Fat Burning – Is Your Body Trained to Burn Fat or Sugar?
Most athletes don’t know if they are good at fat burning or a sugar burner. This is important information. If you enjoy endurance sports and value your health you want to be a fat burner. Fat burners are called keto-adapted or fat-adapted.
An athlete who is good at burning fat can train for hours with little or no fuel. If you are a sugar burner, you have to take in large amounts of carbohydrates to progress in your race. Sugar burning athletes who don’t consume enough carbohydrates will bonk. As a sugar burner, your race may also include gas, bloating, and nausea. The worst case scenario may also include a DNF after your name. The number one reason Ultra runners DNF is due to stomach issues.
Fat Burning – Why You Need to Know?
Poor performance isn’t the only reason to avoid being a sugar burner. Negative side effects include insulin resistance, pre-diabetes, and reactive hypoglycemia. A simple search in PubMed might give you an idea of why insulin resistance is no Bueno.
Some athletes have fortunate genetics and are predisposed to burn fat. Other’s have to train their body to do so. One of the main advantages of fat burning is that the body spares sugar stored in the body. This gives the endurance athlete a competitive advantage because it keeps the gut happy.
Fat Burning – How to Test
How do you know if you’re a sugar-burner or a fat-burner? There are some tests that will give a ballpark figure about fat burning. The “Respiratory Quotient” (RQ) or the Respiratory Equivalency Ratio” (RER). They aren’t exactly measures of the same thing but close. You can read about RQ here and RER here.
Once you know your RQ rate you have a better idea of what how well your body burns fat during exercise. If you find you’re a sugar burner, it is a good idea to work on how you train and to fuel your body to burn fat.
A VO2max test or metabolic assessment will give you your RQ. A web search should point you in the right direction to get the test done. Many medical clinics and universities offer the test. There are many health clubs, bike shops, and running and triathlon stores that offer the test.
The tests run anywhere between $150 to $250 in the US. Cyclist and triathletes should do the test on a trainer with their bike. Runners can use a treadmill.
The typical test protocol starts with a very easy effort. The intensity increases the intensity every few minutes. The test continues until you fatigue and can no longer continue. Good data requires that the test like a race. I recommend you rest for a couple of days before.
Fat Burning Test Results
There will be several pieces of information resulting from such a test. One is RQ. As the intensity of the test increases, you will burn more carbohydrate (glycogen) for fuel. The RQ estimates how much of the energy came from carbs and how much fat. Use the table to determine your percent of energy burned from these two nutrients.
The good fat-burning athlete will start the test with an RQ of around 0.80. This means that they are using about 33% carbohydrate and 67% fat for fuel. That’s good.
A similar fit sugar-burning athlete may start the test at the same low intensity but with an RQ of 0.90. At this RQ he or she is burning 67% carbs and 33% fat. When both athletes reach their anaerobic or lactate thresholds they will be at about 1.00 RQ. This means 100% carbohydrate and 0% fat.
Notice that the sugar burner has a much narrower RQ range (0.90-1.00) than the fat-burner (0.80-1.00). At moderate intensities, the sugar burner runs the risk of running low on fuel. If they use sugar faster than their stomach can absorb the fuel, their gut “shutdown.”
If you find out that you are a sugar burner, it is time to make the necessary changes to your training and your diet. It is harder for some athletes to make adjustments due to genetics. This may have to d0 the makeup of slow twitch muscles fibers .
What Determines Your Fat Burning?
The main determiner of RQ is your diet. The higher carbohydrate load in your diet, the more your body will rely on sugar [2,3]. The lower the carbohydrate load, the more fat, and protein in your diet, the lower your RQ will be. Another major influencing factor to your RQ is how you train. As your aerobic fitness improves, so will your RQ ratio. Research shows that as your training volume increases RQ is also reduced .
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- Toubro et al. 1998. Twenty-four-hour respiratory quotient; the role of diet and familial resemblance. J Clinic Endocrin Metab83(8):2758-2764.
- Goedecke et al. 2000. Determinants of the variability in respiratory exchange ratio at rest and during exercise in training athletes. Am J Physiol Endocrinol Metab 276(6):E1325-E1334.
- Hughson and Kowalchuk. 1981. Influence of diet on CO2 production and ventilation in constant-load exercise. Respir Physiol 46(2):149-160.
- Decombaz et al. 1985. Oxidation and metabolic effects of fructose and glucose ingested before exercise. Int J Sports Med 6(5):286-288.
- Kiens et al. 1993. Skeletal muscle utilization during submaximal exercise in man: effect of endurance training. J Physiol 469:459-478.