Carbohydrate Requirements for Training and Competition: Adapting Strategies for Glucose Management and Meeting Fuel and Recovery Demands

A.     Carbohydrate Intake During Exercise: To Fuel or Not to Fuel? Given that everyone with T1D is unique and there is limited research on carbohydrate intake during long-duration exercise for those with T1D, there is no one-size-fits-all guide for insulin adjustments or carbohydrate intake during prolonged endurance exercise. Nevertheless, two key factors generally determine the amount of carbohydrate that an athlete with T1D needs:

1.         Blood Glucose Management: Carbohydrate intake may be required to maintain stable and safe glucose levels, primarily to avoid hypoglycaemia. This differs from athletes without T1D, who mainly need carbohydrates for performance alone.

2.         Performance: Fuelling the workout is essential for maintaining and optimising performance for a given exercise intensity.

In athletes without diabetes, if we look at extreme cases of highly trained endurance athletes, daily carbohydrate requirements can be very high (Table 1). Elite professional cyclists may adjust their daily carbohydrate intake from 2 to 10 grams per kilogram of body mass during training, depending on the goals of the specific training phase.

During extended endurance exercise, athletes need high amounts of carbohydrates to maintain the rate of carbohydrate oxidation necessary to sustain the exercise intensity and avoid glycogen depletion (muscle and liver), as this is a significant cause of fatigue (see Hearris et al. [23] for a detailed review on this topic).

For an athlete with T1D, the amount of carbohydrates required before, during, and after exercise will not only depend on energy balance, but also on blood glucose levels, the type, intensity, and duration of the exercise, and the level of circulating insulin. For example, for long-duration exercise, such as a 4-hour bike ride, it is perhaps safer to start with slightly elevated blood glucose levels (e.g., 8-10 mmol/L), very low circulating levels of insulin at the start, and begin carbohydrate intake (depending on the exercise intensity) as soon as glucose levels start dropping, followed by insulin administration if needed.

Table 1. Guidelines for carbohydrate intake (CHO intake targets per level of activity) by endurance-trained athletes, adapted from Burke et al. [21] and the position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine guidelines [22].

Note: 1) These suggestions are only for meeting energy needs and not for managing glucose levels. Therefore, they need to be adapted on an individual basis for people living with T1D to prioritise managing safe glucose levels. 2) The timing of carbohydrate intake throughout the day may be manipulated to promote high carbohydrate availability for a specific session by consuming carbohydrates before and during the session or during recovery from a previous session.

Based on our experience, many people with T1D who engage in endurance exercise develop their initial carbohydrate intake strategies using general nutritional guidelines developed from studies conducted in individuals without T1D [11, 24, 25]. They then refine these strategies for meeting energy requirements and glucose management based on personal experiences gained through trial and error [26, 27]. For instance, professional cyclists with T1D have been found to consume vast quantities of carbohydrates during races, which is in line with conventional recommendations (up to 60-90 grams per hour) [1, 28]. However, some athletes we have worked with have reported that following “standard recommendations” during exercise is difficult because simultaneously maintaining glucose levels within the target range is such a challenge. As a result, carbohydrate intake can sometimes be (relatively) too low for the given intensity as the athlete aims to avoid hyperglycaemia.

One strategy proposed for guiding carbohydrate needs during endurance exercise is the “reverse approach,” where the athlete consumes a fixed amount of carbohydrates tailored to the duration and intensity of the upcoming event and then builds their insulin dosing strategy around that [8]. Adolfsson et al. [8] put this reverse approach to the test and found that it helped maintain glycaemia during a 90 km cross-country skiing race in a group of athletes with T1D. However, this reverse approach may not be feasible for everyone, as it requires extensive pre-planning and can be disrupted by factors such as competition stress or unexpected weather conditions.

An interesting trial-and-error strategy we have tried with some endurance athletes with T1D is to aim for around 60 grams of carbs per hour (depending on the planned workout). Then, we estimate how much additional insulin this may need and based on this, the athlete will ‘micro-bolus’ around 0.5 to 2 units of insulin every 60 to 120 minutes during exercise to test the approach. They may need to adjust their carbohydrate intake to achieve reasonable glucose control. In the next session, based on the information obtained in the previous session(s), we then adapt the amount of insulin to meet the planned fuelling strategy (i.e., the amount of carbs needed for the given intensity, duration, and training goals). Notably, a common mistake is that patients start stacking insulin boluses if glucose levels are rising, which can lead to severe hypoglycaemia. Therefore, we recommend altering only one variable at a time (for example, adjusting the carbs first).

Lastly, while it is possible and perhaps even beneficial to perform some training in a state of low-carbohydrate availability (as will be discussed in Chapters 6 and 7), we want to stress that for endurance athletes with T1D, it is also important to train in a state of high carbohydrate intake regularly. You will only learn to manage high-carb intake while administering the right amount of insulin through training and experience in such situations!

 

B.     Carbohydrate Types: Glucose, Fructose, Galactose – Their Impact on Sports Performance and Glucose Levels

Carbohydrates exist in different forms, which affect how they are metabolised and impact blood glucose levels [29]. Monosaccharides, also referred to as ‘simple sugars’, are the basic units of carbohydrates and can’t be broken down further. Examples common in the human diet include glucose, fructose, and galactose. Glucose is the primary fuel source for most of our body’s tissues. These simple sugars combine to form more complex carbohydrates, such as disaccharides (e.g., lactose, maltose, sucrose, and isomaltulose) and polysaccharides (e.g., glycogen, cellulose, amylose, and amylopectin). Glycogen is the primary way glucose is stored in our bodies, mainly in the liver and skeletal muscles.  

The characteristics of carbohydrates affect how quickly they are digested, absorbed in the intestines, and metabolised by the liver. This, in turn, influences how fast they reach the muscles and how much they raise blood glucose levels. Different types of carbohydrates use different metabolic pathways, some of which are partially insulin-independent, such as fructose [30, 31].

Two main factors determine how a carbohydrate affects blood glucose levels:

1)       Its chemical structure.

2)       Other food, such as fibre, protein, or fat, consumed alongside it.

The Glycaemic Index (GI) ranks carbohydrates on a scale of 0 to 100, based on how quickly and how much they raise blood glucose levels after consumption. Pure glucose serves as the reference food with a GI of 100 [30]. Foods with a high GI are rapidly digested and absorbed, causing a rapid increase in blood glucose. Low GI foods score less than 55, mid GI foods score between 56 and 69, and high GI foods score over 70. Since glucose has a GI of 100, it is ideal for quickly treating hypoglycaemia.

A handful of studies have examined how different GI carbohydrates affect people with T1D at rest [32-34] and during or after exercise [35-39]. For example, one study found that consuming a low-GI carbohydrate (isomaltulose) before exercise led to a smaller increase in blood glucose and enhanced fat burning compared to consuming a high-GI carbohydrate (dextrose) [36]. Based on existing evidence, low-GI meals appear more effective at preventing high blood glucose and optimising fuel utilisation during exercise.

Low-GI meals can offer protection against hypoglycaemia for approximately 8 hours after exercise, which can be important. Low-GI snacks consumed before bedtime can also help reduce the risk of nocturnal hypoglycaemia following exercise [38]. However, rapid-acting carbohydrates are the best options for treating acute low blood glucose levels.

 

Fructose and Other Glucose Alternatives Other carbohydrates, such as galactose and fructose, can be alternatives to glucose for meeting energy needs. These sugars are metabolised in distinct ways (Figure 2):

Fructose has the same molecular formula as glucose (C6H12O6) but has a different structure, which means it is metabolised differently. After intestinal absorption, fructose is first metabolised in the intestine and the liver [31]. As a result, plasma fructose concentration remains low (<0.5 mmol/L) after fructose ingestion [40], contributing to its low GI of [18]. In cells, fructose converts into compounds like glucose, glycogen, and fatty acids [31, 40, 41].

Although galactose hasn’t been extensively studied in T1D (but studies are ongoing), fructose has shown promise in reducing the drop in glucose levels during exercise [42, 43]. Consuming fructose before exercise may benefit some people with T1D due to its slower impact on glucose levels and partial conversion into other energy sources like lactate and lipids. The steady, slow release of glucose from the liver following fructose intake helps maintain blood glucose stability during exercise while promoting fat utilisation [15]. A study comparing glucose-fructose co-ingestion to glucose alone confirmed this trend of higher fat oxidation [42]. Another study found that ingesting 20 grams of fructose 30 minutes before an hour of cycling at 50% VO2max reduced the risk of hypoglycaemia compared to water alone in people with T1D [43].

These findings suggest that, if consumed in the right way, fructose could offer an alternative strategy to optimise glucose management during exercise in people with T1D. This approach would promote higher fat burning, preserve glycogen, and lower the risk of hypoglycaemia. However, more research is needed to directly compare fructose and glucose, explore the effects of different exercise types, and consider the time of day at which the exercise is being performed.

As a word of caution, consuming large amounts of fructose alone (>50 grams, and as low as 25 grams in some individuals) may cause gastrointestinal distress [44]. Therefore, this approach should be evaluated individually, considering the spacing of doses or combination with glucose [42]. It is also important to note the potential dangers of chronic high-fructose consumption in the context of overeating and inactivity [40, 45, 46]. Chronically high fructose consumption in rodents can lead to insulin resistance, obesity, type 2 diabetes, and high blood pressure [47]. The evidence is less clear in humans but has been shown to cause abnormal blood lipids, metabolic dysfunction-associated steatotic liver disease (MASLD), and impaired liver insulin sensitivity [48], which is therefore linked to Metabolic Syndrome [49]. However, this evidence comes from sedentary individuals, while studies on athletes show contrasting results [50].