We all know someone who proudly claims they ride a bike so they can eat whatever they want, and even some professional athletes are known to binge on candy and fast food. The caloric demands of hard workouts and the elevated metabolic rate that comes with it can make you feel like a vacuum cleaner, ready and able to ingest whatever you want with no obvious consequence. While this is partially true for weight loss, the long-term consequences of careless eating might be more dramatic than you realize, and understanding how your body processes food differently on and off the bike is important to maintaining good health.
For more information on carb and fat metabolism, check out Ask a Cycling Coach Ep 266.
The Basics of Metabolism
Metabolism is extremely complicated, but for our purposes we’ll consider a simplified version of how the body fuels itself. At low intensity, workouts can be fueled largely by lipolysis, or fat metabolism. This capability to burn fat can be improved with training, but generally as workout intensity rises the body places an increasing reliance on the metabolism of carbohydrates (sugars). The body can store a limited amount of sugars in the muscles and liver as glycogen, available as fuel until it runs low.
Most sugar moves from food in the stomach and small intestine to the bloodstream as glucose. On entering circulation glucose temporarily raises blood sugar levels as it is transported around the body; this rise in blood sugar triggers the pancreas to produce and release the hormone insulin.
Insulin suppresses lipolysis (fat burning) and diverts the sugar from the bloodstream into cells, where it is either consumed immediately in the muscles, stored as glycogen in the muscles and liver, or converted to fat for use later. The muscles can store enough glycogen to ride for a maximum of about 2 hours, but these stores are depleted more quickly at high intensity.
Blood glucose levels vary naturally throughout the day due to activity level and diet. The more tightly these levels remain consistent and regulated, the better. Sustained high blood sugar (hyperglycemia) can gradually degrade the body’s ability to control blood sugar levels and lead to negative metabolic consequences including type II diabetes. While low-glycemic foods such as beans and nuts cause a gradual rise in blood sugar and a subtle insulin response, foods high on the glycemic index such as candy, white rice, and white bread enter the bloodstream rapidly and raise blood sugar dramatically. This is why experts generally recommend against eating a diet high in these simple carbohydrates and sugars.
Metabolism During Exercise Is Different
Knowing that elevated blood glucose can be harmful, it might seem strange that most sports nutrition products such as gels and drink mixes are almost pure sugar. However, there is a big difference in how the body fuels itself during intense exercise and how it fuels itself during times of rest. As reliance on sugar as a fuel source increases with exercise intensity, so does the body’s ability to manage blood sugar levels and handle the intake of sugary foods.
Insulin normally controls most of this regulation, but during exercise muscular activity itself induces blood sugar uptake. In other words, exercise partially allows muscles to fuel themselves by directly removing glucose from the blood, bypassing the hormonal regulation system that normally controls this process. Thus, a dose of sugar that would spike your blood sugar and trigger a hormonal response at rest, goes right to the muscles during exercise.
This means that during intense exercise, sugars are the most efficient and effective way to fuel yourself. However, there are caveats to this. Low-intensity exercise such as endurance riding can be fueled almost exclusively through fat metabolism. Ingesting a gel or sugary drink temporarily reduces lipolysis, potentially negating one of the major purposes of long easy rides. In addition, the body’s ability to store an hour or two’s worth of glycogen means for short rides you probably don’t need supplemental sugars anyway, unless your fuel stores are low to begin with or the workout is exceptionally difficult.
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Fitness Is Not a Free Pass
Physiologically, endurance athletes are special. Training improves our glycogen storage capabilities and increases our insulin sensitivity. It also generally improves our overall ability to control our blood glucose levels. These are all good things for our metabolic health, but this is not a free pass to subsist on candy and junk food.
While sugars are processed differently when exercising, the common idea that an athlete can eat whatever they want all the time is based on an unrealistic notion of nutrition. It reduces health to a balance of calories ingested vs. calories burned and doesn’t consider the metabolic cost of a reckless diet; this simplification may be placing athletes at risk. At least one study has suggested that many amateur athletes may tend to over-nourish with foods of poor nutritional quality, and experience chronically elevated blood glucose levels as a result.
Several well-publicized examples exist of trained athletes developing metabolic disorders such as insulin resistance and type II diabetes, revealing that fitness is not always synonymous with health. While it may be relatively easy to achieve energy balance and lose weight through endurance sports, an everyday over-reliance on sugars and high-GI foods can have serious long term costs that take years to develop.
Takeaway: Fuel For More Than Just The Workout
A healthy nutrition strategy should effectively fuel your workouts, but you can’t compartmentalize it from the rest of your life. Instead, what you eat before, during, and after exercising should all work together to maintain a healthier you.
Before your workout, this means quality carbohydrates and macronutrients that will top-up your glycogen stores without spiking your blood sugar or piling on unnecessary calories. During your workout it might mean supplemental sugars if workout intensity and duration demands it; remember that if properly fueled your body carries enough glycogen stores to exercise for at least an hour, and that the fat-burning benefits of easy workouts are temporarily disrupted by taking in sugar. After your workout you should aim to replenish, but remember that quality counts. Don’t squander the metabolic benefits of good fitness by ingesting lots of unnecessary high-glycemic foods.
Most of us will never be pro athletes. While the top step of a local race might be a powerful training goal, your good health as a human being is more important over the long-term. Luckily, this can go hand-in-hand with getting faster. High performance requires a high-quality diet, and with smart choices and common sense your diet can reinforce the lasting benefits of an active lifestyle.
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References/ Further reading:
- Carbohydrates and Blood Sugar. (n.d.). Harvard T.H. Chan School of Public Health. Retrieved July 18, 2020, from https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar
- Dube, S., Errazuriz-Cruzat, I., Basu, A., & Basu, R. (2015). The forgotten role of glucose effectiveness in the regulation of glucose tolerance. Current diabetes reports, 15(6), 605. https://doi.org/10.1007/s11892-015-0605-6
- Han, H. S., Kang, G., Kim, J. S., Choi, B. H., & Koo, S. H. (2016). Regulation of glucose metabolism from a liver-centric perspective. Experimental & molecular medicine, 48(3), e218. https://doi.org/10.1038/emm.2015.122
- Kelesidis, T., Kelesidis, I., Chou, S., & Mantzoros, C. S. (2010). Narrative review: the role of leptin in human physiology: emerging clinical applications. Annals of internal medicine, 152(2), 93–100. https://doi.org/10.7326/0003-4819-152-2-201001190-00008
- Maffetone, P.B., Laursen, P.B. Athletes: Fit but Unhealthy?. Sports Med – Open 2, 24 (2016). https://doi.org/10.1186/s40798-016-0048-x
- Richter, E., Hargreaves, M. (2013). Exercise, GLUT4, and Skeletal Muscle Glucose Uptake. American Journal of Physiology – Physiological Reviews, 93 (3), 993-1017. https://journals.physiology.org/doi/full/10.1152/physrev.00038.2012
- Rui L. (2014). Energy metabolism in the liver. Comprehensive Physiology, 4(1), 177–197. https://doi.org/10.1002/cphy.c130024
- Thomas, D. D., Corkey, B. E., Istfan, N. W., & Apovian, C. M. (2019). Hyperinsulinemia: An Early Indicator of Metabolic Dysfunction. Journal of the Endocrine Society, 3(9), 1727–1747. https://doi.org/10.1210/js.2019-00065
- Thomas, F., Pretty, C. G., Desaive, T., & Chase, J. G. (2016). Blood Glucose Levels of Subelite Athletes During 6 Days of Free Living. Journal of diabetes science and technology, 10(6), 1335–1343. https://doi.org/10.1177/1932296816648344
- Yeager, Selene. Everything You Need to Know About Glycogen Bicycling. April 13, 2013. https://www.bicycling.com/health-nutrition/a20032965/everything-you-need-to-know-about-glycogen/
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