Within HIIT Science, Chapter 4 describes how carbohydrate availability can be manipulated around HIIT sessions to alter the acute physiological strain and subsequent response. Indeed, it’s been repeatedly shown across numerous studies that dietary carbohydrate availability is crucial to be able to perform high-intensity exercise to one’s potential. This view is fundamentally described throughout nearly all sport physiology and nutrition literature (4). And while daily dietary carbohydrate intake recommendations have appeared to soften in recent years (14), targets in the past for athletes have been recommended as high as 12 g/kg/day (= 900g for a 75kg individual) (22).
In more recent times, many have come to appreciate that such a practice, performed repeatedly over time, and especially as athletes age, may contribute to negative health consequences (10,15,16).
Let´s pause a moment and go back to the original research that helped to establish our collective belief on the topic. In 1967, Bergstrom et al. (22), who devised the muscle biopsy technique, showed a close relationship between the depletion of the muscle’s glycogen stores and the occurrence of fatigue during exhaustive tests. A number of other investigations (3,5,12,13,18,19,21) supported the findings. However, as Martin always reminds us, context is God. When we look to these studies in detail, we can all agree that they are acute snapshots derived from carb-adapted humans.
Well, what about the case that’s more apparent today, where individuals are reported to restrict carbohydrate in their diet over long periods of time, resulting in a physiological state known as nutritional ketosis (24)?
To investigate the necessity of carbohydrate in the diet for performing HIIT, and specifically HIIT in the very high intensity exercise domain, our research group performed a 12-week study (8) examining the effect of a very low carbohydrate high fat diet (VLCHF, < 50g / day) on HIIT and 30-15 Intermittent Fitness Test (30-15IFT) performance in moderately trained young individuals. While there was some performance decrease in the 30-15IFT during the first 2 weeks, participants restored their performance in line with that of a control group shortly following (Figure 1).
Furthermore, in line with our previous study (7), there was no performance reduction revealed in a regular graded exercise test to volitional exhaustion (Figure 2).
The explanation may be easier than we think. Previously, the FASTER study by Volek et al. (23) showed that the key to performance with a VLCHF diet is sufficient adaptation time (and consistency). For those of our subjects on the VLCHF diet, fat represented the primary available fuel source. While intramuscular glycogen stores are likely to be initially reduced across the first two weeks of dietary adjustment, the system is likely to be rectified subsequently, where the liver forms adequate quantities of glucose needed to restore former levels of muscle glycogen, using a process known as gluconeogenesis. Here, the liver uses components of fat (glycerol), ketone bodies, lactate, pyruvate and amino acids to make glucose (6). As you might imagine, this so-called “metabolic flexibility” appears to allow for an alternative and a particularly very efficient way of keeping muscle carbohydrate availability high, as evidenced through the striking muscle biopsy data from Volek et al (23; Figure 3).
This occurring, remember, all without the need to consume high levels of carbohydrate. Indeed, carb intake restriction has been shown to be an effective prevention and treatment strategy across a number of health ailments, including neurodegenerative diseases (2), Type 1 Diabetes (20), Type 2 Diabetes (9), metabolic syndrome (11), non-alcoholic fatty liver disease (17) and cancer (1).
To summarize, the sufficient availability of muscle glycogen stores would appear to be an indisputable necessary prerequisite for high-intensity exercise performance. However, the necessity of attaining high muscle glycogen levels by way of dietary carbohydrate intake is in question. Indeed, this does not have to represent the only way of achieving it. As we have shown, long-term carbohydrate intake restriction, in parallel with high fat intake, may be just as effective, and without the worry that’s emerging from the growing evidence of health consequences associated with excessive carb intake. Also of note was that body mass was reduced by 3 kg, and fat oxidation rate was enhanced after 12 weeks for the group adhering to the VLCHF diet.
As we showed, HIIT “without” carbs, in the context of 12-weeks carb-restriction and adaptation, was not a problem.
About the author: Lukas Cipryan is an associate professor at the University of Ostrava (Czechia) specializing in exercise physiology, sport training and nutrition. As a former ice hockey player, he spent nearly 10 years coaching youth ice hockey players while being embedded within professional ice hockey and volleyball teams as a strength & conditioning coach. Currently, in addition to his academic role, he works for the football academy of FC Banik Ostrava. Follow him on Twitter @LukasCipryan.
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