How do you know if you are an efficient fat burner or sugar burner?

It was about a year ago that I realized I was primarily a sugar burner.  Over the past year, I’ve talked a bit about learning how to be more metabolically efficient as an endurance athlete.

It is true that we need higher carbohydrates for energy and to restore glyogen post workout, but most of the women I work with are consuming more than the recommended amount.  I find that endurance runners in particular get higher amounts of sugars and carbohydrates than the average person. This leads to struggles with sugar cravings, fatigue, energy highs and lows or constant hunger.

Why teach your body to burn fat?

Ideally, you want to teach your body to be efficient at using fat for fuel in training for several reasons:

  • you will have an endless source of fuel (up to 80,000 fat calories stored at a given time as opposed to 1500 calories in carbohydrates)
  • you will be much less likely to ever hit the wall or bonk
  • you won’t feel weak or dizzy or run out of energy on a fasted run
  • your body composition will change (less fluffy, more definition)

We talk a lot about this in the runner’s reset programs I lead.  It’s very easy to assume you need to stock up on gels, gus and energy boosting products to be an efficient runner.  But how do you know if you are burning fat or sugar?

7 Signs that you burn sugar not fat

Here are 7 signs that you are mostly a sugar burner:

  • you can’t run more than 3 miles on an empty stomach first thing in the morning without feeling weak
  • you feel like you need food every 2 hours and struggle to go 3-4 hours between meals without feeling shaky or light headed
  • you have intense sugar cravings or struggle with low blood sugar
  • you hit mid-morning or mid-afternoon energy slumps
  • you rely on stimulants like coffee (beyond a cup or two in the morning) to get you through the day
  • during long runs, you feel like you NEED gu, gel, or other fuel or you start to fade
  • you struggle to lose weight

Do any of those symptoms sound like you? If so, you want to take a careful look at your diet and make some changes to balance carbohydrates and sugars with fat and protein.

For more specifics, you can do a metabolic efficiency test to learn at what paces your body crosses over from fat burning (low intensity) to carb/sugar burning (high intensity).  If your diet is especially high in carbohydrates, you may not even be burning fat at the low intensity jog.

You can also use fasted runs to test your body’s dependence on sugars for fuel. We’ll talk more specifically about using fasted runs and balancing your foods in two follow up posts.

Becoming a more efficient fat burner is not only better for your running, it is linked to better health with a lower chance of diabetes, inflammation and “skinny fat” symptoms.  You don’t have to be over weight to be damaging your health with an imbalance of nutrients. If you’re looking for specific guidance, the next runner’s reset begins August 1st and I’m happy to help you determine if it’s a good fit for you.

Have you ever bonked or felt like you ran out of fuel on a run?

Do you have any idea whether your body is burning mostly sugar or mostly fat?




31 thoughts on “How do you know if you are an efficient fat burner or sugar burner?

  1. Oh this is a good one! I have dabbled in metabolic efficiency (as you know) and it was not really a good fit for me. However, I do struggle with some of the things you mentioned like not being able to go more than 3 hours without a meal and sometimes have mid-morning or mid-afternoon “slumps.” I’m actually talking with one of the guys from Inside Tracker today about all my test results so we shall see what he recommends and why.
    Great post that definitely made me stop and think!
    Allie recently posted..The Rundown – Feel the HeatMy Profile

  2. Excellent post!!! This past year I’ve been adding more fasted runs and started eating more healthy fats and less sugar. I’m still kinda new to the whole thing but I think I’m seeing progress. I’m definitely not bonking on my long runs anymore. I’m still struggling with being fluffy around my waist though. Apple shape. Thanks mom. 😉 it seems I would have to NEVER eat sugar to whittle down to being completely lean. But that’s just not living. Ha!
    Jess @ run pink recently posted..Marathon Monday WK3: Marathons, Marriage, Building: 8 Keys to Staying MarriedMy Profile

    • That’s great! I noticed a big change too, especially over 6 months. It doesn’t happen overnight. And the apple piece- I get it. I have the same genetics. I also find for some women that more running is part of the problem because the body translates it as stress which keeps cortisol levels high (ie belly fat). When I stop running for a period of time I typically lean out but I love running too much to care. 🙂
      Laura recently posted..How do you know if you are an efficient fat burner or sugar burner?My Profile

  3. Laura, this is brilliant! I’m really looking forward to your upcoming posts about this topic.
    I almost never use any kind of gu, gel or other sugary carb source, while my boyfriend seems to need fuel ALL THE TIME.
    I prefer fasted runs- I feel like my energy level is pretty stable.
    I can’t wait to read what else you’ve got coming! I wish I could afford you as a running coach! You’re amazing!

  4. Is a ketogenic diet recommended for runners? I would imagine that once you condition your body to burn off fat stores verses sugar stores, your energy becomes longer lasting? However, in order to become “ketogenic” you need to avoid most sugars. Is this counterproductive as a runner?

    • Hi Jason,
      No, ketogenic diet is too low carb for runners. It is possible to run on a very low carb diet but keto is less than 50 grams which is not recommended for long term for anyone, really, unless there are extreme diabetic or other issues to reverse.

      A metabolically efficient diet is bringing protein and carbs into better balance- typically more of a 35/35/30 break down of carbs, protein and fat is sufficient. When I was first shifting my body to more fat burning, I ate lower carb than I was used to but “low” meaning 150-200 grams instead of 250… it certainly doesn’t have to be low carb. How much depends on the individual’s weight, activity level, etc.
      Laura recently posted..How do you know if you are an efficient fat burner or sugar burner?My Profile

  5. Interesting read! I am curious to read your follow-up posts – especially about changes to the diet. I’ve been following more paleo principles for the past 6 weeks and have found that to be positive! It is tough when you really start ramping up mileage for marathon training – the thought of eating more fat and protein over oats or sweet potatoes or a high carbohydrate food is not very appealing.
    Danielle @ Wild Coast Tales recently posted..Alaskan Cruise Part 4: Epic whale sightings and KetchikanMy Profile

    • Hey Danielle! You don’t have to be paleo to get these affects… just a better balance. Most days carbs are still my highest percentage but with closer to equal amounts of protein, and every meal should have a little fat too. I’m hosting a runner’s reset for August where we’ll work on this together in a group and that may be a good fit for you if you want more specifics!

  6. So interesting! I fuel on nothing and can workout for 2 hours pretty hard core. Once I’m done, I fuel but I actually prefer not working out with food. It makes me feel less efficient in my workouts if that makes any sense. I also workout at 5am and there is no way I’m getting up any earlier to eat. I’ve always assumed I was a fat burner because of this, but I do snack a lot. 🙂
    Megan @ Skinny Fitalicious recently posted..Superfood Blueberry Protein Smoothie {GF, Vegan, Paleo, Low Cal}My Profile

  7. Laura, your posts are always so interesting, informative, and clearly explained. I would have thought I was a sugar burner just from how much I like my oats and potatoes, but going through the criteria I sound more like a fat burner. I started training more low on carbs (and racing higher) over the past year and I think that has helped. I can’t wait to read more on this topic!
    Laura @ This Runner’s Recipes recently posted..5 Things to Do Before You Start Training for Your Fall RaceMy Profile

  8. Thank you for a great post, a very interesting read. Based on the signs I definitely point to being a fat burner. Minus I still have some fluff although attempting to lean out (I think it is more to do with not adding enough variety – which is why I now do bootcamps a couple times a week).
    I run up to 13 or so miles fasted with no fuel. I don’t really start to contemplate the need to eat before/during until I am hitting upward of a half marathon.
    Gianna recently posted..Let the Marathoning Begin!My Profile

    • Yes- there is also the issue of getting some strength work and variety in, and varying the low intensity workouts with high intensity (or as some prefer, doing all low intensity like MAF training) to help lean out. Many body types hang on to a little extra for distance training no matter what – unless calories are too long in which case your body can suffer in other ways. Everyone is different, it can take some trial and error to understand your body!
      Laura recently posted..How do you know if you are an efficient fat burner or sugar burner?My Profile

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