Metabolism and Exercise Physiology

VIRTUAL CHEMBOOK, Elmhurst College. Ophardt c. 2003
VIRTUAL CHEMBOOK, Elmhurst College. Ophardt c. 2003                                                       Diagram 1: Metabolism  Proteins, Carbohydrates, Fats, and Lipids are metabolized into tissue, energy stores, or energy (ATP)

GOAL: Apply scientific knowledge about metabolism and exercise physiology to formulate an effective exercise strategy. 

I. Metabolism

Metabolism is the sum of all of the chemical reactions involved in maintaining the cells, and thus the organism. (3) Metabolism consists of catabolism – the breakdown of nutrients for energy, or anabolism – the synthesis of compounds needed by the cells (ie. DNA, RNA, proteins, cell structures)

A. Proteins

-Proteins contain amino acids and are mainly used as building block of tissues in the body, antibodies, hemoglobin, and genetic material (anabolism).

Catabolized to energy ONLY if there are no readily available carbohydrates or fats.

– Protein is not used by the body within 24 hours will be excreted as urea.

– Best dietary sources of protein in descending order are: whole eggs, milk, soybeans, meats, vegetables, and grains.

B. Carbohydrates

– Preferred source of energy for the body

–  Stored in the Liver and Muscles as Glycogen (branches of glucose)

– Catabolized into Energy (ATP) in cell Mitochondria

  • Anaerobically (no Oxygen) – via production of Lactate,
  • Aerobically (with Oxygen) – via the Citric Acid Cycle (Kreb’s Cycle
  •  (Glucose + O2 = CO2 + H2O + Energy)

-Dietary sources are starch, sugar, and cellulose (fiber)

C. Fats

–  Utilized to form cell structure, a protective cushion and heat insulation around vital organs, carry fat soluble Vitamins, and to provide reserve storage for energy.

– Gives more than double the amount of energy as carbohydrates and proteins

– Stored in Adipocytes and Muscles( as triglycerides)

– Catabolized into Energy ONLY AEROBICALLY BY OXIDATION!


A. Mitochondria

The Mitochondria is the part of the cell where Glucose and Fatty acids are converted into into Acetyl –CoA which eventually gets converted to Energy, Adenosine triphosphate (ATP) via the Citric Acid Cycle. ATP is hydrolyzed to ADP which releases the chemical energy necessary for binding of the protein myosin with actin filaments in the muscle to allow myosin to slide along the actin filament leading to mechanical contraction and relaxation of muscles (mechanical energy).

UpToDate July, 2014
UpToDate July, 2014                                                                                                                                            Diagram 2: Cell Mitochondria

B. Oxygen Delivery

Oxygen is essential for the cell to catabolize nutrients into Energy. Therefore, catabolism depends on the effectiveness of the Pulmonary and Cardiovascular System’s ability to provide and deliver Oxygen to the tissues. The delivery of oxygen by the cardiovascular system is summarized by the Fick Equation.

VO2  =  Qt  x  (CaO2  –  CvO2)

 Qt is the cardiac output (product of heart rate multiplied by stroke volume) and (CaO2 – CvO2) is the systemic oxygen extraction or the difference in O2 content between arterial and mixed venous blood.

VO2max, L/minute (the maximum uptake of Oxygen) – reflects the maximal ability of a person to take in, transport, and use oxygen, and it defines that person’s functional Aerobic Capacity. VO2max may be increased with Endurance Training (ET) and possibly High Intensity Interval Training (HIIT), the former has been shown to increase stroke volume and decrease peripheral resistance. (1)

C. Nutrient Utilization During Exercise

As we exercise our muscle cells initially use the circulating plasma glucose for energy and then later turn to using glucose derived from the breakdown of glycogen.  As the concentration of blood glucose falls, insulin drops and glucagon increases leading to the breakdown of liver glycogen stores into glucose which is released into the bloodstream. Simultaneously, as the available blood glucose levels drop during exercise our muscle cells start to convert its own glycogen stores into glucose to use as energy. Note that glucose derived from muscle glycogen cannot be released into the bloodstream. This is why muscle glycogen breakdown cannot prevent hypoglycemia. The average 70 kg man has about 1100 kcal of glycogen stored in muscles and 400-500kcal stored in the liver.

With continued exercise which results in the drop in glucose, insulin levels decrease and glucagon, epinephrine and norepinephrine rise. Epinephrine and norepinephrine stimulate lipolysisthe process in which triglycerides in adipocytes are broken down into Free Fatty Acids (FFA) and Glycerol. FFAs are then used by the muscle cell’s Mitochondria to produce ATP. The process of muscle contraction involves the use of ATP. Losing weight by losing body fat does not mean you lose your fat cells but that they shrink by releasing their fat contents in the form of FFA into the bloodstream during lipolysis – then getting oxidized into energy.

D. Effect of Intensity and Duration of Exercise on the type of substrate (carbohydrates or fats) utilized by exercising muscle cells.

A study performed by the University of Texas and published in the American Journal of Physiology in 1993 looked at which substrates the body utilizes as fuel with regards to exercise intensity levels ( low – 25% VO2max, moderate – 65% VO2max, high – 85% VO2max) and duration.(2) The substrates were either Glucose (from Plasma Glucose or Muscle Glycogen) and fats from FFA (plasma fats) or Muscle Triglycerides (muscle fat stores). The effect of duration of exercise on substrate utilization was only studied in the low and moderate intensity groups.

*Effect of Intensity at 30 minutes

Low – intensity:  utilizes mostly plasma FFA (fat in blood), a very small amount of Muscle Triglycerides (muscle fat), and a very small amount of Plasma Glucose. Burns mostly fats, BUT the least amount of total fats and total calories/kg/min. of the 3 groups

Moderate – intensity:  utilizes half carbohydrates (mainly Muscle Glycogen and a small amount of Plasma Glucose) and half fats (split between Plasma FFA and Muscle Triglycerides). Burns the most fat of the 3 groups.

High – intensity:   utilizes mostly carbohydrates (mainly Muscle Glycogen but also some Plasma Glucose) and also some fats. Burns the most calories/kg/min.

*Effect of Duration (1 -2 hours)

 Low – intensity: utilizes almost exclusively Plasma FFA.  No significant alterations in the relative contribution of the various substrates to energy production over the 2-h exercise period.

Moderate – intensity: Progressive increase on the reliance of Plasma FFA and Glucose as energy substrates. Burns much more total calories/kg/min. than low intensity.

After 1 hour of exercise, muscle stores of glucose (glycogen) and fats (triglycerides) are slowly depleted so the body turns to using fats derived from fat cells and glucose derived from other sources in the body.

STUDY CONCLUSION:  Although you may burn the most calories with high-intensity exercise the study indicates you burn the most fats when you are exercising at moderate – intensity (VO2max 65%). Somewhere between moderate and high intensity is one’s Anaerobic Threshold (AT). AT is the level of exercise intensity at which your body is starting to go into “Oxygen Debt”. Remember, fats can only be utilized to produce energy when Oxygen is available. Above AT you will start burning mostly sugar as opposed to fat and for endurance athletes this is not recommended since the available sugar is limited by what you ingest and by glycogen stores in the liver and muscles. Once you are past AT and do not recover you will go into Lactate Threshold (LT). At LT, there is very little Oxygen available to the cell so its only option for making ATP is to convert Pyruvic Acid to Lactic Acid (above diagram). This Anaerobic Reaction makes very little ATP compared to the Aerobic Reaction in the Citric Acid Cycle and leads to buildup of pain rendering Lactic Acid in the muscles!




  1. CATABOLISM                                                                                                                                                                    Conversion of nutrients into energy is dependent upon numerous complex chemical reactions, enzymes, and hormones as well the ability of the cardiovascular system to deliver and use Oxygen and Water. Expect individual Variations.
  2. BREATHING                                                                                                                                                            During aerobic exercise carbohydrates and fats are combined with oxygen to produce energy                                  Carbohydrate or Fats + O2 = CO2 + H2O + Energy  
  3. HYDRATION                                                                                                                                                            Water is used in Hydrolysis – a reaction required to convert Sucrose into Glucose and ATP to energy.
  4. INTENSITY                                                                                                                                                                      Low intensity – better than nothing                                                                                                              Moderate intensity – burns fat and sugar (where you want to be most of the time)                                  High intensity – burns more sugar than fat (? improve VO2 max )                                                      
  5. DURATION                                                                                                                                                                          After 1-2 hours of exercise body stores of sugar and fats are lower forcing it to utilize fats derived from fat cells. ALWAYS DO AT LEAST 1 HOUR OF CARDIO!
  6. TRAINING                                                                                                                                                                   Over a period of time the body will become more efficient
  7. REFUELING                                                                                                                                                           Choose foods and the quantities that will nurture the process and not sabotage all of your hard work. “An hour workout can be undone by 5 minutes of eating poorly!”



  1. Systrom, Lewis, UpToDate July 2014
  2. Romijn,et al. Am. J. Physiol. 265 (Endocrinol. Metab. 28): E380-E391, 1993
  3. VIRTUAL CHEMBOOK, Elmhurst College. Ophardt c. 2003



Metabolism and Exercise Physiology

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