Is high-intensity exercise without prior nutrition intake deleterious?

From the Guidelines

Performing high-intensity exercise without prior nutrition intake can be deleterious, as it may lead to hypoglycemia, characterized by a blood glucose level below 70 mg/dL, and potentially result in dizziness, fainting, and decreased athletic performance 1.

Key Considerations

• Carbohydrate intake: Consuming a high carbohydrate snack 3–4 h before exercise can mitigate the effects of reduced carbohydrate availability during exercise, particularly in the luteal phase 1.
• Glycogen depletion: High-intensity exercise without prior nutrition intake can lead to glycogen depletion, which can impair exercise performance and increase the risk of hypoglycemia 1.
• Hydration: Adequate hydration is also crucial, as dehydration can have devastating effects on endurance athletes, and a fine balance must be struck between fluid loss and consumption to avoid electrolyte abnormalities 1.

Recommendations

• The American College of Sports Medicine recommends consuming 30–60 g of carbohydrate during endurance events lasting longer than 1 h 1.
• Current recommendations dictate consuming 30–60 g/h carbohydrate for durations 1–2.5 h and perhaps > 90 g/h carbohydrate for exercise durations > 2.5 h 1.
• Carbohydrate loading, such as consuming 10–12 g kg−1 day−1 carbohydrate for 36–48 h before a competition, can improve performance in events lasting > 90 min 1.

From the Research

Effects of High-Intensity Exercise without Prior Nutrition Intake

• The study 2 found that high-intensity interval training (HIT) and moderate-intensity continuous training (MICT) do not lead to detrimental 24-hour blood glucose profiles in individuals with type 1 diabetes, even when exercised after an overnight fast.
• Another study 3 showed that 6 weeks of HIT improved aerobic capacity without a detrimental decline in blood glucose in people with type 1 diabetes.
• A study 4 compared HIIT and MICT protocols in individuals with type 1 diabetes and found that HIIT reduced hypoglycemic events compared to MICT.
• Research 5 also found that short-term HIIT improved cardiorespiratory fitness and tended to decrease blood glucose in overweight and obese young women.

Safety of High-Intensity Interval Training

• The study 2 concluded that HIT can be carried out after an overnight fast with no increased risk of hypoglycemia or hyperglycemia, if the pre-exercise glucose concentration is 7 to 14 mmol/L.
• The research 3 suggested that HIT may be the preferred training mode for some people with type 1 diabetes, as blood glucose levels remained stable during HIT in the fed state.
• A study 4 found that HIIT training resulted in greater glycemic stability, with a reduction of hypoglycemic episodes.
• However, a review 6 discussed the potential detrimental effects of HIIT on metabolic health, but this was not directly related to the issue of prior nutrition intake.