Relevance of the Postprandial Period and Exercise

The postprandial period – the timeframe following a meal – is clinically relevant to address in individuals with T1D [29]. Elevated peak postprandial glucose can negatively impact overall glycaemic management, as measured by HbA1c [29-31], and may contribute to the development of diabetic complications [30-34]. Meals can often result in hyperglycaemic spikes and increased variability, which can significantly impact daily life and diabetes-related stress. Therefore, managing peak postprandial glucose and post-meal glycaemic fluctuations is an important aspect of T1D management. Skeletal muscle plays a key role in maintaining blood glucose homeostasis, as it is a major tissue for glucose disposal both after a meal [35, 36] and during exercise [37-39].  Hence, postprandial exercise may be a good strategy to help with glucose management by reducing post-meal hyperglycaemic excursions. However, although exercising soon after a meal can help mitigate postprandial hyperglycaemia, it may also increase the risk of hypoglycaemia if not appropriately managed.

 

Post-meal Exercise in Individuals with T1D: Challenge Accepted!

From our clinical experience, post-meal exercise is probably one of the most challenging tasks related to exercise and diabetes. Engaging in physical activity shortly after a meal, when bolus insulin has been administered, can present additional challenges for glycaemic management in people with T1D [40, 41]. This is due to higher circulating insulin levels than the post-absorptive state (i.e., the period starting ~4-6 hours after a meal, when absorption of ingested food is largely completed) (Figure 3). During these periods of higher IOB, the exogenous glucose requirements to maintain stable glycaemia are higher, and the risk for hypoglycaemia may increase [41, 42]. Consequently, performing exercise during this timeframe can be particularly difficult for many people with T1D [40].

As a result, there has been growing attention to the impact of the prandial state [41, 43]. An update to exercise recommendations in T1D seemed warranted [41] as existing guidelines on post-meal exercise are based on limited studies and do not specify advice for different exercise modalities [4]. Therefore, as a first step, we reviewed the currently available literature on the glycaemic effects of postprandial exercise in people with T1D [40], including 20 studies investigating various exercise modalities. We found that all modalities caused consistent acute glycaemic declines, with the largest effect size for continuous exercise of high intensity and the smallest for HIIT, depending on the duration and intensity of the exercise bout. Pre-exercise mealtime insulin reductions created higher starting blood glucose levels, thereby protecting against hypoglycaemia, despite similar declines in blood glucose during activity between the different insulin reduction strategies. Lastly, nocturnal hypoglycaemia occurred after higher-intensity postprandial exercise, a risk that could be diminished by a post-exercise snack with concomitant bolus insulin reduction. We refer to the Boxes in the paper by Helleputte and Yardley et al. for all findings and clinical messages per exercise modality, as derived from the studies included in the review [40].

 

Performing Postprandial Exercise: Bring It All Together

Based on our previous research [40, 44] and experience, we can conclude that postprandial exercise of all modalities is very challenging. We have seen individuals with T1D who either develop postprandial hyperglycaemia with a low risk of hypoglycaemia during exercise or who have reasonable glucose control after the meal but low glucose levels during exercise. Therefore, people experiencing difficulties with mealtime glucose management might be advised to avoid exercise in the first two hours after a carbohydrate-rich meal, and post-absorptive exercise should perhaps be preferred, especially in the beginning.

Still, if the necessary precautions are taken, post-meal exercise can be practical, safe, and feasible: it can help lower maximal postprandial glucose excursions, cause a consistent and clinically relevant blood glucose decline during exercise, and – importantly – hypoglycaemia during or shortly after exercise can be avoided if insulin adaptations are performed adequately. In our opinion, there are always challenges with post-meal exercise [44], being: (1) immediate postprandial (pre-exercise) hyperglycaemia due to the insulin reduction for the corresponding meal; (2) if insulin is not reduced enough, possible hypoglycaemia during exercise due to higher levels of IOB compared to postabsorptive state; (3) postexercise hyperglycaemia (especially for short-duration exercise as food has not been entirely digested at the end of exercise, and continued digestion and absorption likely causes or aggravates the rise in glucose), and (4) nocturnal hypoglycaemia due to a carryover effect of increased insulin sensitivity and/or inadequate insulin reductions in the hours after exercise. Therefore, to effectively manage blood glucose levels during and after postprandial exercise, individuals with T1D must engage in a certain level of pre-planning [4, 45]. This includes considering the exact timing of exercise, insulin dosing, and target glucose concentration, mainly when exercise occurs (very) soon after a meal [4]. 

 

 

 

Recommendations: When it Comes to Post-Meal Exercise, It’s All About Planning

Pre-exercise blood glucose concentration and exercise timing should be considered to reduce the risk of both hypo- and hyperglycaemia around exercise [44]. Therefore, the optimal starting glucose and timing of postprandial exercise should be further explored – however, based on others’ and our work, these are our three main recommendations for postprandial exercise:

1.       Mealtime Insulin Reduction: When exercising post-meal, one should substantially reduce insulin with the pre-exercise meal to avoid exercise-induced hypoglycaemia, with the magnitude of the reduction depending on the exercise duration and intensity. Insulin administration should also be applied as early as possible before the meal, as a sufficiently long interval between insulin administration and the meal allows the insulin to develop its full effect towards the meal, minimising the postprandial rise and lowering the IOB during exercise. For specific recommendations on meal insulin reduction, we refer to the paper [44]. Still, in some individuals, consuming additional carbs is sometimes needed between 30 and 120 minutes after starting exercise to cover insulin on board.

2.       Pre-meal Blood Glucose: To achieve the recommended glucose target range of 126 to 180 mg/dL (7.0–10.0 mmol/L) to start exercise [4, 8], we would advise a pre-meal target range of 90 to 150 mg/dL (5.0–8.3 mmol/L), as glucose will substantially rise before exercise due to the meal (with a bolus insulin reduction) and likely also in the hour after exercise. This recommended target range is for exercise of moderate duration (i.e., 30 minutes); a higher starting glucose or perhaps additional carbohydrate intake would be advised for exercise of longer duration.

3.       Timing of Exercise: We would advise not waiting too long after the meal to start exercise (i.e., 45-60 minutes post-meal, or even earlier) [46] since a better effect on post-meal glucose excursion might be expected, and more favourable glucose profiles during and after exercise (i.e., less hyperglycaemia) [47] may be achieved. Starting post-meal exercise close to peak insulin action might seem contradictory in light of hypoglycaemia avoidance. However, if prandial insulin is reduced and the pre-meal blood glucose target range is respected, we believe that for many individuals with T1D, this timeframe can be a safe period to perform exercise as the combination of glucose absorption from the ingested food and bolus insulin simultaneously affects blood glucose [44]. As said earlier, the remaining challenge is avoiding hyperglycaemia around postprandial exercise (especially for short-duration exercise), and optimal exercise timing may also provide a strategy to lower this risk.

 

And What About Insulin Pumps and Automated Insulin Delivery (AID) Systems?

As previously mentioned, insulin pump users have more options for decreasing circulating insulin before, during, and after exercise by adjusting basal rates or target values. However, while automated insulin delivery (AID) systems have demonstrated satisfactory glucose outcomes for different activities in controlled situations [48], postprandial exercise remains a big challenge, even for those using this advanced technology [49, 50]. Unadjusted, automatically delivered insulin boluses in response to rising glucose levels detected by the system will result in elevated circulating insulin levels. This, in turn, can cause rapid glycaemic declines during postprandial exercise [51, 52]. Recommendations for using AID systems around (postprandial) exercise will be discussed as part of Factor 7 (Technology).

Factor 6. Previous Events and Activities and Their Impact on Glucose Management During Exercise

Before you start exercising, people with T1D should consider the following questions:

“When did I last experience significant hypoglycaemia?”

and

“How long ago did I last exercise?”

These questions help account for the potential impact of these previous events on glucose levels during an upcoming workout. Sometimes, it may be better to delay planned physical activity until the next day, especially if there was recent severe hypoglycaemia or strenuous, long-duration exercise.

 

A Recent Episode of Hypoglycaemia

Antecedent hypoglycaemia refers to a previous hypoglycaemia episode within the past 24 hours. This is important to consider, as it lowers the body’s glucose threshold for triggering a counterregulatory response to low glucose values, making another hypoglycaemic event more likely [53]. This can lead to a vicious cycle of repeated lows, making it even harder for the body to regulate glucose levels [54-56].

Hypoglycaemia-associated autonomic failure, or HAAF [57], reduces the body’s adrenaline response to low blood glucose and can cause hypoglycaemia unawareness, where people might not notice or feel the symptoms of low blood glucose. As a result, HAAF can lead to a loss of warning signs and impaired behavioural responses, such as eating carbohydrates to treat a hypo [55].

In summary, a previous episode of hypoglycaemia can make it harder for people with T1D to sense and respond to lows, increasing the risk of severe hypoglycaemia and perpetuating a vicious cycle.

 

A Previous Bout of Exercise

Exercising the day before or a few hours before your next exercise session can affect the body’s response to subsequent hypoglycaemia, similar to having experienced antecedent hypoglycaemia [58]. Galassetti et al. showed that individuals without T1D who did two 90-minute cycle sessions on Day 1 significantly reduced several neuroendocrine responses (hormone releases that help regulate blood glucose) on Day 2 [59]. Therefore, a previous exercise session can temporarily alter the body’s response to hypoglycaemia, which people with T1D should keep in mind.

 

Factor 7. Diabetes Technology

We could probably write an entire book about diabetes technology and sports, so this section is relatively short compared to the available information on this topic. Again, as always, these are only suggestions. They will not work for everyone, so they require individual planning.

 

Traditional Insulin Pumps (Non-Closed-Loop Therapy)

To avoid exercise-induced hypoglycaemia, here are some strategies:

 

Hybrid Closed-Loop Insulin Pumps: A Game Changer for Exercise

Hybrid closed-loop systems (which are the (currently) most advanced automated insulin delivery (AID) systems) can offer strategies to handle exercise better [67] and the issues mentioned above with postexercise glycaemic management [40, 60, 68].

Overall, most exercise recommendations for individuals using insulin pumps in an open-loop fashion also seem to be applicable for those using HCL systems, specifically to decrease bolus insulin dose at the meal if exercise will be performed shortly thereafter (i.e., postprandial exercise), an “exercise announcement” to the AID system, i.e., a temporary glucose target increase that is set 90–120 minutes before exercise onset, and to be still cautious of late-onset hypoglycaemia [51, 67, 69-71]. Considering postprandial exercise, exercise announcement combined with a 1/3 (33%) meal bolus insulin reduction was found to be the most effective for avoiding hypoglycaemia [51].

Still, the timing of meals and associated insulin boluses before and after exercise and starting glucose must be carefully considered when using an HCL system, even with elevated glycaemic targets during exercise. The recent T1DEXI study found that even in HCL users, aerobic, interval, and resistance exercise increased 24-hour TBR [3]. The difficulties experienced by these advanced systems regarding postprandial exercise and pre- and post-exercise hyperglycaemia need further investigation [51, 67, 68, 72]. For example, it was shown that in HCL users, the pre-exercise bolus insulin reduction strategies were also associated with increased time spent in hyperglycaemia around postprandial exercise [51].

The HCL algorithms for before, during, and after exercise need further refinement [67]. Integrating glucagon (dual-hormone pumps) also seems promising for reducing exercise-related hypoglycaemia [52, 73, 74]. However, its added value compared to single-hormone pumps for managing post-meal exercise remains to be demonstrated, as not all studies show a clear advantage [75].

 

Consensus Statement: AID Use Around PhysicalActivity in T1D

The scope of the topic on automated insulin delivery (AID) and exercise is enormous and could be covered in an entire book alone. Therefore, we would refer interested readers to a recently published consensus statement that provides the latest advice on using AID systems around exercise in people with T1D [76]. The consensus paper lists advice for AID use in different situations, including:

Part 2. Recovery and Glucose Management After Exercise in T1D

People with T1D face additional complexities in managing glucose levels after training or competition. For athletes with T1D, managing blood glucose levels after exercise and achieving adequate recovery are closely linked and equally important goals. While those without diabetes can focus solely on optimising physical recovery, those with T1D must carefully consider other factors much more closely, such as:

o    Transient post-exercise hyperglycaemia

o    Elevated risk of nocturnal hypoglycaemia

Below, we discuss nutrition and insulin adaptation considerations during recovery, focusing primarily on blood glucose management.

 

A.                Post-Exercise Insulin Adjustments to Avoid Hypoglycaemia

Given the limited research on insulin adaptations post-exercise, a general recommendation is to:

Practical Recommendations

For people with T1D, the optimal approach to insulin administration after exercise will vary significantly from one person to another, depending on their unique situation. After finishing a bout of exercise, the top priority should be stabilising blood glucose levels within the target range (4–10 mmol/L). Here are some general considerations:

o   Administer insulin correction if needed.

o   Deliver an additional bolus of insulin to cover the carbohydrates and protein consumed during early recovery. This strategy aims to stimulate glycogen resynthesis and muscle protein synthesis.

o   It is important to re-emphasise that while the rate of glycogen resynthesis is important, the athlete with T1D must balance this with the risk of hyperglycaemia and hypoglycaemia.

o   Athletes with T1D should also be aware that increased muscle insulin sensitivity after exercise can persist for up to 48 hours (or even longer after intense exercise bouts). Therefore, athletes should adjust their insulin doses based on individual increases in insulin sensitivity during periods of increased training or competition.

o   Athletes and their support team—whether a coach, trainer, or nutritionist—should aim to collaborate to develop a consistent post-exercise nutrition and insulin administration routine tailored to the athlete's specific needs and variables.

 

B.                Dealing with and Preventing Nighttime Lows after Exercise

As mentioned in Chapter 3, physical activity, especially aerobic exercise, has been shown to increase the risk of nighttime hypoglycaemia in people with T1D due to increased insulin sensitivity [15, 16, 77]. This is a big deal because reports have shown that as many as 50% of severe hypoglycaemia episodes occur overnight [78, 79]. A study by Reddy and colleagues [80] highlighted the effects on sleep quality, showing that people with T1D slept less on nights following aerobic exercise, with a similar trend seen after resistance training. Therefore, it’s essential to be aware of the risk of delayed or nighttime hypoglycaemia after exercise, especially after late afternoon workouts. Research on post-meal exercise also warns that late-onset post-exercise hypoglycaemia can occur overnight; hence, caution is always advised [40, 44].

 

 

Some Strategies for Preventing Nighttime Hypoglycaemia:

1.       Pre-Bedtime Snacks:

o    Consuming a Meal After Exercise: Campbell et al. looked into the effects of having a second, low glycaemic index meal after exercise [81]. In this study, evening exercise was performed 60 minutes after a meal (with only 25% bolus insulin), and a second meal (with a 50% insulin reduction) was eaten 1 hour after exercise. This helped prevent hypoglycaemia for up to 8 hours post-exercise but, unfortunately, did not avoid lows throughout the night. Also, the studies in our review on postprandial exercise indicated that a post-exercise meal with a 50% insulin reduction may prevent hypoglycaemia in the hours thereafter, especially for continuous long-duration exercise [40].

o    Pre-Bedtime Snack: Eating a pre-bedtime snack has been explored to reduce the risk of nighttime hypoglycaemia [81, 82]. While the effectiveness of this strategy is mixed, it may be worth trying based on individual needs and experiences, especially if night-time basal insulin is not (or cannot be) reduced.

 

2.      Basal Insulin Adjustments at Bedtime:

o    For people on multiple daily injections (MDI): Pre-bedtime basal adaptations (e.g., a 10% or 20% reduction in bedtime basal insulin – but only the shorter-acting ones such as insulin detemir) could be a strategy. However, this may have undesired effects the next day [83]. Moreover, such adjustments are not routinely supported for the newer long-acting insulin analogues (degludec) [84]. The precise amount of insulin adjustments will depend on the type, intensity, and timing of exercise.

o    Increase Carb Intake: Alternatively, some athletes with T1D might choose not to adjust their insulin delivery and consume carbohydrates at an elevated rate to maintain blood glucose concentrations in the target range.

 

 

3.      Advanced Technology for Diabetes Management:

 

Part 3. Further Exercise Suggestions for Active Individuals with T1D

Below, we provide some final practical tips for safely managing exercise if you live with T1D.

 

The Final Word

Managing blood glucose levels during and after exercise can be challenging for people with T1D. That’s why a personalised and well-thought-out approach is key. While the latest closed-loop technology is promising, it’s not perfect yet. Therefore, a comprehensive understanding of the factors influencing glucose levels, carbohydrate intake, and insulin management strategies is crucial for maintaining safe glucose levels during and after exercise.