18. Aerobic and anaerobic energy sources


18.1 Cardio exercise versus resistance training


I mentioned previously in the blog that the effects of cardio versus weight training seemed quite different in regards to post exercise recovery. A long cardio session (say 90 minutes and around 1500kcals expended) will leave me feeling relaxed but energized, I will need less sleep yet wake feeling refreshed and ready for anything. It will not have any noticeable affect on my appetite either immediately following the exercise, or the next day.


A big weights session has the opposite effect, even although significantly fewer calories will be expended during the exercise (although I have recently seen a paper that suggests calories expended during resistance training may be significantly underestimated by current thinking). I will need more sleep after the session and will wake up feeling tired and definitely in need of some recovery. I will usually be hungrier too which implies that the post exercise energy requirements are much greater for resistance training than for cardio.


Mike Tuminello in his book "Strength training for fat loss" (published by Human Kinetics, 2014) actually advocates resistance training as a better approach to fat loss than cardio exercise. Although I have significantly increased my exercise and fitness levels over the past year and returned to my pre-cancer levels, I could not claim any scientific method in my approach. I focused initially on just cardio and increased the frequency, duration, and intensity at a rate I felt comfortable with as my strength gradually returned. I did not include resistance training at the outset as I needed to be in a prolonged calorie deficit to reach my fat loss target and the body will not waste effort on building muscle tissue if there is any significant calorie deficit.


The result was that after losing 20kg of fat and dropping to 8.3% body fat I was decidedly scrawny and needed to put back some muscle, so I then added a couple of resistance training sessions per week. After 6 months on this regime I became very aware of the differences between the two types of exercise and recognized why they it is recommended to do both types of exercise for optimum health benefits. I also discovered that losing 20kg of body fat is pretty easy compared to gaining, and maintaining, 6kg of muscle! I tried to understand a bit more about the fundamental energy requirements for anaerobic and aerobic exercise so that I could perhaps become a bit more scientific in my approach to training. I wanted to understand better how the food we eat is turned into mechanical energy via the muscles and discovered that there are three separate pathways for meeting the energy demands of the body.  Read on for more!


18.2 The three metabolic pathways for energy


All the energy we burn during physical activity comes from our food but the intensity and duration of the exercise determines which process the body uses to metabolize the energy from carbohydrate, fat, blood sugar, protein, or stored glucose (glycogen).


At the cellular level, the energy required for all physical activity is provided by a substance called adenosine triphosphate, or ATP for short, but the metabolic pathway by which the ATP is made available to our cells is determined by the intensity and duration of the physical activity, as well as the availability of oxygen in the cells.


The three metabolic pathways available are known as the Phosphagen System, the Glycolysis System, and the Aerobic System. All three result in ATP being made available to provide energy to support physical activity but the first two are primarily used in high intensity, short duration, activity during which oxygen availability is severely restricted (i.e. "anaerobic"), while the latter is for low intensity, long duration, activity when there is abundant oxygen available (i.e "aerobic").


Phospagen :  The body can only store enough ATP for a few seconds of very intense activity so it must synthesize ATP for all out physical effort lasting beyond a few seconds. It does this by breaking down the creatine phosphate stored in the muscles in a process called "The Phosphagen System". This is the fastest way to synthesize ATP in the absence of oxygen but, as the amount of creatine phosphate stored is very limited, this pathway can only be sustained at maximum effort for about 20 seconds.


Glycolysis : If intense activity continues beyond about 20 seconds then ATP has to be synthesized by a different process known as "Glycolysis" which can power intense activity for about two minutes. Glycolysis works by breaking down carbohydrate, from blood glucose or glucose stored in the liver in the form of glycogen, into a substance called pyruvate which releases two molecules of ATP for each molecule of pyruvate. The ATP becomes available quickly for energy output while the pyruvate breaks down into either lactate, if the supply of oxygen is insufficient to meet demands, or into other intermediaries from which more ATP can be synthesized in the cells mitochondria. Glycolysis can maintain high intensity output for about 2 minutes which is approximately the maximum duration for high intensity human exercise. Our energy levels drop off and muscle fatigue sets in which is why we cannot outrun a cheetah.


Aerobic : The Aerobic System provides the energy for most of our waking hours and it relies on an abundance of oxygen being available to enable the synthesis of ATP in the muscle cells from glucose, glycogen, or fat. This process enables a much larger quantity of ATP to be synthesized sustainably over much longer periods than either the Phospagen or Glycolysis Systems but the downside is that it is more complex and takes more time. The Aerobic System is what sustains long distance runners or any exercise of moderate intensity but long duration. For about the first 2-4 hours of aerobic exercise the body will synthesize ATP from glucose, glycogen and fat but, beyond that time, fatty acids and protein may also be used. Protein is apparently not a prime source of energy under normal daily conditions but will be used if the reserves of carbohydrate and fat are running low.


This understanding of the energy sources for aerobic and anaerobic exercise helps to explain the difference I noticed in post exercise recovery between cardio and resistance training. Resistance training is fuelled mainly by glycogen which we store in limited quantities and which the body will actively replenish as soon as possible. In addition the body will be building and repairing muscle tissue which requires additional energy post-exercise. Hence the feeling of increased hunger after a strenuous weights session which, from experience, I know can last up to 24 hours.


Cardio exercise at moderate intensity (defined as a heart rate of between 50-70% of your maximum heart rate) will probably rely on carbohydrate and glycogen for about the first 30 minutes but if you continue beyond that time then you will start to burn fat. I used to think that fat burning on the treadmill was all about going hard but actually it is the opposite. Exercising at 50-70% level now feels a bit like cheating because it seems so easy but that’s what you need to maintain if you want to burn fat. Raise the effort level to 80-90% and you will improve your power and endurance, VO2 max, and lactate threshold but you will be using stored glycogen not stored fat, so if you want to burn fat on a treadmill then do not run too hard.


When I returned to the gym to begin my cardio exercise alongside the 5:2 Fasting program I quickly rebuilt my fitness to the point where I was able to sustain 60-90 minutes of moderate exercise on the treadmill. It turn out (quite by chance!) that my exercise level was actually the optimum one to help me lose fat. However, as I said earlier in the blog, while exercise brings undoubted health benefits it is a hard way to lose significant amounts of fat. I tried an experiment to see if I could verify the fat burn theory. At the time my fat level was stable at 7.5% so I walked on an 18% inclined treadmill and controlled my effort to maintain my heart rate at about 70% of my maximum (measured with a Wahoo Tickr heart rate strap + iPhone app) during 6x90minute sessions over a period of 8 days. To my amazement my previously stable fat level dropped to 6.8%, even although the exercise level seemed ridiculously easy. So there is confirmation of how to burn fat without building up too much of a sweat. However I’ll leave it up to you to decide whether 9 hours of moderate exercise over 8 days to lose 0.5kg of fat is actually worth the effort. For my part I can tell you that Fasting and cutting out sugar and simple carbs is definitely a whole lot easier! However I exercise principally for the huge cardiovascular benefits and any additional fat loss is just a bonus. If you want to take your exercise beyond the basic workout stage then I would recommend “Conditioning for outdoor exercise – Second Edition” by David Musnick MD (published by The Mountaineers Books) as a really useful book.