Introduction

In the previous chapter, we explored the general landscape of low carbohydrate diets (LCDs) and their potential benefits for people living with type 1 diabetes (T1D). Now, we will delve deeper into the specific applications of LCDs in the context of exercise performance and adaptation. This is an incredibly captivating topic (in our opinion!), as striking the right balance could enhance training adaptations and aid in glucose management for elite and recreational athletes with T1D.

Despite ongoing debate among athletes, coaches, and sports scientists regarding low carbohydrate training, there is a notable lack of research addressing athletes with T1D. Studies conducted on individuals without T1D have shown the effectiveness of a periodic “train low, compete high” approach. This strategy involves intentionally undertaking selected training sessions with reduced carbohydrate availability to activate certain molecular pathways, thereby enhancing skeletal muscle adaptation (as reviewed [1-3]).

Under the right circumstances, endurance training with depleted muscle glycogen stores (“training low”) has been shown to increase the expression of numerous genes encoding mitochondrial enzymes, enzyme protein content, and activity. Compared to training with normal or high glycogen stores, it also increases lipid (fat) oxidation rates during submaximal exercise [4-6].

For athletes with T1D, training with reduced carbohydrate availability may offer some other advantages. It could improve daily glycaemic management, characterised by increased time in range and decreased glycaemic variability. Additionally, it could enhance skeletal muscle oxidative capacity. For example, endurance training sessions performed in the fasted state, with low insulin levels on board, could theoretically boost training adaptations through elevated fat oxidation rates and promote a healthier body composition, characterised by reduced fat mass and increased lean mass.

However, we still don’t know much about the long-term effects of training with reduced carbohydrate availability and adopting LCDs on skeletal muscle insulin sensitivity, metabolic health, training adaptation, exercise performance, and glycaemic management during and after exercise in T1D. This chapter aims to shed some light on these important areas, providing insights into how LCDs can be safely and effectively integrated into the training regimens of athletes with T1D.

 

A Background to Carbohydrate Restriction Strategies for Endurance Athletes with T1D

Navigating carbohydrate requirements for athletes with T1D is a multifaceted challenge, influenced by training status and the specific demands of their events. The numerous factors that can affect blood glucose levels in athletes with T1D make estimating carbohydrate needs a complex task. A wealth of scientific evidence in individuals without T1D suggests that a high-carbohydrate diet benefits endurance performance [7-12]. However, periodically engaging in endurance training with reduced carbohydrate availability, followed by competition without carbohydrate restriction (i.e., ‘training low’ but ‘competing high’), has been shown to promote superior training adaptations in skeletal muscle when compared with high carbohydrate availability in athletes without diabetes (reviewed in [2]). These adaptations often include increased cell signalling, gene expression, and training-induced increases in skeletal muscle oxidative capacity. However, these adaptations do not always translate to improved exercise performance [6, 13-16].

Developing a nutritional strategy for elite or high-level athletes with T1D is complex, given the already challenging task of managing blood glucose levels. Athletes require adequate carbohydrates to fuel each training session, enabling them to maintain the intensity necessary to elicit training adaptations. Moreover, they often need to reduce fat mass and preserve or increase lean mass in preparation for competition [17, 18].

The athlete and their coach must be mindful of the training load and competition schedule, as this will impact daily carbohydrate requirements. Sufficient carbohydrate intake is necessary to restore muscle glycogen stores between sessions, which is particularly important for athletes undertaking long-duration or high-intensity training sessions and those competing more than once within a short period [19, 20]. Weight management can be especially challenging for athletes with T1D, as high levels of insulin will increase carbohydrate needs during exercise and suppress the use of endogenous fuel stores. This is particularly important to consider when competing in certain events such as long-distance running or cycling, where the aim is to improve power-to-weight ratio, or in sports with specific weight categories, like boxing and martial arts.

A chronic LCD probably isn’t suitable for high-level athletes with T1D, given the energy demands of their heavy training load (similar to athletes without T1D). However, several ‘train low, compete high’ strategies have been investigated in individuals without T1D [2], including:

Some of these strategies may be beneficial for individuals with T1D, while others may not be practical for reasons related to glucose management. Athletes with T1D must ensure that training intensity is not compromised while creating a metabolic environment that promotes endurance characteristics and performance. They must also maintain safe blood glucose levels that do not impair performance (a lot to think about, we know!).

As highlighted by Bartlett et al. [1], training with low glycogen stores has limitations, even for athletes without T1D. Exercising in a low-carbohydrate state can make it difficult to maintain training intensity, potentially impairing adaptation. Additionally, regularly exercising without or with low carbohydrates can make it harder to efficiently use carbohydrates as fuel when you increase your intake [21]. This could negatively impact performance during competition.

Lastly, any muscle adaptations resulting from these ‘train low’ strategies may bring additional metabolic benefits for individuals with T1D by improving overall skeletal muscle health. This is important because there is evidence that skeletal muscle health – including mass, function, and metabolism – is impaired in people with T1D (see Chapter 8), making it a key therapeutic target [22-25]. Monaco and colleagues proposed the hypothesis that T1D may be a form of accelerated ageing in skeletal muscle [23] due to impairments in mitochondrial structure and function. Although it’s important to note that more research is needed to confirm these theories.

Improvements in skeletal muscle health resulting from augmented mitochondrial protein expression may be particularly important for athletes with T1D, irrespective of any sports performance benefits. This suggests that investigating the effects of “train low” strategies is worthwhile, even if it remains speculative and requires thorough testing. For example, improved cell signalling leading to enhanced mitochondrial structure and function may benefit sports performance and/or metabolic health, as some studies have shown that mitochondrial structure is impaired in people with T1D [22, 26]. The following section outlines specific “train low” strategies and how they might feasibly be incorporated into the training regime of an athlete with T1D.

 

Practical Considerations for Carb Restriction Strategies in Athletes with T1D

In the following sections, we will discuss three popular carbohydrate restriction strategies: fasted exercise, twice-per-day training, and “sleep low, train low”. Our primary focus will be on endurance athletes because this is where these diets are most commonly used, although we recognise this as a simplified approach. Even among elite athletes, training loads can vary significantly, with weekly training hours ranging from 10 to 25 hours and individual sessions lasting between 1 and 6 hours for cyclists and triathletes. Therefore, any approach to training and nutrition will need to be tailored to the individual.

Unfortunately, little research exists on LCDs and sports in individuals with T1D. The adaptations to exercise under carbohydrate-restricted conditions have not been thoroughly studied in people with T1D. As a result, much of this section is based on what we know from people without T1D.

 

1.      Fasted Exercise Training

Fasted exercise, where breakfast is consumed after a morning training session, has been suggested to lead to superior metabolic adaptations compared to training in the fed state (see Wallis and Gonzalez [27] for a detailed review of studies in people without diabetes). Exercising in the overnight fasted state, as opposed to the fed state, has been associated with several responses that may contribute to long-term improvements in lipid and glucose metabolism [28].

Feeding before exercise lowers post-exercise adipose tissue gene expression and reduces lipid utilisation during exercise due to higher insulin concentrations [29-37]. In contrast, fasted exercise promotes:

 

Fasted Exercise and Insulin Management in T1D

Individuals with T1D can slightly modify fuel selection during exercise based on the timing of insulin administration. Reducing or withholding insulin can increase lipid mobilisation and reduce carbohydrate reliance [43] but may also increase glycaemia and ketone production [44].

Emerging evidence suggests that regular training in the fasted state benefits metabolic adaptations in people without T1D [4-6]. Key findings include:

 

Fasted Exercise and Glycaemic Stability in T1D

In individuals with T1D, blood glucose concentration is more stable following fasted exercise performed in the morning [45], possibly due to:

Future Directions on Fasted Exercise and T1D

While fasted exercise represents a promising strategy for people with T1D, research in this area is still limited [27]. Further exploration is needed to determine the potential benefits and optimal implementation of fasted exercise in individuals with T1D, particularly elite athletes with prolonged training sessions.

 

2.     Twice-Per-Day Training

Twice-per-day training is an intriguing and potentially powerful approach. This strategy involves a morning training session followed by several hours of reduced carbohydrate intake before the second training session later that day. This allows the second training session to commence with reduced muscle glycogen, essentially training in a glycogen-depleted state.

Benefits of Twice-Per-Day Training in Athletes Without T1D

In individuals without T1D, 3-10 weeks of twice-per-day training has been shown to yield impressive results compared to once-per-day training, including:

Hansen and colleagues [50] were the first to investigate the twice-per-day approach using a one-legged kicking model. They compared once-per-day training against training twice-per-day, every other day, with the second exercise bout in the twice-per-day condition performed with low muscle glycogen. Subsequent studies on a whole-body level by other researchers [14, 16] confirmed the positive results of twice-per-day training.

 

Challenges and Considerations of Twice-Per-Day Training for Individuals with T1D

For individuals with T1D, twice-per-day training is likely to be challenging from a glucose management standpoint and has yet to be tested in a research environment. Key considerations include:

1. Increased risk of hypoglycaemia: Antecedent exercise (a previous bout of exercise) blunts counter-regulatory responses during subsequent exercise, increasing the risk of hypoglycaemia [52].

2. Glucose monitoring: Twice-per-day training would require regular glucose monitoring throughout training days and increased vigilance overnight to prevent nocturnal hypoglycaemia. Using CGM and AID systems with prolonged activation of exercise modes after training can make this increasingly safe and feasible.

Currently, there is no hard evidence that multiple exercise sessions per day increase the risk of nocturnal hypoglycaemia compared to a single session. That being said, with careful planning, sufficient carbohydrate intake during the second training session, adequate insulin reduction, and close CGM monitoring, athletes with T1D may be able to train twice daily, i.e., allow a second training in the desired reduced muscle glycogen state.

While the potential benefits of twice-per-day training are compelling, the practical challenges and risks associated with glycaemic management must be carefully considered. Future research should focus on developing safe and effective protocols that allow athletes with T1D to leverage the advantages of this training approach while minimising the risks. By doing so, we can provide these athletes with evidence-based strategies to enhance their performance and overall health.

 

3.     Sleep Low, Train Low

The “sleep low, train low” approach involves exercising in the evening, restricting carbohydrate intake overnight, and then exercising in a fasted state the following morning [53]]. In this context, “low” refers to muscle glycogen stores, not hypoglycaemia.

Primary Benefits of Sleep Low, Train Low:

Studies in Athletes without T1D

Marquet et al. [55, 56] found that 1-3 weeks of sleep-low training for elite-level cyclists and triathletes without diabetes improved cycling efficiency, 20 km time trial performance, and 10 km running performance compared to traditional high-carbohydrate training.

Feasibility of Sleep Low, Train Low for Individuals with T1D

No trials have examined the sleep-low, train-low approach in individuals with T1D. Regardless of its potential to enhance training adaptation, this strategy is likely to be challenging for people with T1D due to the high risk of nocturnal hypoglycaemia. However, advances in technologies such as CGMs with low glucose alarms and automated insulin device (AID) systems offer enormous potential for incorporating these strategies, particularly when considering overnight glucose management.

 

Conclusions and Future Directions on Carbohydrate Restriction and Sports Performance in Athletes with T1D

People with T1D face numerous daily challenges in managing their blood glucose levels and effectively navigating these complexities. Nutritional guidance for people with T1D should be firmly grounded in robust scientific evidence and reliable methodologies.

Research in people without diabetes has shown that specific ‘train low’ strategies can be advantageous for metabolic adaptation compared to constantly training with high carbohydrate availability. However, these methods have yet to be evaluated in athletes with T1D, and some strategies present additional challenges that may render them unsuitable for these individuals, regardless of potential performance benefits.

If research in this area is to be pursued, it is paramount to prioritise glycaemic management during and after exercise, as well as overall metabolic health and performance outcomes. Comprehensive studies should be conducted to analyse 24-hour glucose profiles, along with other factors such as ketone monitoring, changes in lipid metabolism, body composition, and HbA1c following periodised training strategies in individuals with T1D.

Despite the limited research on this topic, understanding the complexities of carbohydrate restriction strategies in the context of T1D is vital for optimising athletic performance and overall health. By exploring these strategies and their potential applications, we can empower athletes with T1D to take charge of their health and strive for their performance goals. Advances in technology, such as AID systems, offer enormous potential for incorporating these strategies, particularly in relation to glucose management overnight.