running at a steady speed.
In an extensive review of the research literature on EPOC, Bersheim and Bahr (2003) concluded that “studies in which similar estimated energy cost or similar exercising VO2 have been used to equate continuous aerobic exercise and intermittent resistance exercise, have indicated that resistance exercise produces a greater EPOC response.” For example, one study found that when aerobic cycling (40 minutes at 80 percent Max HR), circuit weight training (4 sets/8 exercises/15 reps at 50 percent 1-RM) and heavy resistance exercise (3 sets/8 exercises at 80-90 percent 1-RM to exhaustion) were compared, heavy resistance exercise produced the biggest EPOC.
7. The EPOC effect from a HIIT or high-intensity strength-training workout can add 6 to 15 percent of the total energy cost of the exercise session.
High-intensity workouts require more energy from the anaerobic pathways and can generate a greater EPOC effect, leading to extended post-exercise energy expenditure. Heavy weight training and HIIT workouts appear to be superior to steady-state running or lower-intensity circuit training in creating EPOC (LaForgia, Withers and Gore, 2006).
Admittedly there is some debate about the significance of the EPOC effect for the average exercise participant because the high-intensity exercise required for EPOC can be extremely challenging. However, if you want results and are up for the challenge, increasing the intensity of your workouts by using heavier weights, shorter rest intervals or high-intensity cardio intervals may be worth the effort. While HIIT or heavy resistance training is effective and beneficial, remember to allow at least 48 hours of recovery time between high-intensity exercise sessions and try to limit yourself to no more than three strenuous workouts per week.
Cortisol is a steroid hormone that regulates a wide range of vital processes throughout the body, including metabolism and the immune response. It also has a very important role in helping the body respond to stress.
What is cortisol?
Cortisol is a steroid hormone, one of the glucocorticoids, made in the cortex of the adrenal glands and then released into the blood, which transports it all round the body. Almost every cell contains receptors for cortisol and so cortisol can have lots of different actions depending on which sort of cells it is acting upon. These effects include controlling the body’s blood sugar levels and thus regulating metabolism, acting as an anti-inflammatory, influencing memory formation, controlling salt and water balance, influencing blood pressure and helping development of the foetus.
How is cortisol controlled?
Blood levels of cortisol vary throughout the day, but generally are higher in the morning when we wake up, and then fall throughout the day. This is called a diurnal rhythm. In people that work at night, this pattern is reversed, so the timing of cortisol release is clearly linked to daily activity patterns. In response to stress, extra cortisol is released to help the body to respond appropriately.
The secretion of cortisol is mainly controlled by three inter-communicating regions of the body; the hypothalamus in the brain, the pituitary gland and the adrenal gland. When cortisol levels in the blood are low, a group of cells in a region of the brain called the hypothalamus releases corticotrophin-releasing hormone, which causes the pituitary gland to secrete another hormone, adrenocorticotropic hormone, into the bloodstream. High levels of adrenocorticotropic hormone are detected in the adrenal glands and stimulate the secretion of cortisol, causing blood levels of cortisol to rise. As the cortisol levels rise, they start to block the release of corticotrophin-releasing hormone from the hypothalamus and adrenocorticotropic hormone from the pituitary. As a result, the adrenocorticotropic hormone levels start to drop, which then leads to a drop in cortisol levels. This is called a negative feedback loop.
What happens if I have too much cortisol?
Too much cortisol over a prolonged period of time can lead to a condition called Cushing's syndrome. This can be caused by a wide range of factors, such as a tumour that produces adrenocorticotropic hormone (and therefore increases cortisol secretion), or taking certain types of drugs. The symptoms include:
In addition, there has been a long-standing association between raised or impaired regulation of cortisol levels and a number of psychiatric conditions such as anxiety and depression. However, the significance of this is not yet clearly understood.
What happens if I have too little cortisol?
Too little cortisol may be due to a problem in the pituitary gland or the adrenal gland (Addison's disease). The onset of symptoms is often very gradual. Symptoms may include fatigue, dizziness, weight loss, muscle weakness, mood changes and the darkening of regions of the skin. Without treatment, this is a potentially life-threatening condition.
Urgent assessment by a specialist hormone doctor called an endocrinologist is required when a diagnosis of Cushing's syndrome or Addison's disease is suspected.
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