Nutrition 101
Everyone knows that eating is essential to life, but many people don’t know what their bodies actually do with the nutrients that they put into them. We know that those burgers and salads that we eat keep us going, but how?
Turning Food Into Energy
Human beings are basically big energy machines. We take in energy and we output energy. The energy exchange process in our body is called metabolism. There are two types of metabolism:
Catabolism, or destructive metabolism. In catabolism, large organic molecules are broken down into smaller molecules. As they are broken down, they release energy.
Anabolism, or constructive metabolism. In anabolism, small molecules are assembled into large molecules. This requires the input of energy.
These breading-down and building-up processes go on nonstop in the body, which means we are always using energy in some way. Respiration, heart function, and new cell formation are examples of processes that use this kind of energy. Some people think that after 8 P.M. at night or when they go to bed this process stops. If this were true, however, people would only live for a day. Metabolism, or energy, enables our involuntary physiological mechanisms to operate around the clock.
In plants, metabolism takes the form of photosynthesis, which is the building of sugars. This is what plants, algae, and some bacteria do to feed themselves. They use the power of the sun to help
them combine carbon dioxide and water to make glucose and oxygen gas.
Since humans depend on outside sources of nutrients – food – to meet their energy needs, the metabolic process is a little different. Here’s what it entails:
First, food – carbohydrate, fat, and protein – is ingested.
The body then uses various enzymes and water to convert food to chemicals that the body can use, such as sugars, amino acids, fatty acids, and so on.
These chemicals are absorbed into the body and transported to the cells, where they are absorbed.
Once the molecules make it into the cells, they are again broken down into even simpler molecules. From here, they might serve as the building blocks for all sorts of cellular activities. Or they might be broken down again. As they are, they eventually turn back into inorganic molecules such as carbon dioxide, water, and ammonia.
The main source of energy in the cells is a molecule called adenosine triphosphate, or ATP. As you’ll see in a minute, this is one important little piece of matter.
Understanding ATP
ATP, or adenosine triphosphate, is a special molecule – technically, an adenosine nucleotide bound to three phosphates – that your body creates to store and use energy. ATP is made by the mitochondria of the cells and is made when and where the cells need it. ATP provides energy for most of the energy-consuming activities of the cell, including construction of proteins from amino acids, the assembly of nucleotides (a type of chemical compound) into DNA and RNA, and carbohydrate and fat synthesis. Among other things, ATP also does the following:
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Moves molecules and ions into and out of cell membranes through a process called active transport
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Assists nerve impulses
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Helps the cells maintain their volume
ATP is also the primary energy-producing molecule that your body uses for muscle contraction. The process that turns ATP into energy is pretty complicated, but basically, when a cell needs energy, it breaks apart the ATP molecule. As ATP breaks apart, it releases energy by bonding with water. At the same time, it loses a phosphate atom and turns into adenosine diphosphate (ADP).
The body can only make enough ATP to last for a few minutes at rest. As the work of the muscle increases, more ATP is consumed, and it leaves the body at a faster rate. In order for the muscles to keep working, ATP must be replaced.
ATP production is so important to the body that it has three different energy systems that create it – the phosphagen system, the glycogen-lactic system, and the aerobic-respiration system. The systems all work together to create energy transfer during exercise, and overlap during the process depending on how long and how hard you’re working out. Your fitness level and oxygen uptake (your body’s ability to use oxygen efficiently) will also determine when each system kicks in. As the intensity levels of your workout fluctuate, your body will switch between all three systems.
As an example, let’s say you’re going to raw a 2,000-meter race. During the first few seconds after you start, your muscle cells will burn off any ATP that they already contain. Then, the phosphagen system kicks in and supplies energy for the next eight to ten seconds. Since your activity is lasting longer than this, the glycogen-lactic acid system kicks in for the next three minutes or so. Finally, the aerobic-respiration system kicks in for the remaining duration of your event.
The Phosphagen Energy System
Muscle cells are limited to the amount of ATP they have at their immediate disposal because the body can only make so much of it at one time. When the body needs to replenish ATP levels quickly, it takes a replacement phosphate atom from another molecule called creatine phosphate (CP), which it uses to replenish the ADP molecule. Presto! A new ATP molecule, courtesy of the phosphagen energy system.
The phosphagen energy system is used during rapid, high-intensity activities that require a short burst of power, such as sprints, lifting heavy weights, jumping, throwing, and diving, when the muscles aren’t supplied by oxygen. Because it lacks oxygen, this energy system can only provide enough energy to sustain these activities for bursts of time – no more than five to ten seconds.
The Glycogen-Lactic Energy System
This energy system uses the glycogen molecules – or carbohydrate stores – that are deposited in the muscles from food metabolism. It comes into play during activities that are moderate to high in intensity and medium in length, such as medium-distance runs and swims. When the muscles need energy, they convert glycogen into glucose, and then into a substance called pyruvate. From here, the pyruvate enters the mitochondria of the cells, where it helps make more ATP.
The glycogen-lactic energy system is also called the glycolytic energy system, or simply glycolysis. For ease, we’ll refer to it as the glycolytic energy system from here on.
The glycolytic energy system, like the phosphagen system, is also anaerobic. However, it can also take place if oxygen is present. This type of glycolytic energy is called “aerobic” because it uses oxygen. When glycolysis takes place where there isn’t enough oxygen, it creates lactic acid, which further breaks down to lactate and hydrogen ions.
Aerobic Respiration
This energy system takes place in the mitochondria of the cells when there is enough oxygen to meet the demands of the activity. For this reason, it is sometimes called the aerobic energy system or mitochondrial respiration. It kicks in when exercise continues beyond several minutes, as it does during sports like rowing, distance running or swimming, or cross-country skiing. To meet the energy needs of the muscles, the body supplies them with oxygen. This allows glucose to be completely broken down into carbon dioxide and water, which results in even more energy release, or ATP production. Glycogen isn’t the only thing that can fuel aerobic respiration, which is why this energy system can keep going for hours if necessary. Fatty acids from the body’s fat reserves and muscles can also be used. If absolutely necessary, the body will even break down proteins into amino acids and use it to make ATP, although it doesn’t like to do this and will only do it in extreme situations.
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As much as we’d like to burn more fat as fuel, it simply won’t happen. Here’s why:
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It takes too long to convert during intense activity.
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The body needs more oxygen to burn it.
Lactic acid hinders the utilization of fat. The less oxygen available, the more the body relies on glucose. Because the intensity is high, more pyruvate is converted into lactic acid, which then slows the oxygen supply needed to use fat as fuel.
However, this does not mean that you should avoid high-intensity workouts when trying to lose body fat. If you work out at higher intensities, you’ll actually burn more fat during a workout. A workout at high intensity lasting only 30 minutes will burn more fat than one for 45 minutes at a moderate intensity. Also remember that when workout out you are burning energy (calories). After you work out, your body will then pull stored fat out of storage to meet your energy needs.
When compared to other energy systems, aerobic respiration and the aerobic forms of glycolytic energy system produce and activate energy much more slowly. Because of this, these slower systems can continue to supply ATP for several hours or longer, as long as nutrients are available.
The following chart compares the three energy systems. Note how a large role carbohydrates play. In fact, even during light to moderate exercise, carbohydrates supply approximately 40 to 60 percent of your total energy. Fat comes next, and protein comes last, generally comprising less than 10 percent of the energy required for exercise.
When talking about how the body uses its energy stores to fuel performance, these three energy systems are often simply referred to as anaerobic pathways and aerobic pathways. As a reminder, the phosphagen energy system is entirely anaerobic; the glycolytic energy system can be both anaerobic and aerobic; and the aerobic respiration system is completely aerobic.
Whenever we start exercising, we mainly use glucose as our fuel source and the anaerobic pathway to create energy. Once we warm up and our heart starts pumping faster (in about 20 minutes or so), oxygen-enriched (aerobic) blood supplies our muscles. At this point, the body starts to use fatty acids, some glucose, and a very small amount of protein in the form of amino acids.
The Shape You're In
When talking about how the body uses its energy stores to fuel performance, these three energy systems are often simply referred to as anaerobic pathways and aerobic pathways. As a reminder, the phosphagen energy system is entirely anaerobic; the glycolytic energy system can be both anaerobic and aerobic; and the aerobic respiration system is completely aerobic.
Whenever we start exercising, we mainly use glucose as our fuel source and the anaerobic pathway to create energy. Once we warm up and our heart starts pumping faster (in about 20 minutes or so), oxygen-enriched (aerobic) blood supplies our muscles. At this point, the body starts to use fatty acids, some glucose, and a very small amount of protein in the form of amino acids.
Stroking Your Body's Energy Burners
Now that you know more about how your body uses the food you eat for fuel, let’s take a quick look at the fuel requirements for short, intermediate, and long-term sports.
Fueling for Short-Term Sports
Since these sports last only up to four minutes, and your energy source is glucose/glycogen, having optimal levels of glycogen in the body is imperative.
Creatine supplementation may be of benefit for these athletes since creatine phosphate also helps supply energy in high demand over a short time period. (Please note this site is an educational site only and does not take place of medical advice. For athletes under 18 years old it is imperative to talk to your parents and a health professional first before supplementing with anything.) To spare the body’s glycogen stores, training before the day of the event should be little to none, and your warm-up the day of the event should not be at intermittent high intervals. If you are competing several times during the day, pre- and post-event snacks or meals will help you keep your carbohydrate stores their fullest for the next time you compete.
Fueling for Intermediate-Term Sports
These sports last anywhere from four to nine minutes, and may be longer. They use glucose/glycogen for their main energy source. Due to their high intensity and short duration, they don’t use fat for energy.
For optimal performance, tapering training and increasing carbohydrates two to three days prior to competition is recommended for optimal glycogen storage. The day of the event, you will use your glycogen stores, so you’ll need to plan for hydrating and refueling during the day.
Fueling for Long-Term Sports
These activities last longer than 90 minutes for continuous aerobic activity. Again, having optimal levels of muscle glycogen is critical for these sports. Well-trained athletes actually store more glycogen than the untrained. You can improve your glycogen stores by:
Following a higher-carbohydrate eating program for the week before the event. (More information within the Academy)
Decreasing your workouts.
Take a couple of days off prior to your event.
Proper hydration and taking in 30 to 60 grams of carbohydrates every hour during competition will help keep energy supply at optimal levels.