The role of diet in intermittent exercise
Not all exercise utilises the same amount of fuel availability (substrates such as Fat, Carbohydrate, Protein) across differing intensities. During exercise the body utilises protein, carbohydrate (CHO) and fat’s in differing degree relative to intensity. At high intensities, such as at ones threshold power or above (Vo2max), carbohydrate is primarily used as its ability to be metabolised is much more immediate compared to either fat or protein. This is important as glycogen, a product of metabolism fueling exercise, is easiest sourced from carbohydrate. Therefore a diet for an athlete should be high in Carbohydrate? Traditional ‘wisdom’ would say yes, however more contemporary research is highlighting how low carb and high fat diets (LCHF) can be used to improve lipolosis (fat metabolisation), reduce stress markers, and improve performance.
Several athletes we have contact with have adopted, to good effect, a LCHF diet, describing increased energy levels, reduction in weight, and reduced hunger gravings. In these athletes, particularly over longer rides where interval work is completed at the 4hr mark, we have also cited how diet and ingestion of carbohydrate becomes paramount. The reasons for this ‘need’ for carbohydrate is outlined herein.
Taking the graph below as exemplar for substrate useage is the red heart rate (HR) trace outlining metabolic effort. On several of the longer climbs threshold levels of HR are indicated which imply the use of carbohydrate as a fuel source. HR is heavily correlated with aerobic power, as seen in the image above. The period before the first climb the HR is at significantly lower levels, around 65% of maximum, where fat metabolism is much more prominent as a substrate. Herein lies the paradox, when exercising at low intensities the body can use fat as a primary fuel source. Coupled with a low fat diet the body improves its ability to burn fat, thus a performance effect can ensue. During base building phases of fitness a LCHF diet may be superior to human performance as the intensities are usually low and large amounts of training volume indicated.
However as the graph below outlines much of the intensive aspects of the ride were done at intensities around 95% of HR maximum. The substrate useage here is heavily dependant on carbohydrate for energy metabolism. Energy metabolism uses glycogen to fuel exercise. Carbohydrate is metabolised quickly by a working body. However we only have around 1-2hrs of Glycogen available before we begin to falter, or ‘hunger-knock’. Therefore intermittent maximal efforts require a consistent source of simple sugars in order to sustain efforts beyond the 1-2hr mark. Failing this ingestion of simple carbs a reduction in HR is often indicated and a corresponding reducition in power imminent. This is the body preserving itself in the face of a lack of resource availability.
Whilst a HFLC diet can increase performance and improve other physiological variables when training for long steady state events, the LCHF diet if replicated during training may result in a reduction in performance when maximal efforts are required across a long time frame.
The graph above also outlines repeated sprints performed toward the end of the ride as evidenced by the spikes in wattage (pink line) and HR: 6x 20second sprints, with 2mins recovery between. This effort requires predominantly carbohydrate for fueling and recovery between efforts. The reason for this is again best outlined in the graph below.
What can be seen here is the advent of each 20second sprint causes a delayed spike in HR for the 30seconds AFTER the sprint effort. This occurs as the body is attepting to metabolise products of anaerobic exercise (the 20second effort can be completed without much of an increase in HR, therfore is AN [without] AEROBIC [oxygen]), and also refuel muscle groups so the anaerobic system can be best utilised again in the future. Here the HR trace trends up wards between each subsequent interval and rises sharply after each interval before taking around a minute to reach a more fat-burning level. So the result of intermittent sprint efforts is heavily dependant on carbohydrate availability especially given the fact these sprints occured at the 4hr mark of the ride. They can only be performaned successfully if carbohydrate stores are available.
So how much carbohydrate is required when riding? Around 60-80gramms of carbohydrate per hour is the upper limit of ingestion for most humans. We would therefore recommend, if riding for longer durations at intensities where maximal metabolic effort is required toward the latter stages of the ride, a rider consume the recommended dosage from the 1.5hr mark onwards, if not sooner.
With respect to dietary advice, well that is more complicated owing to an increase in studies outlining the positive effect of LCHF diets on endurance potential. Traditional research outlines a diet inclusive of large amounts of quality carbohydrate. More research is required to best outline how diet can best be used to improve training during different phases of competition.
Most importantly maximal exercise occurs within the presence of glycogen, which is best sourced from carbohydrate metabolism. So if you want to go fast, carbohydrate is implicated in success.
This article is published by Brad Hall.
-Brad has a degree in Exercise Science and Psychology (Hons.) AMAPS
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