ISSA SN Chapter 15: Calorie Needs and Metabolism

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Chapter Goals:

  • Describe and define the terms that relate to calorie needs and metabolism.
  • Know the calculations for the requirements of caloric needs daily. 
  • Know the role of metabolism in your body.
  • Know the different types of muscle fibers.

Guide to Estimating Calorie Needs

Calories are the unit of measure used for calculating the amounts of energy inside foods. We use this to show how much energy and what kind of energy is found within the contents of the food we eat. One kilocalorie is technically the amount of heat needed to raise the temperature of one kilogram of water by 1 degree Celsius. The calorie is then just a 1000th of a kilocalorie. 

Technically the calories we use in nutrition will be the kilocalorie we use to measure changing degrees of water. The daily caloric needs are measured in the thousands. 

Another measure that scientists may use is going to be the kilojoule. This is done by simply multiplying the number of kilocalories by 4.2. we will mostly see this used in packaging outside the united states. 

The rate of expenditure of the body is known as the metabolic rate. This is someone’s daily caloric expenditure. Another similar measure is the rate of consumption of oxygen. This is due to the direct effect on metabolism caused by oxygen use. 

All of these are known to have a margin of error when using them, so it is good to always take them as close estimates and not as an exact science. The results can always be too high or too low. So, you should constantly track the changes in your body throughout the finding of your values. 

There will never be the same expenditure by a person on a day to day basis. This is due to the many factors that go into energy use. But one thing that will remain mostly constant is going to be the basal metabolic rate or the BMR. This is the rate at which your body will expend energy on maintenance activities like keeping organs functioning, the body alive, and other small features. The BMR is the lowest when you sleep, and most methods will look at it when you are awake and in controlled rest conditions in a normal temperature environment. For a rough estimate of your own basal metabolic rate, we use the calculations:

  • Men’s BMR equals 1 times body weight in kg times 24 hours.
  • Women’s BMR equals 0.9 times body weight in kg times 24 hours.

These are accurate with average people in terms of body fat levels. The higher the body fat percentage, the fewer calories you will burn. Muscle burns more calories than fat to keep it going. 

Increasing your metabolic rate and not overeating will cause you to eliminate fat easily. 

Average Daily Activity Level

This is an hour to hour approach to see your calorie needs daily. We divide the found BMR by 24 to find the per hour amount. Then you can add the number of calories your activity level demands and see what you get in the hours. The lean mass matters here because the burning of calories is greater for the bigger muscles. So, we have some different multipliers to look at and utilize based on this information. 

The energy expenditure guide can be used to find your activity and its effect on how many calories are burned. This is the same kind of information used by the apps and calorie trackers to determine our expenditures while being active. 

Metabolic Equivalent

This term is used in relation to energy. In this, 1 MET equals the energy used when sitting or lying down. This is 3.5 ml of O2 per kg per min. Another way to write it would be as 1 kcal per kg per hour. Under activity conditions, we see the rate of expenditure increase as we did in the previous part of the book. Here we base it on that 1 MET and determine the activity intensity with a number that relates. 

Activities are given values like 4 METS, which would mean 4 times more than when you are working at rest, that 1 MET value of 3.5 ml of O2/kg/min. 

Getting in 500 – 1000 MET minutes is one way e determine the range of beneficial activity for people. So, this is another way we can utilize these measures. 

Overview of Metabolism

The unit we had for digestion brought the facts on nutrients and their relationship with growth, maintenance, energy, repair, and the pure sustenance of life. For any of that to have happened or to happen, we need this chapter on metabolism, which is the breakdown of said nutrients. All of our reactions are carefully controlled, and we prevent the overproduction or the underproduction of the end products in question. The following parts of the chapter focus on this balancing act with our body’s metabolism. We constantly turn certain controls on and off to maintain our current levels.


Homeostasis is how we keep the body’s environment balanced and standard. One of the best examples that most people recognize is the body’s temperature staying at a near constant 98.6 degrees of Fahrenheit. Physical exertion or some outside heat acting on the body tries to change this, but the body uses its mechanisms at hand to drop and/or raise the temperature within to maintain that 98.6 degrees. When outside the good temperature range, the body does not function perfectly. 

Homeostatic Feedback Control Systems

For homeostasis to work, feedback systems are in place and are allowed to turn things on and off essentially. Some metabolic functions that occur for homeostatic control are:

  • The production of hormones and the maintenance of concentration levels for those hormones.
  • The maintenance of the serum levels of oxygen and carbon dioxide.
  • Balancing of pH levels in the cells and the blood.
  • The water content maintenance of the cells and the blood.
  • The glucose levels in the blood and the other nutrients inside of cells.
  • Our metabolic rate.

Homeostasis is quite important for athletes to understand. It is important for people to be in equilibrium with their outside environment stimuli acting on them. An example more relevant to athletes would be when you have spent a lot of time lifting weights and such, your muscles grow bigger out of necessity to adjust to the needs and the stress exerted. The hormone levels change to meet this new demand and many other things like energy production. We can see this also with those who run or do anything regularly. The change occurs over time as the body adjusts to the stress put on it. 

Of course, nutrient homeostasis will play an important part also. Eating too much bad food affects you negatively, as does eating too little food. So everything becomes about this balance, and it works day to day in small steps. 


Many processes all come into play to make up the human body’s metabolism. We categorize the processes as either catabolism or anabolism. These are going to happen at the same time, but the magnitude of them will differ. 

Anabolism is the term we use for building and creating larger molecules from smaller materials. The net result will be a new cell material like a protein, enzyme, or some tissue.

Catabolism is what we describe as the chemical reactions that will break down a molecule that is complex and make it simpler for energy and other processes. Anti-catabolic training stops the breakdown of muscles and body tissues as much as possible. It is bound to happen, but minimizing this is key. 

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Metabolic Set Point

This is the average rate of metabolism that results in a body composition set point. People with slower metabolic rates will store fat more easily, and those with fast metabolisms will likely eat and rarely get fat.

Food and Metabolism

The type of food you eat will influence your metabolism. Diets higher in protein and carbs will increase the BMR. Processing excess fats will need more energy. Diets to reduce fats are low in fat and high in protein. These are just some of the more common examples of using these ideas in diets. 

We use something known as the respiratory quotient or the RQ. This is used for measuring the amounts of fats, carbs, and proteins that are being burned and used for energy. It is also a measure of the ratio of CO2 volume expired and O2 volume consumed. 

RQ = the volume of C02 expired / volume of 02 utilized

An RQ of 1 is for carbs, and 0.7 is for fat. The RQ for protein is 0.8. 

Exercise and Metabolic Responses

Exercise stimulates many metabolic responses affecting the body’s anatomy, physiology, and biochemical makeup. Some changes that result from endurance exercise are:

  • Increases in muscle glycogen storage capacity.
  • Increases in the muscle mitochondria density.
  • Increases in the resting content of ATP in the muscles.
  • Increases in the aerobic enzymes.
  • Increased levels of slow twitch fibers.
  • Decreased levels of fast twitch fibers.
  • And many more similar changes.

Aerobic System Changes

Increases in the density of mitochondria in the slow twitch fibers result in greater energy production from fats. Maximum oxidative capacity develops in all fibers. 

A Greater Aerobic Capacity.

Increases in muscle capacity utilization and fat mobilization result from increased levels of fats and their enzymes and increased blood flow. 

Greater development of slow twitch fibers.

Increases in myoglobin, an iron-protein compound in the muscle that stores and transports oxygen in the muscles. 

Anaerobic System Changes

  • Increases in size and the number of fast twitch muscle fibers.
  • Increases in the tolerance of high levels of blood lactate.
  • Increases in the enzymes involved in the anaerobic phase of the breakdown of glucose. 
  • Increases in the resting levels of muscle stores in ATP, CP, creatine, and glycogen content.
  • Increased growth hormone and testosterone levels after the short bouts of exercise. Usually around 45 – 75 minutes.

Energy Metabolism

Energy metabolism is essentially a series of reactions resulting in the breaking down of food. The result of these reactions is the energy we use for our body. This energy is given off as heat. The body is roughly 20 percent efficient at trapping the produced energy; the remaining 80 percent is put off in the heat. This explains why the body heats up during exercise. 

Physical activity can be put into four basic groups that are based on the energy systems used to support the activity. These are:

  • Strength power: the energy from the immediate ATP stores is used. It is the high intensity things we do in short times of around 0 – 3 seconds. 
  • Sustained power is the energy from the immediate ATP and CP stores. Sprints and football linemen are examples. This is the first 0 – 15 seconds of max effort.
  • Anaerobic power endurance: these activities last one to two minutes, like a 200 or 400 meter dash.
  • Aerobic endurance: this is our aerobic and oxidative energy system. Anything over two minutes falls under this. 

Useful Terms:

ATP, or adenosine triphosphate, is the main energy storage form in the human body. It is used for many of the biosynthetic processes that need energy and it is going to be essential for muscle contractions.

CP or creatine phosphate is found within the muscle tissues and it is stored as a source for muscle contractions. We do not store much; it is only used for immediate replenishment of ATP and fueling other high intensity muscle contractions. 

Glycolysis is the process of metabolism that creates energy by splitting the glucose molecule for the formation of pyruvic or lactic acid and the production of some other ATP molecules.

Krebs Cycle is part of the oxidative part of energy production where the carbon chains from glucose, fat, and protein breakdown get used for more ATP production. This is used in endurance activities. It takes the place of the mitochondria. 

Gluconeogenesis is the cellular breakdown of the stored glycogen for energy use. 

Glycogen depletion has some factors that athletes experience relating to fatigue.

ATP and CP depletion is faster than they are formed; thus, we run out.

Metabolic acidosis and a decrease in the pH levels are associated with the increase in acid forming hydrogen ion concentration.

Alterations in the central nervous system neurotransmitters like serotonin and dopamine.

  • Neural activity decreases.
  • There are imbalances in ions like electrolytes.
  • Oxygen depletes and there is a reduction of aerobic energy being produced. 
ISSA SN Chapter 15: Calorie Needs and Metabolism 4
ISSA SN Chapter 15: Calorie Needs and Metabolism 5
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Tyler Read - Certified Personal Trainer with PTPioneer

Tyler Read

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