 There has been a lot of talk over the last few years about the fat burning potential of doing cardio in a fasted state. There are a few questions that should come to mind when considering this as an option:
*All nutritional and fitness info must be discussed with your doctor before any changes to diet or exercise regime are implemented. What is fasted vs fed? As we go throughout the day – and night – our body goes through a cycle of fed vs fasted states, as illustrated by this chart: (read more: Insulin: Friend of Foe?)  As you can see, it is only once our blood sugar levels return to normal – thanks to insulin – that we are considered in a fasted state. How long this takes depends largely upon what and how much we ate, but as you can see from this chart it would be about 4-6 hours since eating to be considered in a fasting state:  This chart is based upon three meals a day, any snacks in between meals would obviously delay the fasted state. Most often someone desiring to do fasted cardio would do so first thing in the morning, thus ensuring that they are in a fasted state. Why is fasted cardio beneficial for fat loss? When we are in a fed state, insulin is still active in our body; insulin is responsible for sweeping nutrients (including glucose) into our cells, but while it is doing this it impairs the breakdown of fatty acids. This means that the higher the insulin level in your blood while you work out, the less your body is going to be able to release fat from storage to potentially use it as energy. Logic therefore dictates that the lower your insulin level while you train, the more efficient your body will be at tapping into fat stores and releasing them into your blood stream to be used for energy; this is done through a process called lipolysis ”. In fact, research has shown this to be true, in both trained and untrained individuals. What kind of fasted cardio is best for fat loss? Traditionally Steady State (SS) cardio has been used more often for fasted cardio fat loss. Steady State (SS) cardio is the traditional approach to cardio; get your heart rate up to a certain level and maintain it for 20-30 minutes, or more. In order to get cardiovascular benefits your HR should be between 60-85% of your maximum heart rate (MHR), depending on your fitness level, however, for maximum fat burning potential you should keep your HR at around 65-70% of your MHR, around 120-140 bpm for most people. Keeping our HR lower we are sure to stay in the aerobic zone (vs anaerobic), meaning that we are working out at a low enough intensity to use oxygen for energy and not our muscle glycogen. At this lower intensity we also actually use a higher percentage of calories from fat; 50% vs 35% during higher intensities. Keep in mind however that even though you burn a higher percentage of calories from fat at SS, you actually burn more fat calories overall doing HIIT (for the same time period). (Read more Cardio: HIIT vs SS vs Zone 2) The other factor to consider is that while your body is releasing fatty acids into your blood stream during lipolysis, you still need to flush them from your blood stream in a process called fat oxidation. This is the process where the fatty acids are essentially “burned off”; if they are not completely burned off, whatever remains in your blood will return to storage in your fat cells. SS cardio doesn’t burn energy (calories) at a very high rate, so unless you are doing 2hrs of cardio, SS might leave some unused fatty acids left un-burned, and therefore re-stored at the end of the workout. Fasted HIIT for fat loss So what about doing HIIT cardio in a fasted state? HIIT stands for High Intensity Interval Training; it is basically any activity you can perform at a maximum level for a short period of time, followed by a short rest period. The idea is to get your HR up to between 80-90% of your HRM, then rest long enough for your HR to go down to 50-60% of your HRM. A great example is sprinting; sprint at 90% of your MHR then rest, enabling your HR to go down to 50-60% of your MHR. Done properly, HIIT can help with fat loss for many reasons (Read more Cardio: HIIT vs SS vs Zone 2), including the fact that it burns more calories overall despite its shorter duration. So we’ve established that we need to prompt both the process of lipolysis as well as fat oxidation; SS is better for lipolysis due to its low intensity, so what about fat oxidation? Some say that fasted SS is not a good idea as you are only getting lipolysis benefits, and others say that fasted HIIT is not the best game plan as you are breaking down fat faster than you can burn it through fat oxidation. If this happens then your body will turn to other sources of energy, ie glycogen. Studies have however shown that the higher the intensity of workout, the higher the rate of fat oxidation, thus better enabling us to burn the fat as we break it down. With the higher calorie-burning potential of HIIT, the after-burn benefits as well as higher fat oxidation levels, fasted HIIT cardio would be your best bet for fat loss potential. Fasted weight lifting Weight training is more similar to HIIT than SS as they are both anaerobic activities, relying more upon glycogen than oxygen for energy. This means that it is not generally great at mobilizing your fat stores until you have depleted your glycogen stores. Doing your weight training in a fasted state can tip the scales in the direction of more lipolysis, giving you potentially more fat burning potential. However, it can also tip the scales in the direction of catabolising your muscles. If you are working out at an intense level, you run the risk of mobilizing fat faster than your body can use it as it must convert it to glycose to be able to use it. A quicker way for your body to draw upon extra energy is to use glucose, either from within our blood, or in storage in our muscles as glycogen. Without pre-workout nutrients there is no glucose in your blood and your body will have to go directly to its glycogen stores to find extra energy. This loss of glycogen in the muscle can lead to the catabolism (breaking down) of your muscle to be used for energy. Consider cortisol Another factor to consider is cortisol, as both high intensity workouts and long lower intensity workouts will raise our cortisol levels. Cortisol is a hormone released from the adrenal gland when it perceives stress (as well as performing other functions); when your energy levels drop deplete during training your body perceives this as stress, which results in higher cortisol production. Cortisol’s job is to find fuel to help fight the stress, so it will break down protein in the muscle in order to release glucose into the bloodstream to be used as energy. Cortisol will also increase insulin production in order to protect our fat stores as this is valuable energy we may need to fight the stress. (read more Insulin: Friend or Foe?) Cortisol is highest in the morning due to the cortisol awakening response; eating helps to lower your cortisol level so you start your workout at a neutral level. Without food you begin your training with your cortisol at an elevated level which puts you at risk for catabolising muscle mass. Energy levels fasted vs fed If you have ever done a fasted workout, be it cardio or weight training then you have probably noticed a decrease in energy vs a fed workout. Our bodies are very adaptive, and over time we can adjust to a fasted workout, however this obviously depends upon the individual. If you have a decrease in energy during your workout, this will affect your results; less weight lifted, less sets, shorter sprints, slower run, etc. Less effective workout, less calories burned. As the number one factor affecting weight loss is a calorie surplus, your focus should be on creating that surplus. How much difference can fasted workouts make? There is a cornucopia of research devoted to this study, and they all differ in results, partly due to the fact that they all use different parameters; trained, untrained, men, women, SS, HIIT, calorie deficit, calorie stable……the list goes on. There is one study from 2014 however that uses parameters that I would consider the average person trying to lose weight:
This is a recap of some of their results:
The study is rather limited (20 subject, 4 weeks) but it gives us a pretty good idea of what kind of results a typical fat-loss-seeking individual can expect. The study is a bit long but well worth the read: Body composition changes associated with fasted versus non-fasted aerobic exercise There are studies that have shown that you burn more calories doing fasted cardio, ranging from about 5-17%. So lets look at how much difference that makes:
With these numbers in mind, is it worth risking muscle loss to lose the extra 2lb a year? Better alternative to fasted While the traditional approach to pre-workout nutrition is to consume both protein and carbs, a better approach for fat burning may be to consume protein only. In one study Timing protein intake increases energy expenditure 24 h after resistance training they found that the subjects who consumed protein only vs protein and carbs had increased resting energy expenditure (REE) 24hrs after training. This means that for at least 24hrs after training the protein only group was still burning more calories that the protein + carb group. The potential reasons for this are:
Ideally, consume 10g BCAAs (the three amino acids most directly linked to protein synthesis) or 30g whey protein (providing similar amounts of BCAAs) immediately prior to training (read more about BCAAs). Key takeaways:
*All nutritional and fitness info must be discussed with your doctor before any changes to diet or exercise regime are implemented. Change nothing and nothing will change; without change, there can be no progress! Related articles: Our Metabolism: Can we change it? Insulin: Friend or Foe? Nutrition 101: Master the basics Pre & Post Workout Nutrition: Maximize your Training Gains with the Correct Nutrition  Comments are closed.
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