Fuel Sources During Exercise, how to burn fat, how to fuel for a marathon, and what is interval training!

On Friday I was hiking with a friend, who is also a holistic nutritionist (and a really darn good one, if you are looking for someone to guide you through your transition to healthier eating, I highly recommend her!).  You can see pictures of our hike here on my Instagram account:  https://instagram.com/p/4aow8FEWDm/?taken-by=optimal_health_coaching 11335176_1700651906831329_851638458_n Naturally, the topic of fitness, health, and nutrition came up, and specifically we ended up discussing sources of fuel during exercise, a.k.a, what does your body burn during your exercise sessions. It also reminded me of a question a few months back. A client asked me why her friend training for a half marathon wasn’t carb loading at her dinner the night before. This blog today will clear up some of these questions, and more, including:

  • Is there such thing as a “fat burning” zone in activity
  • Should I carb load before a race?
  • Do I need to eat carbs as an athlete?
  • What will you use as fuel in a marathon?
  • Should I just do low intensity cardio to be burn fat?
  • What should I eat during long days of hiking?

First let’s dive into what we use as fuel during exercise, then we will tackle more specific questions. There are 3 main options for what we can use as fuel in exercise.

  • Carbohydrates:  Also known as sugars and starches, carbs are stored in the muscle, liver, and in the blood as glucose/glycogen. For each gram of carbohydrate we store in our body we get 4 calories. Carbs are easier to mobilize as a source of fuel, which is why they are preferred in more intense activity’s (you will read about that below).  Although they are easy to mobilize, they only store 4 calories per gram, so we don’t get as much energy from a gram of carbs as we do a gram of fat.
  • Fats:  All types of fat (saturdated and unsaturated) are stored as intramuscular triglycerides (fat in the muscle), subcutaneous (fat under the skin) as well as free fatty acids in the blood (as it’s broken down and released for use). Each gram of fat contains 9 calories, and in total we have about 70 000 calories stored in the body (but this obviously depends on the amount of fat you as an individual have). There are more steps involved in burning fat on the body, and oxygen is required (this is outlined below), because it is harder to mobilize it is not the desired source of fuel for intense activity.
  • Protein– Rarely used as fuel, generally only 2 % contribution, we won’t focus on this. Unless you are an elite cardio athlete you don’t have to worry about burning your protein as fuel.
  • – Additionally we use phosphocreatine, and blood lactate as other sources.

To understand what we use as fuel, we first need to understand the energy systems that our body uses during exercise. When exercise is short (less than 10 seconds) and intense (80-100 % of your maximum heart rate), we use our ATP- PC pathway. We won’t dig into this pathway too deeply, but it is important to know that the fuel for extremely short and extremely intense activity is phosphocreatine (PC). *  Maximum hear rate= 220- your age= ________ When our exercise is roughly less than 2 minutes, and highly intense (80-100 % of max heart rate), then we use our anaerobic pathways (lactic acid system). Anaerobic means “No oxygen”, exercising so intensely that there is not sufficient oxygen supply to our muscles, and the muscle must use another source of fuel for energy. This system relies also on phosphocreatine, and also glycolysis. Glycolysis is the splitting of sugars, sugars (also known as carbs), so glycolysis is using carbs stored in our muscle and liver for fuel. When we exercise anaerobically without the use of oxygen, eventually there is a build up of by products in our muscles (also known as lactic acid) that we are unable to fully clear them without sufficient oxygen. As a result we either need to stop or slow down our intensity level (which would shift us into aerobic respiration, discussed next). Also, we only store 2000 calories MAXIMUM of glycogen (carbs) in our body at one time. So let’s say you are completely carb loaded (you have 2000 calories stored in your body), and you do anaerobic training (interval training), if the burn in your muscles (increase in lactic acid and other byproducts) doesn’t cause you to slow down, eventually you can use up all the glycogen in your body, and will have to start tapping into another energy source for fuel, discussed next. The energy system that takes over for long term, less intense activity (less than 75 % of your maximum heart rate) is the aerobic energy system. Aerobic means we use oxygen to create the fuel breakdown we need for exercise. This process is called oxidative phosphorylation, and we can use fat or carbohydrate to fuel it (and protein..). We have an almost unlimited amount of fat stores on the body that we can use to fuel aerobic activity, if the intensity is low enough to allow us to use oxygen to do so.

fuel graph

This graph shows how the energy systems change as the time involved in an activity increases.

“Fat burning zone” You know on the treadmill how you can see the heart rates, and there is a chart telling you if you stay in a certain heart rate you will be in the fat burning zone? Well yes, there is some truth to this. If you exercise between 40 and 75 % of your maximum heart rate, you will be providing your body with enough oxygen to burn either fat or carbs as fuel. Remember before, I said fat is harder to mobilize, meaning there are more steps in the process to use it as fuel. When we work out less intensely, and can use oxygen, this is prime time for use of fat stores. Does this mean you should always stay in the fat burning zone? No, not all the time, and I will explain why in the next paragraph on interval training. The graph below demonstrates this concept further. As the intensity increases during activity (higher heart rate) the amount of fat utilized decreases, and carbs increase, as you tap into a different energy system. intensity Your body switches into ‘carb burning’ when the intensity increases. As I mentioned before, there are only about 2000 calories of stored glycogen (carbs) in the body for fuel. When it is depleted the body is forced to switch into burning more fat (which is abundant). fuel sources “Interval Training” On top of doing some lower intensity cardio, if you are looking for increased fitness, or to improve your physical appearance (less body fat) you will also want to do some interval training. Interval training means working super hard for 20-45 seconds, followed by a 10-30 second rest, or easy activity. The purpose of interval training is to work so hard during the ‘hard’ interval that you tap into the anaerobic energy system. You should be going ALL OUT on the hard intervals, and really easy on the rest intervals. This is the most metabolically demanding cardio you can do. What I mean by that is if you interval training for 10-30 minutes you have put yourself into a state of oxygen deprivation. Because you worked so hard on the ‘hard’ intervals (anaerobic training, lack of oxygen), on each rest interval your body was trying tremendously hard to catch up on the lack of oxygen you created. After you are done your session your body is in an oxygen dept (scientifically known as EPOC- excess post exercise oxygen consumption). So for hours after you finish interval training your body’s metabolism works incredibly hard to bring it back to homeostasis, or a level where there is enough oxygen for all the functions of the cell. interval training So, yes it’s great to work in the fat burning zone (going for walks, light bike rides, yoga classes), but there is just as much merit (or more) to doing interval training to tap into the metabolic demands of your body, if you are looking to lean out and burn body fat. A strong endurance athlete needs to have experience working in this anaerobic range as well. Should I only do low intensity cardio to burn fat? No. There are 3500 calories in one pound of body fat. To burn one pound you will have to jog for roughly 3-5 hours (assuming around 800-1000 calories burned per hour , on top of eating a pristine diet consisting of no extra calories. To burn 1 pound using interval training: Interval training perhaps only burns a few hundred calories while you do it (for 10-30 minutes), but it spikes your metabolism so greatly that you burn hundreds of calories for the few hours after. So you could exercise for 20 minutes and burn almost as much as with one hour of jogging. On top of that, interval training (especially when combined with strength training) is going to add muscle to your body. Muscle is a calorie burning organ! The more muscle, the more calorie burn, which means as someone who does interval training you will burn hundreds of more calories per day. Jogging makes you hungrier, but doesn’t necessarily boost your metabolism or add muscle to your body. Although we haven’t discussed it too much in this article, weight training (another form of anaerobic exercise when done correctly) is CRUCIAL for sculpting a lean, strong body. The effect of intensity and heart rate on fuel sources So as you can see, the higher intensity the activity, the more carbohydrates are used up, and the longer duration and lower intensity the more fat is used. Both are important when trying to deplete fat and ‘lean out’.   The answer to “should I carb load before a race” in answered in a description of heart rate and intensities. Should I carb load before a race? As we described, there are only 2000 calories total in the body at one time for use. 2000 calories divided by 4 calories per gram= 500 grams total storage in the body of carbs. If you eat a regular diet of healthy whole foods 3 meals a day including complex and simple carbs, you most likely have full carbohydrate stores already. Usually, the week before a race you are tapering (meaning running much less and much less intensely). This allows the body to do a variety of things, including fill up it’s carbohydrate stores. Therefore with a proper daily diet for the weeks before a race and during training, ensure you include complex and simple carbs in your dinner the night before, and some for breakfast and you have most likely met your 500 g total. You most definitely would not want to be carb depleted before a race, but there is no need to eat thousands of calories of pasta if your carb stores are already full (if you already have 2000 carb calories stored in the body any extra fuel is just stored as fat). Let’s put this into perspective. If someone my weight, age, and height runs a 5 km race in 30 minutes I am only burning about 300-500 calories. Even if all of these calories are from carbs, as long as I ensure I refuel them after, I will be fine. Now let’s look at a half marathon. If I run the 21 km in 2 hours, I will burn maximum maybe 2000 calories. So this means, by then end of my race I might be coming close to using all my carbs. BUT HERE’s THE KICKER…it all depends on how HARD I am working. If I am highly trained cardiovascularly and I run the 2 hour half marathon and this is a LOW intensity race for me, meaning my heart rate doesn’t go over 70-75 % of my maximum heart rate, then technically I could also be using some fat as a source of fuel in my workout. If I am untrained and I run the same race, most likely my heart will be flying through the roof, the intenisity will feel high, and as we saw before I will be using more carbohydrates as fuel. So here’s what I suggest. Train well, and train your body using both the ‘fat burning zone’ , and the interval training protocol. This way, as you go to compete you may be fine with just using up what you have stored in your body, and what you have taught your body to burn. BUT- I do recommend bringing something with you on your race in event that you are not having the best race, and can feel that your carb stores are depleting (you may start feeling light headed), and you need something quick. The best thing to bring with you is something small, portable, and easy to digest. Liquids or gels are most often to use, because they are easier to digest. When you run there is less blood being shunted to the digestive system (it’s all going to the muscles) so you need something that can permeate the digestive system without relying on breaking it down. Most running research done shows that a mix of glucose and fructose combined (two types of sugars) is better than just ingesting one. This is why commercial gels, drinks, and sports drinks are so widely used, they contain both these sugars. You need to find one that works for you, either homemade or store bought! As we think about ‘burning out’ on a half marathon near the end, or running a full marathon it’s important to also factor in 3 things. 1) It takes time to digest these sugars, and this depends on the individiual. 2) We can only digest 1 gram of carb per minute. 3) You need to replace your carbs before they are less than 30-50 % used up. Here’s what I suggest. 45 minutes into a race, consume your gel/liquid/sugar drink. This way if it takes 15-20 minutes to enter the blood stream, your body will be not crash. If you are running a half marathon you might want 1-2 times of refueling. For a full, you can take another gel/liquid/drink every 30-45 minutes (as they contain about 20 grams of sugar, so it will take you roughly half an hour to burn it off). Now remember, if you are well trained and can keep your heart rate lower (therefore working at a lower intensity while running at the same speed), you are going to have the option to mobilize some fat. This is why it’s important to train in this fat burning zone and get your body used to using fat as fuel. A.k.a when you go for runs, don’t go ‘all out’ and kill yourself, train yourself to run faster at less intensity and use fat as fuel! Should you carb load for regular exercise? No.   Unless you are competing or looking for a performance goal in the gym (or if you are training for more than an hour a day intensely), just eat a balanced diet including a variety of carbs everyday. Have a simple sugar (fruit) 1-2 hours before exercise, and refuel with protein and carbs immediately after, followed by a well balanced meal 1-2 hours later and you are fine.  Remember, any more than 2000 calories of carbs in the body is simply stored as fat! If you are hiking for a full day, or doing some kind of full day active event I suggest bringing the following: fruist and healthy simple sugars, electrolytes, salt, and whole balanced meals. If you keep the intensity moderate there should be no need to rely on things like gels etc. Takeaways:

  • Carbs are used in higher intensity exercise.
  • Fats are used in lower intensity exercise.
  • Train in both the fat burning zone, the interval training protocol, and weight training for optimal body composition.
  • For marathons and other performance races, train your body effectively so you can mobilize fat during activity and don’t need to load up on endurance gels to stay alive (but bring them to be safe, and to maintain a proper blood sugar).

Well I hope this helped some of you. If you have specific questions please post them below, and I am happy to help!

ps stay tuned for my next blogs on:

– can you be fat adapted?  And use more fat for exercise

–  what happens when we wear shoes our whole life and what can we do about it.

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