Fat Burners and Sugar Burners

Interesting stuff in regards to long races. I'm intrigued by how drinking juice before a race instead of sports drink can help you.


by Joe Friel

Are you a fat burner or a sugar burner? Most athletes don’t know, yet this is valuable information, especially if you compete in four-plus-hour races. A limiting factor for such events is carbohydrate intake. If you don’t take in enough sugar during the race you are likely to run low which ultimately means your name in the results will be followed by the letters ”DNF” (did not finish). On the other hand, take in too much sugar and your gut can’t process it possibly resulting in bloating and nausea.

To further complicate the matter, there is a fair amount of individuality when it comes to using carbohydrate during such events. Some people’s bodies burn more carbohydrate as a percentage of total calories used. They are “sugar burners” and need to be very concerned with carbohydrate intake. The “fat burner” has a body that prefers to use fat for fuel and so spares sugar stored in the body. This person is metabolically designed for long endurance. That may be the result of fortunate genetics or effective training.

How do you know if you’re a sugar-burner or a fat-burner? And how do you determine if you are taking in the right amount of carbohydrate? Ultimately you need to experiment during your long workouts to find what works best for you. But you can narrow it down by discovering your “Respiratory Quotient” (RQ) which is sometimes also called 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 your carb needs are during exercise. If you find you’re a sugar burner it may even be possible to change your body so that it relies more heavily on fat. More on this later.

RQ may be found by doing a metabolic assessment or VO₂max test. It used to be that you had to go to a medical clinic or university lab to have such a test done, but now there are boutique testing centers popping up around the country in health clubs, bike shops, and running and triathlon stores. You can probably find such a test facility somewhere near where you live. The test generally costs between $150 to $250. It can be done on your own bike using the test center’s indoor trainer (this is preferable to using a stationary bike) or on a treadmill for running. If you’re a triathlete and can afford only one test I’d suggest doing it on the bike as your nutrition here generally has a greater impact on your performance than when running due to the duration of the bike portion.

The typical test protocol starts you at a very easy effort and increases the intensity every few minutes until you fatigue and can no longer continue. In order to get good data you need to treat the test like a race by resting for a couple of days before. Doing this test tired will muddle the results and what you do with them.

There will be several pieces of information resulting from such a test. One is RQ. As the intensity of the test increases you will gradually burn more carbohydrate (glycogen) for fuel. The RQ closely estimates how much of the energy came from carbs and how much fat. The following table may be used to determine your percent of energy burned from these two nutrients throughout the test.

RQ	Carb %	Fat %
0.71	0.0	100.0
0.71	1.1	98.9
0.72	4.8	95.2
0.73	8.4	91.6
0.74	12.0	88.0
0.75	15.6	84.4
0.76	19.2	80.8
0.77	22.8	77.2
0.78	26.3	73.7
0.79	29.9	70.1
0.80	33.4	66.6
0.81	36.9	63.1
0.82	40.3	59.7
0.83	43.8	56.2
0.84	47.2	52.8
0.85	50.7	49.3
0.86	54.1	45.9
0.87	57.5	42.5
0.88	60.8	39.2
0.89	64.2	35.8
0.90	67.5	32.5
0.91	70.8	29.2
0.92	74.1	25.9
0.93	77.4	22.6
0.94	80.7	19.3
0.95	84.0	16.0
0.96	87.2	12.8
0.97	90.4	9.6
0.98	93.6	6.4
0.99	96.8	3.2
1.00	100.0	0.0

The fat-burner will start the test with an RQ of around 0.80 meaning that he or she is using about 33% carbohydrate and 67% fat for fuel. That’s good. I like to see that in those I coach. An otherwise similarly fit sugar-burner 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. That’s not so good. I see very few of these but they are out there. When both athletes reach their anaerobic or lactate thresholds they will be at about 1.00 RQ which means 100% carbohydrate and 0% fat. (Total fatigue will end the test at an RQ of about 1.1 to 1.2 for both.)

Notice that the sugar burner has a much narrower RQ range (0.90-1.00) than the fat-burner (0.80-1.00). So at moderate intensities, as are common in long-distance events, the sugar burner needs to be very aware of carbohydrate intake as he or she risks running low on this precious fuel. Some athletes are such gigantic sugar burners that they find it difficult to take in enough carbohydrate during the competition. They’re using sugar faster than their stomachs can process it from sports drinks. That often means a gut “shutdown.”

If you discover from a test that you are a sugar burner you may be able to modify this condition somewhat. I say “may” because there is some research indicating that there is an element of genetics involved [1]. This probably has to do at least in part with your muscle make up, especially your percentage of slow twitch or “endurance,” muscles [2].

Another chief determiner of RQ is your diet. Simply put, the more high glycemic load carbohydrate foods you eat (starches are the most prevalent in this category) the more your body will rely on sugar during exercise [2,3]. Conversely, the more fat and protein in your diet the lower your RQ will be. Eating starch or taking in glucose in a sports drink before the start of the test or the race may also shift your RQ to the sugar-burning side whereas fruit juice will not have this effect [4]. (Lesson: Best not to rely on sports drinks or starches before races.) The other known influencers of RQ are related to training. As you become more aerobically fit your RQ will drop [5], and related to that, research has shown that as your training volume increases RQ is also reduced [2].

It is even possible to determine how much carb you need to take in during a race from a test. All you need to do is find your goal race intensity—heart rate, power or pace—for your race in the test’s raw data results and determine, also from the results, how many calories you were burning at that point. Then check RQ at that same intensity to see what percentage of those calories came from sugar. You will need to replace most of this expended energy during long events (this is not a big deal for short races). The test technician can help you figure this out.

Knowing your RQ and, more importantly, keeping it on the low side through diet and training has the potential to improve your performance in long-distance endurance events.

References

1. Toubro et al. 1998. Twenty-four-hour respiratory quotient; the role of diet and familial resemblance. J Clinic Endocrin Metab 83(8):2758-2764.
2. 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.
3. Hughson and Kowalchuk. 1981. Influence of diet on CO2 production and ventilation in constant-load exercise. Respir Physiol 46(2):149-160.
4. Decombaz et al. 1985. Oxidation and metabolic effects of fructose and glucose ingested before exercise. Int J Sports Med 6(5):286-288.
5. Kiens et al. 1993. Skeletal muscle utilization during submaximal exercise in man: effect of endurance training. J Physiol 469:459-478.