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Exercise on Sugar or Fat Burning Fuel

Monday, July 30, 2012

Those of us who grew up in the 80s and 90s are familiar with the conventional wisdom advice that we need carbohydrates for energy. Without carbohydrates, we'll run out of energy during exercise and performance will suffer.

I am not a high performance athlete. I have no intention of running a marathon. I know many Sparklers who aspire to this challenge, and I applaud them. That feat of aerobic exercise is a major accomplishment for many who started from couch potato-ville.

I don't enjoy running. I never have. In high school/college, I played soccer, which was mostly sprinting and agility. As I got older and fatter, I disliked running even more as it put stress on my knees, shins and feet.

Even though I don't run, I am pretty active. I enjoy hiking, cycling, kayaking, long walks on beaches, snorkeling and diving.

This has not changed with my lower carbohydrate diet.

A brief simplified explanation on how our bodies turns food into energy.

GLUCOSE - Carbs and sucrose are broken down to get at the glucose molecules. Starches (potatoes and rice) require no breakdown and are used immediately as glucose. Insulin is released in order to provide cells with energy. Excess glucose is stored in fat cells. High blood sugar is toxic, so glucose is given priority for 'use it or store it' to maintain a neutral blood sugar range.

FRUCTOSE - Unlike glucose, it is broken down and metabolized primarily in the liver. Fructose decreases insulin and leptin, which are responsible for decreasing appetite, and increases ghrelin, which increases appetite. You get hungrier eating high levels of fructose. Unused fructose is stored in the liver and other organs. High fructose consumption, either from syrups or excessive fruit consumption, can lead to non alcoholic fatty liver disease and visceral fat. High fructose consumption can also lead to a type of IBS called fructose malabsorption through promoting overgrowth of bad gut bacteria and fungus.

GLYCOGEN - The quantity of calorie rich carbs didn't come about until the advent of agriculture. Our species evolved to use or store carbs when available. Glycogen is stored muscle energy. Whenever we need a quick burst of energy, like when sprinting or picking up something very heavy, glycogen is fired to provide muscle energy quickly. The more muscle endurance you train, the better your glycogen store. Glycogen replenishment is the highest priority destination for carbs. Depleted glycogen will always be replenished first, before it is stored as fat, thus is it highly desirable. However, if you don't exercise, you never deplete glycogen, and it is more likely to be stored as fat. This is one reason why people who are very active have better carb tolerance. More glycogen storage means a beneficial place for those carbs to go.

There is a downside, though. Total glycogen depletion will cause a massive energy drop. Runners know this as "the wall". Burn out all your glycogen, and your muscles can't move.

TRIGLYCERIDES - They have a bad rap due to the low-fat theory claiming fat increases risk of cardiovascular disease. The higher your triglycerides, the increased risk of metabolic syndrome and heart disease. This is all true. Fat consumption does create triglycerides. But...there is a missing piece of information. It is a fuel source. And high T's are a symptom, not a cause. With healthy metabolic function, our bodies turn dietary fat into triglycerides to burn as fuel. The problem is with elevated insulin levels in an unhealthy metabolic body, we cannot use triglycerides. Dominate insulin hormone over glucagon hormone prevents fat burn. If you have no insulin resistance or eat low carbs, then dietary fat turned into triglycerides in the liver will be burned as fuel, not floating around at excessive levels in the blood stream.

KETONES - When there is excess glucose and glycogen stores are full, insulin stuffs it into fat cells. In a healthy functioning metabolism, when there is a lack of glucose in the bloodstream, this stored energy is retrieved from fat cells and broken into ketones. It is a byproduct of burning stored fat. I'll spare the details and just say that it is broken into 3 different pieces - 2 of which are used for energy and the 3rd is discarded. Look up Ketones on wikipedia if you're interested in the whys and hows.

Contrary to misinformation, burning ketones is not dangerous. Not unless you are a Type I diabetic who lacks insulin production to prevent excess ketone buildup. The rest of us discard unused ketones through urine. Burning ketones is a very natural state. Every single one of us goes into ketosis when we sleep and wake up. If we didn't, we would never be able to sleep because we'd have to constantly eat food. Ketosis is how people who are fasting or starving survive lack of food. It is a crucial survival mechanism.

PROTEIN - Not typically used as a fuel source. Our bodies prefer to use it as building blocks. Since protein consumption was precious back in our early paleo-neolithic days, protein would not be squandered as a fuel source. Our bodies prefer to burn either carbs or fat for fuel. In extreme carb deprivation (or protein over consumption), our livers are capable of turning up to 60% of protein into glucose through a process called gluconeogenesis. This process is how Inuit Eskimos and Mongolians are able to survive on an almost exclusive protein/fat diet. Our livers can create all the glucose we need to survive. There is no such thing as an essential carbohydrate. Eskimos and Mongolians would never have survived their environments otherwise.

In terms of exercise performance, you can either be a sugar burner, using primarily glucose,fructose and glycogen for fuel, or a fat burner, using triglycerides and ketones for fuel, or a combo of both.

I'm not going to discuss how this is accomplished for now as I want to focus on exercise performance as a mostly 'fat burner'.

As someone attempting to lose bodyfat, obviously burning more stored fat through ketosis is desirable.

Once I shifted from sugar burn, how did my exercise performance fare on fat burn?

There was an adjustment period. My exercise performance initially declined. It seemed a lot harder to move my muscles and keep them going. If I stopped there, then I probably would have said low-carb sucks for exercise.

However, after I got over the hump, it was like replacing a gasoline engine with diesel. Little sluggish on the accelerator, but excellent fuel economy over the long haul.

Again, I am not a high performance marathon runner. Most of the exercises I enjoy doing don't push my heart rate beyond 70% max. I rarely dip into the "sprint" zone of 90%+ where mostly glycogen is burned. Because I am using a high percentage of bodyfat (of which I have 28% of my mass stored), I have what seems to be a near inexhaustible source of fuel for long distance activities. I have gone for hours hiking, cycling, snorkeling and diving without feeling hungry or needing to eat.

Read those last two again. Snorkeling and diving. If I ran out of energy doing these, it would be pretty dangerous, right?

No problems with a fat-energy storage tank in the caboose. :P

Here are the long distance activities I enjoy on a regular basis. I do all of these without the normal advice of carb-loading before hand. I eat a breakfast of about 25%carbs/50% fat/25% protein.



Climbing Mayan Ruins:


These are my typical weekend/vacation activities.

On weekdays, I workout at my apartment gym for 30-45 minutes 4x per week.

I work out fasted in the morning - no breakfast. I don't experience an energy crash during or after my workout - remember, I'm fat-burn adapted. I'm burning my fat and glycogen reserves.

And no, I'm not burning lean mass. When properly fat adapted, that would only happen if glycogen is exhausted.

One hour after my workout, I eat breakfast with protein and fat (usually eggs and yogurt or cheese), and a small piece of fruit to replenish my partially depleted glycogen reserves.

I perform cardio, weight training, and HIIT all while fasted. Something the exercise experts say not to do because I'll experience an energy crash. If I was a sugar burner, yes, this would happen. High carb sugar burning exercisers should not workout fasted. If you aren't efficiently burning fat, then you won't make it through a workout this way. Your body can't burn triglycerides and ketones as efficiently as glucose without being conditioned.

I have heard from people who run regularly or participate in marathons/triathlons that low carb diets gives them poor performance. I don't doubt their experience. If they say that's true, then I believe them. Again, I don't perform at that level. Most of my activities run in the range of about 50-80% heart rate max, and not often more than 2-3 hours. If you engage in more intense activities, I don't have any good advice for you. I've never run a marathon, so I don't know how to eat for one.

However, to say that people can't exercise on low carb due to low energy is not true. It depends on whether you burn primarily sugar or fat for fuel.

I wrote this blog with the intention of being an anecdotal account. Your mileage may vary. If you want a very good detailed, scientific account of how high carb to low carb exercise performance fared, read this blog post from EatingAcademy.com:


(Thanks to SalonKitty for the suggestion to write this blog!)
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Member Comments About This Blog Post
    Awesome blog -- I 'liked' it but I'm sure it won't be picked by sparkpeople... I get same results as you. I can go do my aqua aerobics without eating breakfast and have no issues. Love low carb and the eatingacademy website!!!
    2124 days ago
    Love your blogs. Always informative, and your tracking of so many variables is an inspiration.
    2125 days ago
    How low are your carbs during the day? I may need to start counting carbs again and monitoring that intake.
    2125 days ago
  • ACTIVE_AT_60
    IN-credibly well written blog!!! Thank you for posting it. Five stars!!!!

    We can live without carbs but not without lipids and protein.
    2125 days ago
    Wow! You've really nailed it in your write up! Such detail! Going to hang onto this for the low-carb detractors!

    2125 days ago
    Very interesting blogs you have. emoticon
    2125 days ago
    Loved this! Thank you for sharing so much detail!
    2125 days ago
    Egailtaire, I agree.

    Definitely now that I am not eating lots of carbs, I don't 'bonk' like I used to. I guess this is fat adaptation! Now I just get more and more tired and slower rather than actually hitting a wall.

    In my younger and carbier days I would routinely go for 6-8 hour bike rides that absolutely required sugary drinks and energy bars or I would find myself sitting under a tree after 4 hours feeling nauseous and barely knowing who I was. That does not happen now.
    2125 days ago
    + 1 to Bearclaw and Houndlover - 3 - 4 times over the bike riding season I do 3 and 4 hour off-road rides, which include some flat, but mostly lots of ups and downs, which means my heart-rate can go back and forth from 55% of max to 85% multiple times in an hour.

    In theory I should need to be burning both fat and glycogen. Since starting a primal eating regimen last winter and becoming what I believe to be "fat-adapted" I have done a 3 hour ride while fasted and no carb-loading.

    I did not experience an energy crash, either during or after the ride - although I was hungry when I was done. In comparison, in previous years, when I did anything approaching two hours, I would have to eat and/or experience an energy crash. A few years ago I had to stop a ride just before the 3 hour mark because I was exhausted and didn't know if I would even get home - I hadn't carb loaded before the ride and didn't eat enough on the ride.

    I don't think about food related to riding anymore - I just go ride and eat when I get back - when I stop to think about it I find it amazing the difference in my body's response - wouldn't believe it if I wasn't experiencing it - which may be why it is so hard for others to believe it is possible.

    I plan on doing a 4 hour trail ride in the next couple of weeks - will let you know how that goes.

    Stay Strong
    2125 days ago
    Nice blog, I have made the same experience as you and Bearclaw. I can run for about 3-4 hours without having to eat before my glycogen runs out. The trick is to keep my heartrate at a modest 135 bpm (about 75% of max.)
    I use Phil Maffetone's book The Big Book of Endurance Training and Racing as a guide and highly recommend it.
    2125 days ago
    Great blog!

    I think the fuel source really comes down to duration of exercise for me. Most of us (including low carbers like me) burn calories faster while exercising than we can generate by burning fat. But.....we don't 'run out' of quickly available fuel until we have been active for 3+ hours. So, if I am going for a 4-hour bike ride or mountain hike, I will take something carby with me like one of those protein bars that actually have more sugar in them than protein or I stop and have some chocolate milk or ice cream. This is extremely rare lately. But, for anything less than 3 hours I just eat my normal low-carb stuff.
    2125 days ago
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