Metabolic Pathways & Energy Requirements

Energy Requirements & Metabolism

FOOD = FUEL:  The macronutrients our bodies require for optimal health is carbohydrate (CHO), fat, protein. It is important that we consume the right proportions of each macronutrient in order to get the energy that sustains our body.  When we eliminate one of the macronutrients for fear it will make us fat we ultimately diminish the efficiency of one or several of our organ systems. The truth is, too much of ANYTHING and not enough exercise will make you gain weight.

  • If calories consumed is greater than what you expend you will gain weight.
  • If calories consumed is less than what you expend you will lose weight.

How Is The Food We Eat Transferred to Energy?

Energy Pathways: This is how our body transforms the calories (fuel) we eat into energy.  There are three pathways the body transfers food into energy (ATP). Two of these pathways do not rely on oxygen (O2) and so we call these “anaerobic” the last pathway does rely on O2 and so we call this aerobic or oxidative pathway.

1. FASTEST ENERGY: ATP/Creatine Phosphate cycle (ATP/CP)

This anaerobic pathway only lasts 3 to 15 seconds of very intense work. The ATP/CP pathway is dominant during VERY heavy strength training and explosive power exercises. For example: A 100m sprint or as in weight training, 5 reps at 85% 1RM would tax this system.   You use up all your ATP/CP in 15 seconds and it takes 3 min for the body to regenerate that ATP/CP before you’re ready to go again.  This is why your muscles NEED to rest for 3 min between sets when you are performing strength or explosive power exercises in order to give your best effort.

2. FAST ENERGY: Anaerobic Glycolysis (glyco = sugar and lysis = breakdown –> so glycolysis means sugar breakdown)

This is the other anaerobic pathway and it ranges from the end of the ATP/CP (~15 sec) to about 3minutes of very hard work.  Anaerobic glycolysis relies on carbohydrate as fuel.  The term for carbohydrate stored in the muscle cells and liver is called glycogen.  We also have carbohydrate floating in the blood stream and this is called glucose.  Performing very hard work such as 1-min “all-out” efforts on the treadmill or any resistance exercise of 8+ repetitions is using this pathway.  You probably feel your muscles aching or “burning” then you know you are in this pathway. The reason why you feel the muscle “burn” is because a by-product of metabolizing glycogen is something called lactate + hydrogen ions.  The Lactate + hydrogen ions are accumulating faster then they can be cleared by the blood and so whenever hydrogen ions accumulate we get an acid LACTIC ACID.  This is what people like to call “lactic acid“, but it is important to realize that “lactic acid” is 2 separate components: LACTATE + Hydrogen ions.  It is the H+ ions that lead to the pain associated with muscle fatigue. The lactate isn’t a big deal, we actually circulate lactate through the blood and transfer it to other working muscles to use it for more fuel!

3. SLOW SUSTAINED ENERGY: Aerobic (Oxidative) Pathway

Begins at about 3 minutes to hours and hours of physical activity!  The oxidative pathway metabolizes all three fuel sources: glycogen/glucose, fat, protein. The CHO and FAT come from the muscle, liver, blood, and fat cells to bring a constant supply of energy to the working muscles.  After 2 hours of continuous exercise you have depleted most of your glycogen stores, and results in the “bonk” feeling. This is why endurance athletes must drink sports drinks and eat gels/bars while training to prevent glycogen levels from dropping.  The more intensely you exercise the faster you deplete.

a.    While at rest the body uses this pathway as well, and the contribution of the fuel used is mostly from FAT and some glycogen and very little from protein.

b.    The easier and longer the exercise session the greater the contribution of fat.  This is because you are able to bring in enough oxygen to break down (oxidize) a fat molecule (triglyceride).  Triglycerides are very large carbon chain molecules and so they require a lot of available oxygen to break these bonds.

c.    As exercise intensity increases –> heart rate increases –> breathing increases = greater oxygen demand!  This causes a transfer of fuel from FAT dominant to CHO dominant because a carbohydrate is a smaller molecule and easier to break down when oxygen demands increase.

Each energy pathway can be separated into distinct time periods, but in reality during a typical exercise session you are actually using all three pathways.  Lets say you are in a Spinning class: When you initially begin the first few pedal strokes come from ATP/CP then after a few seconds your body realizes the intensity isn’t that much, it doesn’t waste all the stored creatine phosphate, so it kicks into anaerobic glycolysis for the next 3 minutes because it takes that long for the aerobic system to get going. You may actually feel some muscle fatigue sensation during the initial few minutes because you are in anaerobic glycolysis and waiting for your aerobic engine it get going (BTW this period is called oxygen deficit). After several minutes at a slow pace the body has enough oxygen available to start breaking down a mixture of carbs and fat.  Now lets say the instructor tells you to increase your resistance so you’re climbing a hill, so for 30-60s you have to push hard to get up the hill. During that minute you are supplementing with anaerobic glycolysis, but when you recover on the top of the hill you switch back to the oxidative pathway. Lets say now you are doing “jumps” in and out of the saddle, every time you jump you are using the ATP/CP system, because you need more energy as fast as possible to generate enough force to pick your body up out of the saddle.  The oxidative system can’t give you that energy fast enough.  As the minutes go by you are steadily depleting your glycogen stores, as glycogen becomes less available you take your fuel from more FAT and PROTEIN. As a result of relying on less efficient fuel sources you have to slow down your pace to meet the oxygen demand.  Depending on your pre-exercise glycogen levels will dictate how long you can go before you “hit the wall” or “bonk”. The more topped off your glycogen the further you can go. On average this is about 90 minutes to 2 hours.  A typical Spinning class is 45-60min, so it is unlikely you ever “bonk” in spin unless you took it without eating anything for several hours prior to the class.

Conversion of energy sources over time: Chronic aerobic training.

Chronic aerobic training does not alter the total amount of energy expended but rather the proportion of energy derived from carbohydrate (CHO) and fat.  One result of training is the energy derived from fat increases and the energy derived from CHO decreases.  If two ladies, one is regularly active and the other sedentary, are running on a treadmill at the same speed of 6.0mph, the active woman is not working as hard as the sedentary woman, because her oxygen demand is lower therefore the contribution of fat to total energy expenditure is greater than the sedentary person who’s oxygen demand is much higher and is burning through all of her glycogen stores.

Active Individual Nutritional Needs
NOTE: these recommendations are based on general guidelines from the most recent position stand from the American College of Sports Medicine and the American Dietetic Association regarding nutrition and athletic performance.  These values are indictated for an individual who averages 3-4 days/week of cardiovascular and 2-3 days/week of resistance exercise.  Daily requirements fluctuate depending on the specific activity level, age, and gender of the individual.  For personalized nutritional information please seek expert advice from a Registered Dietition.

Carbohydrate (CHO): 4.0 – 7.0g/kg Divide your weight in lbs by 2.2 to get kg Example: 130lbs/2.2 = 59kg. 59kg x 6.0 = 354g of CHO/day

  • Carbohydrate is the preferred fuel, and the body uses this energy source first and foremost.  The average person and strength athletes need only about 4 – 5g/kg per day  An endurance athlete in heavy training or competition may need up to 7g/kg per day, but for most average training loads (8-10 hours/wk) then 5-6 g/kg may be all that is needed.  During the off-season carbohydrate needs should drop to normal levels of 4-5g/kg to prevent weight gain.

Protein: 0.8 – 2.2 g/kg (a strength athlete would need closer to 2g/kg whereas an endurance athlete needs slightly less 1.2 to 1.5g/kg per day. A sedentary person only needs 0.8g/kg per day)

  • High quality source with all essential amino acids is best, such as lean animal sources. Pre and post heavy strength sessions you need fast absorbing protein supplement such as whey protein isolate that includes the branch-chain amino acids (leucine, isoleucine, valine). A whey protein drink is required for an athlete within 60min of training in order to maintain a positive nitrogen balance to facilitate synthesis of lean tissue.   For endurance athlete protein requirements are not as high as carbohydrate requirements.  So after an endurance session, carbs should be emphasized however protein is also essential to off-set the catabolic effects of long distance training.

Fat: 20% – 35% total energy intake.

  • Consuming <20% does not benefit body functions or exercise performance.  Fat is a source of energy, fat-soluble vitamins (A, E, D, K), and essential fatty acids are important.


  • Dehydration in excess of 2-3% body mass decreases exercise performance.  Adequate fluid intake before, during, and after exercise is important for optimal health.  After exercise drink 16-24 oz for every pound of body weight lost from dehydration.

Pre-exercise snack

  • Should be relatively low in fat and fiber to facilitate gastric emptying and minimize GI distress, be high in CHO for maintenance of blood glucose, be moderate in protein, and should be familiar and taste good to the individual.

During exercise

  • Primary goals for nutrient consumption are to replace fluid losses and provide CHO for maintenance of glucose levels.  This is especially important for endurance events lasting longer than 1hour or when in an extreme environment (heat, cold, high altitude).

After Exercise Nutrition (within 60min! Timing is EVERYTHING) 

Endurance Trained Athlete:

  • Dietary goals are to provide adequate fluids, electrolytes, CHO to replace glycogen, water, and ensure rapid recovery.  A CHO intake of 1.0g/kg body weight during the first 30min and again every 2 hours for 4-6 hours will replace glycogen stores.  Protein consumed after exercise will provide amino acids for repair of muscle tissue.

Strength Trained Athlete

  • Whey + Casein combo protein supplement with a little carbohydrate may be the best combination for post strength training recovery nutrition.  Protein amount should be between 10-20grams in one sitting and then again 2 hours later.  Research is showing that frequent feedings of smaller protein doses is better than one massive feeding because your digestive and renal system can only handle so much at one time.  


  • No vitamin and mineral supplements are required if the individual is consuming adequate energy from a variety of foods to maintain body weight.  A multivitamin may be appropriate if the individual is dieting, or habitually eliminating foods or food groups, is ill or recovering from injury, or has a single micronutrient deficiency.


  • May be at risk for low intakes of energy, protein, fat, and key micronutrients such as iron, calcium, vitamin D, riboflavin, zinc, and vitamin B12.


ACSM, AHA, ADC Joint Position Statement. Nutrition and Athletic Performance 2009

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4 Responses to Metabolic Pathways & Energy Requirements

  1. I was suggested this web site through my cousin.

    I’m now not certain whether this put up is written via him as no one else recognise such certain about my trouble. You’re wonderful!
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  2. Ashwin says:

    How do you approach getting to ‘race weight’? Do you have target you shoot for during the competitive season.

    • Thanks for the question! First off, an athlete would need to establish what their ideal weight is during optimal performance. This can be done through identifying the weight the athlete was during a “personal best” season in which the athlete had his/her best performances. It is common and not necessarily unhealthy to gain 4-5lbs in the off-season when training is not too heavy. However if an athlete gained extra pounds during a hiatus from the sport, then the most recommended approach to attaining “race weight” would be focus on weight loss during the off-season (>12 weeks prior to race season). I recommend the athlete to discuss his/her energy needs with a registered dietitian and calorie restriction must be modest to not be chronically depleted and sacrifice training intensity. Weight loss should be on average <1lb a week to avoid decrements in training performance.

      For me personally, as my training program increases in volume and intensity through March and April (pre-season) I lean out a couple pounds to put me at my optimal competing weight. For me performance tends to be best at 112lbs (no less however as this has led to decreases in performance).

      Hope that helps!

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