A recent study investigated the effects of progressively increasing training on various health and performance measures, shedding light on the potential drawbacks of excessive training. The research suggests that going overboard with your workouts may have adverse effects on your overall well-being.

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Methodology

To conduct the study, the researchers enlisted 11 participants, both male and female, who engaged in regular endurance and strength training. While not professional athletes, these individuals maintained an active lifestyle. The participants underwent a 4-week training program that incorporated high-intensity interval training (HIIT) using a cycle ergometer. Each week, the number of sessions and the duration of some sessions increased, resulting in a higher weekly training volume. A recovery week with reduced training volume was also included. Throughout the study, the participants underwent various assessments before and after the program, as well as at the end of each week, including blood samples and muscle biopsies.

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Findings

As expected, the overall training program led to performance improvements, evident in increased power output. However, while performance consistently improved on a weekly basis, there was no further improvement in power output during the week with the highest training volume. This finding aligns with previous research, indicating that excessive training, also known as overreaching, can hinder training progress. Nonetheless, after the recovery period, performance reached its peak, a phenomenon known as "super-compensation."

The study's primary focus was on examining changes in muscle mitochondria. Mitochondria, often referred to as the cellular "powerhouse," play a crucial role in generating the energy (ATP) required for aerobic or oxidative exercise. Most types of exercise training, including endurance training and HIIT, increase the number of mitochondria, as well as the proteins and enzymes responsible for energy production. These adaptations enhance performance by enabling greater energy production in the muscles.

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Mitochondrial Function and Overreaching

While many mitochondrial markers showed week-to-week increases, their growth plateaued or even declined after the week of excessive training. Several of these markers are associated with good health and are dysregulated in conditions such as type-2 diabetes. One notable marker is intrinsic mitochondrial respiration (IMR), which indicates the capacity of mitochondria to function and produce energy. Astonishingly, the week with the highest training volume resulted in a 40% reduction in IMR. Type-2 diabetics typically exhibit lower IMR levels, although it remains unclear whether diabetes causes impaired mitochondrial function or vice versa.

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Glucose Tolerance and Overreaching

To explore the potential impact of IMR changes on insulin sensitivity, the participants underwent an oral glucose tolerance test (OGTT). This test involves consuming a concentrated glucose solution and monitoring blood glucose levels. During the training program, the blood glucose response to the OGTT remained consistent from week to week. However, after the excessive training week, blood glucose levels rose, suggesting decreased effectiveness in removing glucose from the bloodstream. This decrease in insulin sensitivity parallels the underlying mechanisms of type-2 diabetes.The study revealed that excessive training not only caused molecular-level mitochondrial dysfunction but also impaired glucose tolerance, a significant marker of metabolic health. Furthermore, the improvement in exercise performance was blunted.

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Follow-up

Blood Glucose in Elite Athletes: In a secondary investigation within the same study, the researchers analyzed blood glucose levels in elite athletes to assess whether the observed changes with excessive training have real-world implications. Continuous glucose monitoring was employed to measure blood glucose levels 24 hours a day over a span of two weeks. The results demonstrated that while the average blood glucose concentrations throughout the day were nearly identical between elite athletes and healthy non-elite volunteers, there were significant differences. The elite athletes spent a considerably longer time in hyperglycemic (high blood glucose) and hypoglycemic (low blood glucose) conditions compared to the control group.

Prolonged periods of elevated blood glucose levels are associated with type-2 diabetes, making it intriguing that these elite athletes exhibited worse blood glucose control than the control group, despite engaging in more training and exercise, which typically improves insulin sensitivity. However, it is worth noting that elite athletes often engage in frequent overreaching during their training, a factor now understood to cause mitochondrial dysfunction and potentially affect blood glucose regulation.

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Conclusion

The study's findings indicate that individuals who are not high-level athletes are unlikely to engage in the same level of training as the participants who underwent excessive training for a week. The excessive training regimen involved daily sessions of high-intensity exercise, accumulating to a training volume exceeding 150 minutes per week. However, in this study, a week of low-volume training allowed most health markers and mitochondrial function to recover. For elite athletes who may regularly push their limits to maximize performance, intensive training is unlikely to pose a significant long-term health concern.

Previous research even suggests that former endurance athletes exhibit lower mortality rates and longer life expectancies than the general population.In summary, the study highlights that a period of overreaching can cause a decline in health markers, mitochondrial function, and glucose tolerance. However, careful management of training volume and incorporating recovery periods can help mitigate these negative effects, ensuring a balance between exercise intensity and long-term well-being.

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Scientific references

1. Flockhart M, Nilsson LC, Tais S, Ekblom B, Apro W, Larsen FJ. Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers. Cell Metab. 2021.
2. Ruiz JR, Fiuza-Luces C, Garatachea N, Lucia A. Reduced mortality in former elite endurance athletes. Int J Sports Physiol Perform. 2014;9:1046-9.