This is an excerpt from Science and Application of High-Intensity Interval Training.
It's no secret that we all have to eat to survive. You wouldn't be reading this otherwise. Nutrition provides needed energy, the water that makes up the body, as well as everything we need to run our bodies (micronutrients, etc.). While this book is not about nutrition, nutrition clearly has an influence on the physiological effect of an HIIT bout, which is why we are covering it. While there are likely other subtle effects that can be caused by manipulating other dietary nutrients, including supplements, we will focus on what we believe are the key nutrient manipulations influencing physiological effects. These include the dietary carbohydrate availability, hydration status, and caffeine intake. Chapter 7 additionally explores some of the common pitfalls and detrimental effects a poor diet has on performance. Like environment, nutrition is an additional area that can help us make further benefits in training, if well manipulated.
Carbohydrates are plant-derived sugars that come in a variety of forms. At the simplest level, and the level to which all carbohydrates are broken down eventually, is the compound glucose, a six-carbon molecule. Glucose is a primary metabolic fuel used by all cells to give us energy. Specifically it is the fuel of use for the process of anaerobic and aerobic glycolysis (chapter 3).
Notwithstanding the profound individual variance in the blood glucose response to the consumption of foodstuffs, there are some general principles we might appreciate around the topic of nutrition, acknowledging that refinement and optimization will exist at a monitored individual level. In the context of most athletes habituated to a mixed-Western diet high in carbohydrates, the act of lowering carbohydrate availability can create stress on (among others) muscle and nerve cells. The resultant lowered level of blood glucose, termed hypoglycemia, acutely creates a perceived feeling of fatigue and a reduced motivation to exercise. Ultimately this global effect of the lowered basal blood glucose level reduces systemic central nervous system activation. Under such conditions, acute high-intensity exercise capacity is typically reduced.
While high-intensity exercise capacity is typically lowered in the acute state, such a state can create an advantaged adaptive condition. Lower blood glucose concentrations reduce glycolytic capacity, which drives AMPk signaling for associated benefits (mitochondrial biogenesis and enhanced fat oxidation rates; chapter 3). The condition also makes training harder, without increasing mechanical (musculoskeletal and neuromuscular) load, which is something we often want during a training session. Alternatively, the condition can be nonoptimal in the context of the training condition, and could acutely impair aspects such as mood, confidence, coordination, agility, judgment, game intelligence, learning, and so forth.
A number of methods can be used to lower carbohydrate availability. The most obvious is to lower carbohydrate consumption in the diet. This can be challenging for many athletes as it requires nutrition knowledge, and as mentioned, the individual response to foodstuffs is extremely variable. The social aspect of food (i.e., habits, eating what others eat, eating what someone serves you, etc.) and access to appropriate foods (low-carbohydrate foods can be difficult to source in many places) additionally make this intervention difficult. Typically, when lowering carbohydrate content from the diet, additional calories need to come from somewhere, and generally speaking this largely will come from fat, a concept that can be difficult for many to appreciate based on tradition, ingrained mind models, or instilled beliefs surrounding the effect of dietary fat on health. Regardless, a number of methods can be used to manipulate systemic levels of carbohydrate content or blood glucose, including eating low-carb high-fat (LCHF), fasting, and sleeping low.
Low-Carbohydrate High-Fat (LCHF) Diet
The low-carbohydrate high-fat diet is just what it sounds like. With protein levels similar to that of a typical mixed Western-based or high-carbohydrate diet (about 20%), fat calories are increased toward 70%, with carbohydrate content falling to 10%, or below 100 g in a given day. The LCHF diet has received considerable attention in social media, but few studies have assessed its effects. In most studies that have examined the impact of an LCHF diet on high-intensity performance, the time course has been relatively short (3 d to 3 wk). For example, when Havemann et al. examined high- versus low-carbohydrate diets over 6 d in cyclists, overall 100 km performance time was not different; however, high-intensity 1 km sprint performance during the time trial was impaired on the high-fat relative to the high-carbohydrate diet. As well, the authors found significant increases in HR and effort perception with the LCHF diet after 6 d. However, as with training adaptation, which requires long periods of adaptation, dietary adaptation may follow a similar time course. To address this discrepancy in the literature, Cipryan et al. recently had recreationally trained participants change from their habitual mixed Western-based diet to a ketogenic (<50 g CHO) diet over a 4 wk period. After this 4 wk period, performance and cardiorespiratory responses during a graded exercise test and HIIT were not impaired in the LCHF diet group. The LCHF group did, however, substantially increase their rates of fat oxidation. While more research is needed, it appears that short-duration time course adaptation periods can compromise HIIT performance, with adaptation requiring at least 4 wk. Additionally, LCHF diets substantially enhance fat oxidation, which is of importance for fatigue resistance and prolonged exercise performance.
Fasted training is simply that—training in the fasted state. This can occur any time we decide not to eat for a given time period. Under the fasted condition, blood glucose and insulin levels drop and fat oxidation rates increase to meet the energetic demands. When we train under such a condition, we drive further the fat adaptations previously mentioned with the LCHF diet and with high-intensity training even more so (increased AMPk signaling). Anecdotally speaking, this is a common approach to training for many athletes, especially endurance specialists.
When we feed in the evening (typical) and we go to sleep, we are in effect fasting. When we wake, still in the fasted state before breakfast, we can train without breakfast and we are in effect typically already 8 h fasted. Lowering carbohydrate content in the evening meal, which can subsequently lower muscle glycogen levels, termed sleeping low, can enhance the adaptations further. Indeed, Marquet et al. have shown significant improvements in submaximal cycling economy, supramaximal cycling capacity, and 10 km running time in trained endurance athletes using such an approach.
We all know that athletes train, and many more than once in a day. When training occurs without refueling carbohydrate levels after the first session, muscle and liver glycogen levels may be only partially restored, and as a result, the same adaptations are driven in the subsequent session. As such, the signals for increased mitochondrial biogenesis (i.e., AMPk; chapter 3) and fat oxidation rates are enhanced.
When athletes train, specifically when they train in the heat, it's often emphasized that they need to maintain hydration status. While that might be true in some contexts, there are times in the healthy athlete context where exactly the opposite approach can enhance a physiological state. Indeed, a lowered hydration status, which acutely lowers plasma volume, creates stress in a number of different areas, including the heart and the kidneys. The endocrine system responds by retaining sodium at the level of the kidney (heightened aldosterone and arginine vasopressin levels), water follows, and plasma volume is increased. The enhanced plasma volume can be highly beneficial in a number of contexts, from creating partial heat acclimation to enhancing stroke volume through increased ejection fraction, thereby enabling cardiac stability. For example, Garrett et al. showed that dehydration increased the desired adaptations from a short-term (5 d) heat acclimation protocol (90 min cycling in 35°C, 60% RH) in well-trained males. Aldosterone was enhanced more with dehydration, which was positively related to the plasma volume expansion, which tended to be larger in the dehydration condition. Thus, plasma volume expansion may be more pronounced with permissive dehydration and is one further factor that can be considered around HIIT to skin the cat. Of course, caution must be warranted when applying such maneuvers as physiological strain is likely to additionally be high, which could jeopardize other aspects of performance, such as skill development and tactical learning within the team sport context.
While there are, in our opinion, only a very small number of silver bullet solutions to solving the performance puzzle relative to the number of commercial claims, caffeine supplementation does show relatively consistent positive effects on exercise performance across a number of different conditions. In particular, caffeine couples well with train-low strategies to enhance acute HIIT performance. As described, under conditions of low-carbohydrate availability in athletes adapted to traditional Western diets (typical) or in any condition in which muscle glycogen content is substantially lowered (i.e., two-a-day training), HIIT performance (but not necessarily the acute signal/response) is compromised. In any such condition when HIIT is planned, caffeine supplementation, typically taken in a variety of forms (including coffee and tea) approximately 1 h prior to the session, will enhance HIIT performance and potentially the acute metabolic signal in the larger motor units recruited. For example, Lane et al. showed that caffeine ingestion (3 mg∙kg∙BM-1) taken 1 h prior to an HIIT session (8 × 5 min maximal sustainable pace, 1 min recovery) conducted in a low-carbohydrate state enhanced power output by 3.5%. Thus, such a strategy may be particularly useful in athletes reluctant to train low for various reasons.