What is Super About a “SuperStarch” if it Doesn’t Help Performance? #superstarch #endurancenutrition
Fat metabolism versus carb metabolism in endurance competition.
A growing trend in recent years has been the search for ways to push the body towards a relative increase in fatty acid oxidation, at the expense of carbohydrate oxidation, in endurance competition. Certain sports supplement products have been developed with this thought process in mind, with the creation of slow-acting carbs that attenuate the insulin spike, leading to increased mobilization of fatty acids. The logic is at least partly based on the observation that, in endurance athletes, training adaptations appear to lead the body to an increased reliance on the metabolism of fat as a fuel source.
But is this really true? Or is there a component of a “fat is bad therefore burning fat must be good” thought process at work?
To begin with, let’s examine from a physiological standpoint, whether or not fat is really your body’s best choice for substrate utilization during endurance activity.
Fat exists in your body for a reason. It is an excellent storage form for energy. For a given amount of mass, fat contains more than twice the amount of stored energy than carbohydrates (9.3 kcal/gram versus 4.1 kcal/gram) (1). If your body had to store it’s spare energy in the form of carbohydrates, you would weigh a whole lot more. So in those times when your body has extra energy to spare, it lays down fat because this is the most efficient means of longer term storage.
However, fat is NOT the most efficient fuel for energy USAGE (1, 2). Fat requires more than twice the amount of oxygen than carbohydrates to metabolize the same amount of mass (1.96 liters O2 per gram fat compared to 0.81 liters O2 per gram of carbohydrate) (1). For every liter of oxygen you take in, you burn 0.51 grams of fat producing 4.74 kcal energy. In contrast, for every 1 liter oxygen you take in, you burn 1.23 grams of carbohydrate and produce 5.0 kcal energy.
Again, fat is excellent for STORAGE, carbohydrates are excellent for USAGE. And the natural process is for your body to interconvert between the two forms of energy depending on its acute needs. When your body is in a “fed” state, insulin goes up (among other hormonal changes) and your body shifts to a strategy of laying down fat for storage. However, in the “starved” state, insulin levels go down and fat is converted back into usable energy.
Attempts to fool your body into believing it is in the “starved” state, by suppressing the insulin response, will only lead to you utilizing a less efficient energy system.
And there are other considerations with a shift towards fat usage in exercise. Fatty acids compete with tryptophan for protein binding (5). With increasing levels of fatty acids, more tryptophan is displaced, becoming “free”. This free tryptophan is then able to cross the blood brain barrier and contribute to serotonin synthesis, a major player in central fatigue (6, 7). More fatty acids in the blood stream could therefore, theoretically at least, contribute to increasing central fatigue during exercise.
So what does the evidence show?
Studies have demonstrated that slower-acting, lower glycemic index carbohydrates, will attenuate the insulin spike and lead to higher utilization of fatty acids during endurance exercise (8, 9). What they have all failed to show is a performance benefit. Similarly, attempts to modify the diet to increase reliance on fats during endurance competition have also failed to show performance benefits (10, 11).
So, yes, taking forms of carbohydrates that attenuate the insulin spike will lead to an increased reliance on fatty acid metabolism compared to carbs, but this does not appear to help you perform better. And this is exactly what you would expect given what I discussed above.
Why, then, does endurance training lead to an increased reliance on fatty acid metabolism, as the evidence shows (4)? The answer is that these results are misleading, because in most of these studies the subjects are tested at the same absolute work level, not the same relative work level.
For example, imagine if at the beginning of a study a subject is tested at 65% of his or her VO2 max (i.e. moderate to high intensity). The metabolism would be mediated through a certain combination of carb and fatty acid oxidation, as would be expected during this level of intensity of exercise. After several weeks of endurance training, the subject is again tested at the same absolute work load and the percentage of fatty acid metabolism is now found to be higher, and the percentage of carbohydrate metabolism lower. The conclusion is therefore that with endurance training the body has learned to burn fatty acids in preference to carbs.
But this is a flawed assumption, because it doesn’t account for the fact that the VO2 max has increased. As a result, the subject is now exercising at a lower percentage of their VO2 max (and thus a lower relative intensity) and that is the reason for the increased reliance on fat, not that their body has trained itself to use fat preferentially.
This was demonstrated by Bergman et al (3), who examined the metabolic response of nine male subjects before and after nine weeks endurance training. They first tested the subjects prior to the training, at both 45% and 65% VO2 max. They then re-tested the subjects after nine weeks of training, testing at both the same absolute intensity (i.e. 65% of pre-training VO2 max) and the same relative intensity (i.e. 65% of post-training VO2 max). What they found was that, with the increased VO2 max that occurred through training, the total amount of carbohydrate- and fat-based fuels increased, but the relative proportion of substrate utilization did not change when exercising at the same relative intensity.
Attempts at making your body shift to a higher percentage of fat metabolism are therefore not mimicking what takes place during training. And you are not allocating the resources of your body in an effective manner when you do this.
Fat has many important roles in your body. But it is not an efficient source of energy utilization during intense endurance competition. If your goal is to lose weight and improve lean body mass, then exercising at a lower intensity (i.e. “fat-burning”) for longer periods will help accomplish this. No “super” supplements necessary. But if what you want is to improve performance times or to enhance stamina during prolonged high-intensity competition, then artificially shifting towards higher fatty acid metabolism is simply not the way to go. U Can do better.
1. Nelson, Philip. Biological Physics, W. H. Freeman, 2004.
2. Krogh, A., and Lindhard, J. (1920). Relative value of fat and carbohydrate as sources of muscular energy. Biochem. J. 14, 290–363.
3. Bergman BC, Butterfield GE, Wolfel EE, et al. Evaluation of exercise and training on muscle lipid metabolism. Am J Physiol. 1999;276:E106-17
4. Hawley JA, Maughan RJ, Hargreaves M. Exercise metabolism: historical perspective. Cell Metab. 2015;22(1):12-17
5. Curzon G, Friedel J, Knott PJ. The effect of fatty acids on the binding of tryptophan to plasma protein. Nature. 1973;242:198–200.
6. Meeusen R, Watson P, Hasegawa H, Roelands B, Piacentini MF. Central fatigue: the serotonin hypothesis and beyond. Sports Med. 2006;36(10):881-909.
7. Blomstrand E: A role for branched-chain amino acids in reducing central fatigue. J Nutr 2006, 136(2):544S-547S.
8. Roberts MD, Lockwood C, Dalbo VJ, Volek J, Kerksick CM. Ingestion of a high-molecular-weight hydrothermally modified waxy maize starch alters metabolic responses to prolonged exercise in trained cyclists. Nutrition. 2011 Jun;27(6):659-65.
9. Sands AL, Leidy HJ, Hamaker BR, Maguire P, Campbell WW. Consumption of the slow-digesting waxy maize starch leads to blunted plasma glucose and insulin response but does not influence energy expenditure or appetite in humans. Nutr Res. 2009 Jun;29(6):383-90.
10. Havemann L, West SJ, Goedecke JH, Macdonald IA, St Clair Gibson A, Noakes TD, Lambert EV. Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance. J Appl Physiol 2006 Jan;100(1):194-202.
11. Burke LM. Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon? Sports Med. 2015 Nov;45 Suppl 1:S33-49.