How to become a fat burning machine, lessons from athletes

Grant Schofield

Why become a fat burner?

In a previous post, Why some people stay skinny and others get fat I talked about how high carb foods lead to high insulin levels which effectively turn down, or off, your ability to burn fat. There is a longer downward spiral from years of this. But that was that post.

In this post, I want to look at exactly what you can do to turn yourself into a fat burning machine. This is not only about turning the fat burning on and creating a situation where you can drive a homeostasis for a steady and healthy weight, but is also about well-being and energy. My experience, as well as the overwhelming blogashpere and research itself, shows that an important benefit of fat adaptation is a much more stable energy level and well-being/mood.

We’ll look specifically at endurance athletes first. They want the same things that those who have problems with metabolic dysfunction want. They want to burn fat, not carbs, because humans have such a limited supply of carbs but much much bigger supplies of fat to draw upon. When athletes going long distances run out of carbs they are said “to hit the wall” or “bonk” (French for the noise your head makes when it hits the road?) So we’ll look at some of our lab results with an athlete later.

It’s not about treating carbs as evil and trying to run your body without carbs. In some ways it’s the exact opposite – it’s about sparing the glucose you do have. It’s about getting your body to do what it is designed to do under usual evolutionary conditions – burn fat and have enough energy to move all day and/or make short intense bursts. Becoming an efficient fat burner allows you to do just this.

Carb burners v Fat burners

If your metabolism is set up to predominately burn carbs, then you’ll most likely have many more highs and lows throughout the day energy wise. I call the lows “falling off the glucose cliff”. That’s when your glucose dependent brain cries out for more fuel – in the form of simple carbs usually. The cycle continues. If that’s you, then this blog could change your life for the better!

There are many plausible or proven health benefits here to. There is the obvious one of easily being able to control your weight. But far beyond that is reducing the damage high sugar, and high insulin, and sugar burning (glycolysis) do in your body. All of these are inflammatory and cause oxidative stress. These are the causative mechanisms behind chronic disease development including heart disease and stroke, diabetes, cancer, and brain (dys)function.

Fat burning in endurance exercise

One lab-based method we use to measure fat burning vs carb burning is the respiratory exchange ration (RER), also known as the respiratory quotient. This is the ratio between the amount of carbon dioxide exhaled and oxygen inhaled, which provides an indication of which substrate (fat, carbohydrate or a mix) is being used for fuel. We do this using our breath by breath gas analysis system in our Metabolic and Exercise Science Clinic at the Human Potential Centre at AUT Millennium Campus.

The RER varies between 0.7 (100% fat burning) through to ≥1.0 (100% Carb burning). A ratio of 0.85 has been labelled the metabolic efficiency point, when the body burns half of each. We try to determine what exercise output can be maintained for half and half. Bear in mind that everyone will eventually burn 100% carbs if the exercise is intense enough, but the higher the intensity at which fat is still the predominant energy source the better. However, what we are most concerned with is the ability to burn fat at rest and at lower to moderate intensities of exercise. This is great for weight loss. It’s great for health. But it’s also great if you are an athlete trying to do longer distance events like the Ironman triathlon.

We recently had a high level triathlete in our lab. We measured his RER before and after a 10 week training block going into this year’s Ironman NZ. We also transferred him to a low carb high fat (LCHF) diet for the period of that training. He was training about 20 hours a week and came in to the 10 week block relatively fit, albeit slightly heavier than he wished.

Some stats:

  1. Start weight 86 kg, post weight 78 kg. Good weight loss while reporting eating until full. No deprivation of food or calories if needed. Reduced calorie consumption on long training sessions. Few if any gels or sports drinks.
  2. Pre RER @ 270 W bike = 0.93, post RER @300 W = 0.82. This translates to a change in fat utilization from 23% of fuel to 60% of fuel at the same power output, for a lighter overall weight (power per kilo was also increased).
  3. Metabolic efficiency point (50/50 fuel use) improved from 180 W to 300 W. This shows the massive increase in efficiency we saw with a switch to a LCHF diet.

If you know anything about endurance training and racing then you’ll know that these results are outstanding. To the point of being spectacular. The limiting factor in longer races is not maximal output, but how fast you can go while conserving muscle and liver glycogen (carbs). You need to maximize your fat burning and preserve your very limited supply of carbs as much as possible.

Most athletes try to get around this problem by eating extra carbs during training and racing. This can work to an extent, but perversely raises insulin and shuts down your body’s ability to burn fat. It’s almost impossible to eat and digest enough carbohydrate to actually race these events well. You need a good degree of fat burning.

Anyway, this illustrates what can be achieved with a LCHF diet; good, effortless weight loss and spectacular performance gains.

If you’re not an athlete, and I’m assuming most of you are not, then the same principles still apply. We can hook you up to the gas analysis system and determine your “metabolic efficiency” at rest. We can assess just how much of a carb burner you are and we can track your progress if you decide to become a fat burning machine through a LCHF diet.

Fat adaptation v ketosis

There’s a special type of low carbohydrate diet called a ketotic diet. I want to explore that in the next post. Stay tuned.


Thanks to Helen Kilding for her help with this blog

Author: Prof. Grant Schofield

I am Grant Schofield, Professor of Public Health at Auckland University of Technology and director of the university's Human Potential Centre (HPC) located at the Millennium Campus in Auckland, New Zealand. My research and teaching interests are in wellbeing and chronic disease prevention especially reducing the risk and eventual mortality and morbidity from obesity, cardiovascular disease, and diabetes. I live by the motto "be the best you can be" and have a strong commitment to peak performance in which I also do consulting work. I’ve been interested in human health and performance for my whole career. I started in psychology, went into sport and exercise psychology, then into public health, especially physical activity, then obesity. There have been some twists and turns along the way, which are the reasons for why I do what I do – you can read about those in my first blog entry. I want to know how we can be the best we can be. This crosses disciplines such as biology, medicine, pubic health, and productivity management. The cornerstones are nutrition, exercise, sleep, neuroscience, psychology and wellbeing. In my blog, I cover these topics under the broad heading of the Science of Human Potential.

21 thoughts on “How to become a fat burning machine, lessons from athletes”

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  2. Very nice piece. I recently started training for the next Dublin city marathon. This is my first but I have found it so hard to reach beyond 15k before hitting a wall. I’m pretty fit as I have always trained throughout my life but endurance – long distance training is a different animal to working-out. Time to re-access my dietary intake. Big thanks for sharing.

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  8. Excellent article. Just found your blog on Google after reading your recent article in Stuff. Do you by any chance have a list of good/bad foods on another article? Being a lazy bugger it would be good to know what foods are/aren’t recommended as I’d like to try this out for my own endurance training.

    Keep up the good work

  9. Hi, you have written “fuel at the same power output” but for the pre-state you write 270W and for the post-state yout write 300W. Is one of the numers wrong or what do you mean with “same power output”? Thank’s for your great articles, kind regards, Sebastian

  10. Hi, I am a masters athlete – middle distance runner. My coach Bruce Milne encourages us to burn fat acid for fuel. He was trained by Arthur Lydiard and is a level 3 coach and a physio therapist. He has had international exposure and has been a National Coach, Olympian and Commonwealth Games Coach. His athletes have won many NZ titles, amongst others he coaches a long distance 100 km runner who is a world class performer. He believes in training intelligently and fueling appropriately – still controversial however has worked for his athletes for the past 40 odd years. If you want to know more about athletes and there training to use fat for fuel then I would contact Bruce for scientific and historical proof of taking an athlete to optimal performance based on these principals.

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