ISSA Nutritionist Chapter 3: Metabolism and Energy Balance 1

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    Chapter Goals:

    • Be able to explain the human energy balance and the components that go into our intake and expenditure. 
    • Know the metabolism of humans at a cellular level.
    • Be able to differentiate between both aerobic and anaerobic energy systems within the body.
    • Find the body’s energy currency and know how it works within our energy systems.


    Metabolism is the way that the human body converts the consumed foods into energy sources in order for us to power physiological processes. 

    The energy requirements of the body for bodily functions and physical activity are met by the digestion and absorption of nutrients and the presence of oxygen. The balance here between the consumption of nutrients and the demand for energy is known as the energy balance. 

    Nutrition and Energy Balance

    Metabolism is a very complicated chemical process that happens within the cells of the body. 

    When it comes to nutrition, it is important to understand these processes and the balance of energy. 

    In terms of nutrition, energy is provided by a calorie. We write this technically as a kilocalorie, or a kcal. This is the energy needed to raise the temperature of 1 gram of water by 1 degree Celsius at a pressure of 1 atmosphere. 

    The calorie content for the various macronutrients are as follows for every 1 gram:

    Cards are around four kcals

    Proteins are around four kcals

    Fats are around nine kcals

    Stored fat from carb sources are around 3.27 kcals

    Alcohol is around seven kcals

    Energy intake

    Total intake of calories in a 24 hour period is going to be used to measure energy balance. 

    If someone is gaining weight over time, this means they are consuming an excess of calories in comparison to what they are expending. On the contrary, if someone loses weight over time, they would be consuming fewer calories than they are expending.

    Excess calories lead to gaining weight, and deficiency in calories leads to a loss of weight.

    Research shows the average American takes in around 3,600 calories within a day, which is an increase of about 24% from the average of 2,880 calories a day in 1961.

    Current guidelines for adults from the US department of health and human services has declared that the average adult female needs 1,600 – 2,400 calories each day, and the average adult male needs 2,000 – 3,000 calories each day. 

    Energy intake should be measured by surveying someone’s food consumption in a 24 hour period. While the average human may consume 3,500 – 3,600 calories per day, the actual self-reported calories per day are more along the lines of 2,639 for men and 1,793 for women. 

    Energy expenditure

    This is measured in many ways, with the classic way being a calorimeter, which measures heat production. 

    Direct calorimetry uses an insulated chamber to measure the heat added to the ambient environment, while indirect calorimetry measures oxygen consumed and carbon dioxide produced. 

    The energy expenditure is a result of the cumulation of four main processes in the body, which are RMR, TEF, physical activity, and physical growth. 

    Resting Metabolic Rate

    This is the energy needed to support cardiac function and respiration, and also to repair the internal organs, maintain the temperature of the body, and balance water and ion concentration across the cell membranes. 

    The RMR is correlated to both body size and gender. Determining this RMR for someone is nearly impossible. We do have some formulas that help to predict it, though:

    Females: RMR = 248 x weight (0.4356) – (5.09 x age)

    Males: RMR = 293 x weight (0.4330) – (5.92 x age)

    Thermic Effect of Food

    The TEF accounts for the loss of heat that results from energy consumed when the body digests carbs, fats, and proteins.

    We also refer to this as diet-induced thermogenesis, and the thermic effect is going to vary depending on the macronutrient. 

    Physical Activity

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    This is second only to the RMR in terms of the effect it has on daily energy expenditure.

    This category includes the movement of the body, and it is directly related to the size of the person’s body and their level of physical conditioning. 

    The more that someone moves, the more energy they are going to expend. 


    The body is always growing and changing in small ways. We have millions of cells dying every day, and of course millions replacing them.

    For babies, infants, and other youth, we see their bodies maturing and growing at a more rapid rate, meaning more cells are being created than dying daily. 

    The energy cost of physical growth varies at different stages of life, but it is an important factor in the total overall expenditure of the body.

    The aspect of growth applies especially to the pregnant women that are supporting their own cell growth and also the growth of their fetus.

    Energy Balance

    Creating an energy balance involves more than simple management of weight.

    A positive energy balance means that more energy is being consumed than is being expended, and this would lead to gaining weight.

    A negative energy balance means that more energy is being expended than is being consumed, and this would lead to losing weight. 

    The body has a habit of naturally seeking a homeostatic balance of the energy system, and this can be quite evident in weight gain patterns. Over time we see that people who gain body weight will often have periods of their weight plateauing, regardless of the presence of positive energy balances. 

    Cellular Energy Currency

    Cellular metabolism can often be referred to as cellular respiration, and this is where a series of reactions occur that convert nutrients into the cellular energy currency of ATP, or adenosine triphosphate. 

    Adenosine is made up of adenosine and ribose. And this is then attached to three phosphates, and the two bonds between the three phosphates store and release energy.

    The body has all of the raw materials needed for the production of ATP.

    Converting ATP into Energy

    ATP alone does not provide cellular energy. This energy is stored in the bonds between the three phosphates in the ATP molecule, and those bonds need to be broken in the presence of water to release the energy.

    ATP to adenosine diphosphate plus energy

    The first step is for the adenosine triphosphate to break down to a simpler counterpart of adenosine diphosphate. 

    The break requires the enzyme, which is ATPase, which breaks that second and third phosphate for the release of stored energy. 

    ATP – ADP Cycle

    This is the process of allowing the cleaved ADP to be recycled back into functional ATP in the cell. The process of reattaching phosphate, which we call rephosphorylation, will require the enzyme of ATP synthase.

    The breakdown of ATP to release the stored energy is known as ATP hydrolysis. 

    Muscles constantly generate protons during basic cellular metabolism, whether you are at rest or in an activity. The body manages the hydrogen ions easily during rest or light activity by moving them to the mitochondria of the cells, where the energy is harnessed to resynthesize ATP with the use of O2 and the formation of water. 

    ADP to Adenosine Monophosphate

    In the compound of ADP, there are still two phosphate ions left, and the bond they have will still contain stored energy. During some extreme circumstances, we can use this remaining bond to generate needed cellular energy.

    For an all-out sprint of 10 – 15 seconds, the body will have energy needs that exceed what the ATP-ADP cycle is able to actually provide. 

    The Energy Systems

    The body has three different energy systems, and they can be classified as short-term, intermediate-term, and long-term energy systems. The systems will overlap in everything that a human does.

    Overview of the Energy Systems

    Muscle tissues have enough stored ATP to last only a couple of seconds, and then in order to quickly make more ATP, muscles turn to phosphocreatine. 

    This entire phosphocreatine process provides an instant source of energy, or up to 30 seconds worth. 

    This system using phosphocreatine is known as the phosphagen system. 

    As the muscle tissue continues to use phosphocreatine, glycolysis, where ATP is made from glucose, emerges as the main source for our energy. 

    This happens around 7 seconds into a run. 

    In glycolysis, a series of reactions allows the body to break down the glucose molecule into two pyruvate molecules, and this produces a small amount of ATP for a short amount of time, around 2 minutes. 

    The phosphagen system and glycolysis are both anaerobic processes, meaning they do not use oxygen in their processes. 

    When oxygen is added to anaerobic glycolysis, there are a few complex series of steps added to break down the pyruvate unit and generate ATP in the mitochondria. 

    Aerobic metabolism is now able to manufacture ATP for longer periods of time, and this introduces the use of fat as a fuel source. 

    The primary sources of energy will be glucose, fatty acids, lactate, and ketones.

    Phosphagen System

    Every cell in the body has cytoplasm, and in muscle cells, we refer to it as sarcoplasm. This serves the purpose of cytoplasm in normal cells. 

    The sarcoplasm has the actions of the phosphagen system taking place. 

    Phosphocreatine includes one phosphate molecule connected to one molecule of creatine. Creatine is naturally found in muscle tissue, and keeping average stores of creatine will depend on the overall mass of muscles. 

    Replenishing phosphocreatine is important since it is used at the onset of activity. 

    Naturally, this molecule is synthesized in the kidney, pancreas, and liver from the amino acids methionine, glycine, and arginine.

    Creatine is stored in muscle tissue, but also it has been found as a major substrate in the brain. 

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    Glucose is the next fastest source for ATP, and this is going to be stored in the form of glycogen in the human body. 

    Anaerobic Glycolysis

    Unlike the phosphagen system, glycolysis occurs in the cytoplasm of almost all cells in the human body, and not just the muscle cells like with creatine. 

    Glycolysis is acidifying to the muscle, unlike the phosphagen system, which is an alkalinizing process. 

    Aerobic Metabolism

    Aerobic metabolism produces ATP when oxygen is present. The process of aerobic metabolism begins with acetyl coenzyme A after the pyruvate molecule enters the mitochondria. This molecule is required to be formed regardless of what the fuel is going to be.

    The second stage here is the electron transport chain, which is also known as oxidative phosphorylation. This forms most of the ATP that is made during aerobic metabolism.


    These cell organelles are essential for the generation of chemical energy needed for the cellular processes. Some cells in the human body have more mitochondria than other ones do. 

    The specialized organelles have a dual membrane. The outer membrane acts as a skin, while the inner one is folded like that of the intestines. 

    The inner matrix of the mitochondria is filled with enzymes, water, and proteins, as well as the unique DNA and ribosomes of the organelle.

    Glucose for fuel

    Glycolysis splits the glucose into a pair of pyruvates in the sarcoplasm of the muscle cell. With sufficient oxygen, pyruvate moves from the sarcoplasm into the mitochondria, where aerobic metabolism starts. 

    Fatty Acids for fuel

    Fat is stored throughout the body in three different locations. 

    In the midsection, the area between the abdominals and organs, this fat is called visceral fat. 

    Right beneath the skin is what is known as the subcutaneous fat, which is the most widely distributed fat in the body. Inside the muscles we have intramuscular fat. 

    Fat is stored in all three of the locations as triglycerides. 

    Lactate for fuel

    Muscle tissue produces lactate in short term contraction, even when there is sufficient oxygen. Lactate can be used for energy by remaining in the muscle or moving to other areas. 

    Ketones for fuel

    Ketones are an acidic by-product of fatty acid metabolism, produced in the liver when glucose is not available. 

    These ketones are a normal part of the human metabolism and usually is well-controlled by the hormones of insulin and glucagon. 

    Trouble arises when starvation, severe illnesses, infection, or the presence of chronic diseases like diabetes forces the liver to metabolize large amounts of fatty acids. 

    Ketones have an effect in extreme circumstances, like:

    • A diet that is low in carbs
    • A very low-calorie diet
    • An extreme physical endurance event

    Ketones are a key source for energy in the brain in these aforementioned circumstances.


    Regardless of the source of the fuel, acetyl-CoA is the central metabolite initiating the aerobic metabolism process within the mitochondria. 

    The Citric Acid Cycle

    The Krebs cycle is also referred to as the citric acid cycle, named for one of its by-products. 

    The cycle consists of eight consecutive steps in a closed loop, meaning that the final step of the process re-creates the compound that is sed in the first step of the cycle. 

    The Electron Transport Chain

    This is the stage where most of the ATP is produced in aerobic metabolism.

    The fuel production form various fuel sources:

    Glucose results in around 30 – 32 ATP

    Fatty acids result in around 106 ATP

    Lactate results in around 30 – 32 ATP

    Ketones result in around 22 ATP

    ISSA Nutritionist Chapter 3: Metabolism and Energy Balance 2
    ISSA Nutritionist Chapter 3: Metabolism and Energy Balance 3
    ISSA Nutritionist Chapter 3: Metabolism and Energy Balance 4

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