Aerobic and Anaerobic Metabolism: How Exercise Affects Our Body's Metabolism

The previous article covered metabolic differences, which are influenced by various factors. However, one universal factor is aging. Individuals in their mid-20s typically have a fast metabolism if they are healthy, fit, and have plenty of lean mass. But, as we age, we may notice changes such as slower digestion, reduced alcohol tolerance, increased sensitivity to certain foods, longer recovery times, and decreased fertility and hormone production. Additionally, we may notice an increase in fat and a decrease in muscle mass, resulting in a reduction in Resting Metabolic Rate (RMR) of around 2-4% per decade after the age of 25. This decline is partly due to the loss of metabolically active lean mass. Lifestyle choices play a significant role in "body age" and our biological aging process. While we cannot reverse the aging process, we can control it to some extent through smart nutrition and preserving muscle mass, leading to healthy aging.

The benefits of movement and exercise on our bodies and brains have been well-documented. Regular exercise has positive effects on our metabolism and promotes health, wellness, and long life. The metabolic effects of exercise are due to the repeated contractions of skeletal muscles that require energy for fuel. The more intense the exercise, the more energy is required per minute, as is the case with high-intensity activity such as sprinting, which also demands more energy for recovery and repair after exercise. Different exercise intensities and types affect our physiology differently, and intense exercise seems to prevent muscle loss and preserve our RMR (resting metabolic rate) most effectively. Exercise affects our body's metabolism in several ways. When we begin to contract our muscles during exercise, muscle cells use up most of their available ATP within three seconds. The ATP-PCr system then kicks in to provide more ATP, which lasts for about 10 seconds. As ATP regeneration takes time, we start to slow down a bit. As we continue to exercise and deplete our ATP-PCr stores, the glycolytic system starts providing most of the energy transfer for ATP regeneration, which lasts for about 90-120 seconds, depending on the intensity of the exercise. This system generates ATP more slowly than the ATP-PCr system, so we have to slow down a bit more. If we continue to exercise, the oxidative phosphorylation system will then provide most of the energy transfer for ATP regeneration. However, as the oxidative systems are slower than the anaerobic systems, we'll have to slow down again. Waiting between short sprints, as in interval training, lets our ATP-PCr system quickly replenish the available ATP so we can blast off again. Different exercise intensities and types affect our physiology differently, with high-intensity exercise being most effective in preserving muscle and metabolism with age. High-intensity exercise is relative and varies from person to person because our body must adapt to the demands of exercise. It includes anything that gets our heart pounding, sucks our oxygen, gets our whole body working, and maybe even lights our muscles on fire a little. The metabolic effects of high-intensity exercise include muscle damage, increased energy production, challenged defenses and immunity, increased circulation of oxygen and nutrients, stress on our skeleton and connective tissues, and even more brain activity as our motor cortex and senses process incoming signals. Our body must deal with these demands, repair and recover itself afterward, and nutrition plays a critical role in this process. Energy demands of muscle Our muscles play a significant role in our body's metabolism, and there are three types of muscle fibers present in humans - slow-twitch (type I), fast-twitch A (type IIA), and fast-twitch B (type IIB). These fiber types vary in their contractile speed, fatigue resistance, diameter, capillary, and mitochondrial density, and myoglobin content. Slow-twitch fibers are dark, while fast-twitch fibers are lighter, indicating a unique composition of contractile proteins, organelles, and myoglobin. Different types of exercise can affect our muscle fiber types in various ways.

Metabolic testing, which measures the amount of oxygen consumed during a given activity, can help us determine the energy cost of exercise. The intensity of exercise and the number of muscles involved affects the amount of oxygen we consume and, therefore, the amount of energy we use. For instance, during sleep, a person consumes around 3.5 mL of oxygen per kilogram per minute, which is equivalent to 1 MET. Walking briskly requires five times the amount of oxygen consumed while sleeping, which is 5 METs. In contrast, an all-out sprint to escape danger can result in 10 METs, indicating a near-maximal effort that involves inhaling 2.45 L of oxygen per minute.

We can use METs to estimate the amount of energy we use during exercise. By multiplying the MET intensity by our body weight in kilograms, we can calculate our energy expenditure per hour of activity. For example, if a person weighing 70 kg cycles at 10 METs, they would be expending around 700 kcal per hour. However, different individuals have different oxygen consumption levels and metabolic demands, and METs are just one way to estimate energy expenditure. A side note: Embarking on a fitness journey can be an exciting but overwhelming experience. With so much conflicting information out there, it's easy to get lost in a sea of conflicting advice and end up feeling frustrated or disillusioned. That's why I've created a series of articles on fitness - to provide a science-based overview of the science of fitness, and to help anyone who's serious about getting fit do so in a safe and effective way. By reading these articles, you'll gain a deeper understanding of the biology of fitness, the different approaches you can take, and how to navigate the maze of information out there so you can make informed choices about what's best for you. Whether you're a complete beginner or an experienced athlete, this series is for you. So, let's get started on your journey toward a healthier and happier you! You can begin here: The Big Picture of Fitness. Oxygen consumption: Oxygen consumption is an essential process for the body during exercise. When we move, our muscle activity initially lags behind the amount of oxygen our body is consuming, leading to an oxygen deficit. This means that our muscles require more oxygen than we are taking in to meet the demand for energy. In response, the body uses anaerobic energy transfer from the ATP-PCr and the glycolytic systems to power our movements. Even low-intensity activities such as walking will be fueled by anaerobic energy production at first.

The concept of oxygen deficit can be compared to a startup company borrowing money to run its business. A small startup with minimal overhead costs will not require much debt to get going and will soon stand on its own. In contrast, a startup with big dreams and high machinery requirements will need more money and will incur a lot of debt before breaking even.

The intensity of exercise determines the extent of the oxygen deficit. Low-intensity exercises require minimal borrowing of energy from other energy systems to get started, leading to a small oxygen deficit. In contrast, high-intensity activities such as high-rep squat sets or Ultimate Fighting will incur a significant energy and oxygen debt. The higher the intensity, the greater the deficit.

Eventually, once we slow down or find a steady pace we can do for a long time, our oxygen consumption catches up. However, we still owe oxygen, and once we stop exercising, we keep consuming oxygen to pay that debt back. This period of increased oxygen consumption and energy demand has been called the period of "oxygen debt" or EPOC (excess post-exercise oxygen consumption). It can last for a few minutes or even a few hours, and for intense training, it can last up to two days.

During EPOC, the body needs to repay the oxygen debt for three crucial jobs: metabolizing additional nutrients, replenishing energy stores, and reloading depleted oxygen stores in the muscle and blood. Additionally, a higher body temperature after exercise, a harder working heart and respiratory muscles, more metabolism-boosting hormones, converting energy transfer products such as lactate into other things, more protein synthesis, and helping stressed and damaged muscles recover all contribute to EPOC.

The intensity of exercise directly impacts the size and duration of EPOC values. Higher-intensity exercise leads to more oxygen consumption and energy use during and after exercise and stimulates muscle mass growth. In contrast, lower-intensity exercise expends less energy and oxygen, resulting in a small oxygen debt afterward that decreases with training adaptations.

However, for practical purposes, most people benefit from a mix of higher- and lower-intensity exercise. Lower-intensity exercise can facilitate recovery from higher-intensity exercise while still expending energy and maintaining fitness. Anaerobic and aerobic exercises are two types of physical activities that use different energy systems in our bodies. There are three main energy systems: the ATP-PCr system, the glycolytic pathway, and the oxidative phosphorylative pathway. All three systems work together during exercise, but their level of activity and the amount of ATP contribute to determining whether the exercise is anaerobic (without oxygen) or aerobic (with oxygen).

The ATP-PCr and glycolytic systems are anaerobic, and they provide most of the energy needed for high-intensity exercise, while the oxidative system is aerobic and uses oxygen to produce energy. As exercise intensity increases, the glycolytic system produces more pyruvate than the mitochondria can handle, leading to the production of lactate and increased levels of hydrogen ions in the bloodstream. This is called the anaerobic threshold (AT), beyond which the body can't sustain the exercise for long.

By reducing the exercise intensity below the AT, we can continue to exercise indefinitely using the aerobic pathway of ATP supply and efficiently using lactate as an energy substrate to keep hydrogen ion levels low. However, if we stay above the AT, exercise becomes more anaerobic, and we can only sustain it for a short time. In the end, Exercise is a vital part of a healthy lifestyle and has numerous benefits for our body and brain. Different types and intensities of exercise affect our body's metabolism in various ways, and intense exercise seems to be the most effective in preserving muscle and metabolism with age. In this article, we explore the difference between aerobic and anaerobic metabolism and how exercise affects our body's metabolism. We will also talk about Macronutrients in the next article: The Big Three: A Guide to Macronutrients and Their Role in Your Health.