How Do You Measure Energy In Regards to Nutrition?
Energy is the capacity or potential to perform work. Work is one form of energy, known as mechanical energy. Work is performed from the hydrolysis (breakdown) of something called adenosine triphosphate (ATP). ATP is the energy currency in the body and powers all forms of mechanical work. ATP is created by the breakdown of food, which is why it is crucial to be well-nourished and provide the body the nutrients it requires. Approximately 40% of the potential energy in the food consumed is transferred to ATP.
The sun is the ultimate source of energy. The energy from the sun is then used by plants to produce carbohydrates, proteins, and fats. Humans and animals both consume plants. The nutrients obtained from these food sources are then transformed into a usable source of immediate energy (e.g., exercise) or to provide a storage form of chemical energy.
Forms of Energy
- Nuclear
- Chemical
- Light
- Electrical
- Mechanical
- Heat
Measuring Energy
What is a calorie?
There are a variety of ways to express energy. However, the most common measurement in the metric system used in the United States is the calorie. Everyone knows that foods have calories, but few know how to define a calorie. A calorie is a measure of heat. More precisely, it is the amount of heat needed to raise the temperature of one gram of water one degree Celsius. In fact, a calorie is such a small unit of energy, it is instead often referred to as a kilocalorie (which is equal to 1,000 small calories because kilo = 1000) and is commonly abbreviated kcal.
What is a kilojoule (kJ)?
The term kJ is not often used in the United States, but is more common in other parts of the world and is actually the proper unit for energy in the International Unit System. 1 kcal is equivalent to 4.2 kJ.
What is ATP?
When food and nutrients are consumed, they are transformed in the body to a useful source of energy through a number of biological processes. Each macronutrient (carbohydrates, proteins, and fats) provides various amounts of ATP. This transformed energy is then available for immediate use in the form of ATP. The ATP molecule is composed of three components (tri, meaning three). At the center is a sugar molecule (ribose) and attached is a string of three phosphate molecules.
Ribose (the sugar molecule)
These phosphates are the key to the activity of ATP. The potential energy in the bonds holding together the sugar molecule and the phosphate groups is termed a high-energy phosphate bond.
During the breakdown (hydrolysis) of ATP, an enzyme catalyzes the reaction when ATP joins with water. During the process, the outermost phosphate bond splits releasing a considerable amount of free energy (which is then available for work). This energy can be used to build proteins, contract muscles, etc. Although this appears to be a laborious process, this occurs rapidly and does not require oxygen. The remaining molecule is adenosine diphosphate (ADP) because there are only two phosphate molecules remaining (di, meaning two). Remember, the outermost phosphate bond was split.
ATP is often referred to as the universal energy donor because it is required by all cells in the body. ATP carries potential energy to provide energy to all cells of the body as needed. It is the immediate source of energy for muscular contraction. Muscular exercise requires a constant supply of ATP to provide the energy needed for contraction. Subsequently, metabolic pathways must exist to produce ATP.
There are three metabolic pathways that can be used for the formation of ATP:
- Phosphocreatine (PCr) system (or phosphagen system)
- Glycolysis (or lactic acid system)
- Oxidative (with oxygen)
Anaerobic Pathways
Phosphocreatine (PCr) system
Phosphocreatine (PCr) functions as a store of high-energy phosphate in muscle (the amount of PCr is proportional to the muscle mass). PCr itself cannot be used as an immediate source of energy, but can rapidly replenish ATP. PCr (and ATP) are both found in very small amounts in the body, however, and are therefore used up within about 5 to 10 seconds. Subsequently, the PCr system is the first energy system used for the replenishment of ATP, but only for high-intensity, short-duration activities. Activities like sprinting, weight lifting, or any other activity with short bursts of speed (lineman exploding off the line to block their opponent, for example). Remember from above that when ATP is hydrolyzed, it splits the last phosphate group from the molecule leaving only two phosphate molecules. This splitting of ATP results in ADP, so one phosphate is gone. If activity continues, more ATP needs to be produced, so the phosphate needs to be replenished.
Now think of the name phosphocreatine. PCr is able to donate a phosphate group to the ADP to essentially replenish ATP and continue this process until both ATP and PCr stores are totally depleted. ATP then needs to come from another metabolic system or pathway.
Glycolysis (or lactic acid system)
Glycolysis has two major functions, one of which is to generate energy in the form of ATP. It does this through the use of muscle glycogen (the storage form of carbohydrate in muscles and liver) to help replace ATP rapidly. Muscle glycogen needs to first be broken down to glucose (this process is called glycolysis).
The rate of glycolysis is governed by the energy needs of a cell. Remember that PCr only lasts a very short time, so when activities last longer than 5 to 10 seconds, the next best source for ATP is glycogen. The total ATP yield from the complete breakdown of one glucose molecule is 38; however, ATP is required for the process itself, so the net ATP yield is 36 molecules of ATP.
Glycolysis can occur with or without oxygen, which is referred to as aerobic (with oxygen) or anaerobic (without oxygen). Aerobic glycolysis occurs if oxygen is present (think of walking, where there is an increase in heart rate, but not to the point of heavy panting). On the contrary, if inadequate oxygen is available, this is anaerobic glycolysis. Anaerobic glycolysis is used in sports or activities that last 30 to 120 seconds, such as a 200 – 800 meter run or 100 – 200 meter swim.
Aerobic Pathway
Oxidative Phosphorylation
Oxidative Phosphorylation cannot produce ATP directly; it makes use of stored energy (glycogen, free fatty acids, muscle, and adipose cell triglycerides, and even protein) and can convert this into very large quantities of ATP. Each fat molecule that is catabolized for energy produces 460 molecules of ATP. If you compare that 460 to the 36 ATP produced from each molecule of glucose, it should be apparent that fat can provide a much greater amount of energy than glucose. However, the catabolism of fat requires more complex metabolic pathways in the body before any ATP is produced, making it more time-consuming (as such it predominates during longer duration activities). Traditional aerobic exercises (step aerobics, jogging, walking, etc) are the types of exercise that use the oxidative (aerobic) system because the body is able to get sufficient amounts of oxygen (even though breathing becomes more rapid). The major benefit of this system is that it can produce very large quantities of ATP, even though it takes longer to do so.
Key Takeaways:
- ATP-PCr System provides enough energy to fuel ~ 5-10 seconds of activity
- Glycolysis fuels activities that last ~ 30-120 seconds in duration and produces 36 ATP
- Oxidative phosphorylation is used in longer duration activities, such as a marathon, and produces ~ 460 molecules of ATP. of ATP. This pathway is able to produce energy from fats and proteins.
Energy Efficiency of Macronutrients
Energy Equivalent of Macronutrients:
- Carbohydrate: 4 kcal/g
- Protein: 4 kcal/g
- Fat: 9 kcal/g
- Alcohol: 7 kcal/g
Carbohydrates are the first macronutrient source of energy. Carbohydrate is the only nutrient whose stored energy can be used to generate ATP anaerobically. Carbohydrates are stored as glycogen in both the muscles and liver and can provide approximately 2000 kilocalories of energy. Glycogen is ultimately converted to ATP; however, there is a limited amount of stored glycogen. It can be compared to filling half of the gas tank in your car; it will work for shorter distances (like a half-marathon equivalent), but you will ultimately need to refuel once that distance is increased (full marathon, for example).
The type of diet someone follows (high carbohydrate or low carbohydrate) also determines the level of glycogen storage. Higher carbohydrate diets would allow for great levels of storage, while the opposite would be true for low carbohydrate diets.
Fat is the next macronutrient used for energy production. Aerobic ATP production can last for an extremely long period of time. No matter how lean an individual is, there is enough stored adipose and muscle triglyceride in the body to produce ATP for many hours. In fact, the actual fuel reserves from stored fat in an average young adult male provide approximately 90,000 to 110,000 kilocalories of energy (or the equivalent of running roughly 40 Marathons)! Compare that to the average carbohydrate energy reserves which provide around 2000 kilocalories. Remember, too, each molecule of fat provides 460 ATP compared to the meager 36 ATP from glucose. Fat clearly provides a much greater amount of fuel than glucose, even though it is also a longer process to metabolize each fat molecule.
Protein can also be used for energy, but this is truly a last resort. Protein is typically only used during times of starvation, extremely low-carbohydrate diets when it is one of the predominant dietary sources of energy, and sometimes during extreme physical training. Protein is a very inefficient source of energy. Certain amino acids must be converted to a form that can readily enter the necessary pathways for energy release. The structure of all amino acids includes a nitrogen molecule, which first must be removed before being used for energy. This process is called deamination and it leaves something behind called a “carbon skeleton.” This carbon skeleton can then be used in a specific metabolic pathway called the Kreb’s cycle to ultimately be synthesized into glucose and finally, ATP. This process is called gluconeogenesis (the creation of glucose from non-glucose sources).
Alcohol is often classified as a nutrient because it does provide calories; however, its use before (including the evening before), during, or immediately after activity or exercise is not recommended. Alcohol is not used as a major energy source during any activity. In fact, it may actually be detrimental to performance because of its effects as a diuretic and because it can interfere with liver function.
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