I want to give you guys some credit and believe people have some semblance of intelligence so I will always give you the facts, although the info is sometimes hard to take in at first read; it will always be truthful, factual and more importantly applicable to either fat loss, muscle building or dietary excellence.
In this article we are going to take on the issue of fat loss and what exercise intensity optimizes fat breakdown. We have seen this covered in the mag's from time to time but it's usually for the non compus mentus and although an easy read you come away as un-enlightened as when you first started the article...let me prove it to you....
- Low intensity exercise burns more fat than high intensity exercise!
- The calories I burn during exercise are the most important part of fat loss!
- If I work above 70% max effort I won't burn fat!
- Running or cycling burn just the same amount of fat!
Well, if you are agreeing with all or any of the above then you better read this article if you want to change those body-fat levels and get a real understanding of the "science-behind-fat loss."
Sources Of Fuel
The sources of fuel in the human body comprise of three main sources as we all know include: carbohydrates, protein and fats. But it is the latter which is the core of our focus this time. Fat, adipose (a cell filled with triglycerides) or lipid as it is more commonly referenced in the literature; is composed of many different storage forms; as glycogen is to carbs, so lipids are to intramuscular triglycerides. Lipids are distributed into a variety of organs and tissues but for the purposes of simplicity and clarity those most relevant to exercise and body composition can be split into two parts; intramuscular (within the muscle cell) and extramuscular (blood, adipose tissue and the liver).
Crossover Concept & VO2
An understanding of what is known as the "crossover concept" in physiological circles is vital if you are to grasp the basics behind energy usage in relation to exercise and lipolysis (fat breakdown) [1]. It's a fairly simple concept to grasp in that as exercise intensity increases, the proportion of carb to lipid utilized as an energy source goes up (see fig.1) and vice versa. So when exercise intensity decreases we liberate a greater proportion of fats as our energy source.....but the story does not end there!
Simplistic Explanations Lead To Confusion
When trying to grasp how the whole issue of how much lipid or carbs is "burned" during exercise, and practices that have been used to derive understanding of the theory and testing, these figures would be useful.
Now percentage wise e.g. 40% carbs used and 60% lipid is easy to calculate. The formula is based on something called the respiratory exchange ratio and is the CO2 (carbon dioxide) exhaled / O2 (oxygen) uptake [2]. This is analyzed as you are sitting at rest or exercising and basically the air you breathe in and exhale out is analyzed by a machine (indirect Calorimetry) we won't go into it too much but once you have the values it's a simple division of two numbers.
However, all this really tells us is a percentage value; now for me, percentage means nothing. For example if you have two groups, and one group burns off 0.01g of fat and the other burns 100% more it's like WOW! Not really, as this extra 100% only equates to 1g. Is this amount a value that would make a real difference to your goal of bodyweight/composition change? "No way." Yet percentage increase is used all the time in the nutrition industry, i.e. product "x" increases fat loss 200% more than placebo; it's a nice play on figures so be aware!!! What we really need to know is the total grams burned at "X" intensity, because it's the total caloric loss that will make the difference when it comes to body composition change.
So let's compare some figures using real data... but just before I do, I just want to explain what the term VO2 means. It is basically a measure of exercise intensity, and again is something measured using indirect calorimetry. It is the volume of air expired, usually quoted per minute and in milliliters(mLs) relative to your bodyweight, hence: VO2max = 60mL/kg (this would mean the maximum amount of oxygen you can take up and use is 60 mLs per minute per kg of bodyweight). In trying to give you a mental picture for what this value would mean in the gym I guess the best example would be running on a treadmill. As VO2 relates to heart rate, and is usually 10% lower than heart rate; try to imagine yourself running at 70% of your max heart rate. At this level you may be working at about 60% of your VO2max.
You can calculate your VO2max by using our calculator page.
In figure 2 we see that at 30% VO2max you are burning about 70% carbs or 45g/min and about 30% fat or 9g/min fat. At 70% VO2max your carbohydrate use increases to about 80% (135g/min) and the contribution to energy supply from fat drops by about 10%. However, the total amount fat burned per minute increases to 12.6g/min about a 30% increase. So what about total calories? Well in figure 3 we can see over a 45 min exercise period the total caloric expenditure from fat and carbs combined is a massive 60% during higher intensity (70% VO2max) vs. low intensity (30% VO2max) exercise.
Therefore the suggested maximal fat burning zone is about 60-65% of VO2max or 70-75% of Max heart rate.
This data is from a real subject and I have included 2 other figures (Fig.4 & 5) so that you can see where in this subject fat oxidation reaches it maximum and carbohydrate oxidation is still on the increase. In essence there is a plateau in lipid oxidation at around the 70-75% VO2max point.
Note -- The submax test was run over 6 stages, although data exists to show fat oxidation does not reach its peak until 20 minutes once exercise has started. Therefore it could be argued that each stage of exercise (30% VO2max and 70% VO2max) should last 20 minutes for a fare comparison. The author believes the difference between the test protocol used to calculate the graphs in this article (3 minutes per stage) and the 20 min protocols would give similar trends in fat and carbohydrate oxidation based on previous studies.
Post-Exercise Burn: A BIGGER Part Of The Fat Loss Puzzle.
The above sort of slams the low intensity hypothesis already but I just want to place a few more nails in the coffin. Post-exercise oxygen consumption is one way scientists assess if we have had a shift in resting metabolic rate because of the exercise we have undertaken. When we need to replace lost fuels, build new muscle or carry out any process that requires energy (usually in the form of the high energy phosphate--ATP), the little power houses of our muscles known as mitochondria (where ATP is produced) need oxygen to carry out many of their functions. As such, we can measure any change in this oxygen use as mentioned earlier by measuring oxygen consumption from our mouths "via indirect calorimetry". So what does the research show?
There has been, again, a lot of debate about which exercise burns more calories i.e. Low vs. High intensity, but also about what the total impact of the exercise intensity has on caloric expenditure over both the exercise period and more importantly post exercise.
There is now a vast quantity of research showing that post-exercise elevation of oxygen consumption is supplied primarily by fats and is elevated to a greater degree following high intensity aerobic exercise.
Let me explain why! Briefly as you recall earlier during moderately intense exercise we use a large quantity of muscle glycogen and even though we use quite a bit of lipids in the form of intramuscular triglycerides our fat stores are massively larger than our glycogen stores.
Thus, muscle recognizes that glycogen stores are low so one of its main actions post exercise is to replenish these stores, this process requires energy [3]. However, if we have only completed low intensity work which has little impact on muscle glycogen stores, (i.e. can be supplied primarily from blood glucose) then post exercise metabolic demand is smaller leading to a lower caloric deficit and less weight loss [4]. Another bonus of glycogen depleting exercise is that we can use peripheral fat stores such as subcutaneous fat to replace depleted intramuscular triglyceride stores [5]; another advantage for positive changes in body composition.
A final note on the high intensity work as the way to go for fat loss, is training on an empty stomach, i.e. first thing in the morning. This further forces your body to use fat stores more readily as blood glucose and muscle glycogen will be low after a night of sleep and no food. Many of you will know this already but what you might not know is by training in a fasted state up-regulates your ability to use fats as fuel, so carrying out any exercise in this physiological state will automatically increase the proportion of fat utilized as your driving energy source [6]. This occurs through increasing oxidative enzymes like citrate synthase and total mitochondrial number as in endurance athletes. I would also suggest no carbs for at least 30-60 mins post exercise to maximize these effects.
There is also the issue of growth hormone release during & post exercise and it's relationship to lipolysis. However, it is outside the realms of this issue so I look forward to covering it in the future. Until then, an excellent review can be found by Dr Djurhuus et al (2004) from University Hospital of Aarhus, Department of Endocrinology, Denmark.
What Exercise Is Best?
A question that has not really been addressed; but you know most, all of us have seen it, and probably have all done it.... you know what I'm talking about walking into the gym and thinking what piece of cardio equipment today? Well maybe its time we took a look at exactly what equipment we are using and if it's going to make a difference regarding our fat loss. Due to the word limit, I am going to pick two fav's to make my case "running vs. cycling".
The latest studies by Dr Achten from Birmingham University in the UK say it all really [7]. The aim of the study was to assess if fat oxidation rates are higher during running compared with cycling over a wide range of intensities. Gas exchange and heart rate (HR) recordings were performed throughout exercise.
The results were clear from the outset, as maximal fat oxidation rates were significantly higher during running compared with cycling. Value wise over a 45min period at the same intensity running would burn 30g (270cals) of fat whilst cycling would burn a mere 21.15g (84.6cals). For all exercise intensities assessed (50-80% VO2max) running had far higher fat burning ability than cycling, so why the big difference? Well my guess is that it's a matter of working a greater amount of muscle mass so you get greater lipolysis occurring in limbs other than the legs as in cycling. So my advice is to look for aerobic exercise equipment that elicits the greatest movement of the whole body such as versa climbers and indoor rowers.
In conclusion, in both exercise types in the Achten et al study showed 62-65% VO2max, which is around 72-75% heart rate max (roughly Hr max is 220-age + 10 beats), was the maximal fat burning zone. So set those heart rate monitors guys*.
The good news is that this maximal fat burning zone was similar for both exercise types, so no recalculating for each exercise type. On a personal note, reaching a high % heart rate maximum on the bike is more difficult when untrained than reaching it on a treadmill. The subjects in the study could do it but they were moderately trained and in trained cyclists we see much higher muscle recruitment patterns allowing them to work at higher intensity work loads.
*Note. I suggest finding a max protocol to run on the treadmill or bike to calculate max heart rate.
Reality Bites: Especially For The Resistance Athlete.
In summing up, I really hope this article helps you take advantage of the current literature on fat loss and exercise intensity. There may be a few factors that will influence fat burning and selection of a level to optimize fat loss such as: endurance athletes can work at higher intensities and still burn fat as an energy source. However I am guessing most of you reading this are not massive endurance athletes and have the goal of losing body fat whilst gaining lean muscle, therefore all the "TAKE HOME" recommendations I have given below should fit like a glove....
So until next time...
Take Home Points
Maximum fat burning rate occurs during moderate intensity at about 60 - 65% VO2max which corresponds to about 75% max heart rate for most people. Duration * intensity = maximal fat burning.
Total caloric expenditure for moderate intensity exercise again is size dependent (larger people expend more calories because they are moving larger masses and working larger muscles).
The longer the duration of exercise, the total caloric expenditure tends to be higher due to intramuscular glycogen depletion that occurs. I suggest exercise lasting between 20 – 50mins is optimal for fat loss. The longer the better.
Exercise type will make a difference to muscle mass and fat loss. Use exercise that utilises the most muscle mass i.e. legs and arms.
Post-exercise caloric expenditure will be higher following higher intensity exercise due in part to "depleted muscle glycogen, increased catecholamine and growth hormone release."
Train on an empty stomach, ideally 1st thing in the morning when blood glucose levels are low. If this is not a possibility, exercise following at least a 3-4hr fast.
Restrict post exercise carbohydrate intake to optimize fat use as the primary energy source.
REFERENCES
[1] Brooks,G.A., "Importance of the 'crossover' concept in exercise metabolism," Clin Exp Pharmacol Physiol, 24(11) (1997): 889 - 995.
[2] Waldau, T., Larsen, V.H., Parbst, H., Bonde, J., "Assessment of the respiratory exchange ratio in mechanically ventilated patients by a standard anaesthetic gas analyser," Acta Anaesthesiol Scand. 46(10) (2002): 1242 - 1250.
[3] Marliss, E.B., Simantirakis. E., Miles, P.D., Hunt, R., Gougeon, R., Purdon, C., Halter, J.B., Vranic, M., "Glucose turnover and its regulation during intense exercise and recovery in normal male subjects," Clin Invest Med. 15(5) (1992): 406 - 419.
[4] Laforgia, J., Withers, R.T., Shipp, N.J., Gore, C.J., "Comparison of energy expenditure elevations after submaximal and supramaximal running," J Appl Physiol. 82(2) (1997): 661 - 666.
[5] Burns, T.W., Boyer, P.A., Terry, B.E., Langley, P.E., Robison, G.A., "The effect of fasting on the adrenergic receptor activity of human adipocytes," J Lab Clin Med. 94(3) (1979): 387 - 394.
[6] Dohm G.L., Beeker, R.T., Israel,. RG., Tapscott, E.B., "Metabolic responses to exercise after fasting," J Appl Physiol. 61(4) (1986): 1363 - 1368.
[7] Achten, J., Venables, M.C., Jeukendrup, A.E., "Fat oxidation rates are higher during running compared with cycling over a wide range of intensities," Metabolism. 52(6) (2003): 747 - 752.
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