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Trommelen J, Groen B, Hamer H, de Groot LCPGM, van Loon LJ. *Exogenous insulin does not increase muscle protein synthesis rate when administered systemically: A systematic review*. *European Journal of Endocrinology.*
Background:
Insulin is widely known for its role in regulating muscle protein metabolism. However, the effectiveness of exogenous insulin in boosting muscle protein synthesis (MPS) has been debated, with inconsistent results in human studies. Some research suggests that insulin may be less effective when administered outside of a specific metabolic context, while others propose that it could be beneficial under certain conditions.
Objective:
The goal of this systematic review was to determine whether exogenous insulin administration increases MPS rates in both young and older adults.
Study Design:
This was a systematic review of clinical trials that assessed the impact of exogenous insulin on muscle protein synthesis. The analysis was divided into different models that excluded certain factors to better isolate insulin’s effect on MPS:
- Model 1: Trials with concurrent hyperaminoacidemia (high amino acid levels).
- Model 2: Trials with insulin-induced hypoaminoacidemia (low amino acid levels).
- Model 3: Trials with supraphysiological insulin concentrations (insulin levels much higher than normal).
- Model 4: Trials involving older, more insulin-resistant subjects.
Key Findings:
1. Amino Acids and Insulin Together: When both exogenous insulin and amino acids were administered, muscle protein synthesis was effectively increased. However, this effect was primarily attributed to the elevated amino acid levels, not insulin alone.
2. Hypoaminoacidemia and Insulin: Exogenous insulin administration without adequate amino acids led to reduced amino acid levels, which inhibited any potential insulin-stimulatory effect on MPS.
3. Supraphysiological Insulin Levels: At insulin concentrations exceeding 50,000 pmol/L, exogenous insulin did appear to enhance muscle protein synthesis. This suggests that insulin may have an effect when administered at extremely high levels.
4. Age-Related Factors: In older adults, the effectiveness of exogenous insulin in promoting MPS appeared to be diminished due to anabolic resistance, a common issue in aging individuals that limits the body's ability to build muscle.
5. Young, Healthy Adults: For healthy, young adults, the review concluded that exogenous insulin administered systemically does not increase muscle protein synthesis under typical conditions.
Conclusion:
The data suggest that while exogenous insulin can increase muscle protein synthesis when combined with amino acid supplementation, it does not independently promote muscle growth unless in specific contexts (e.g., extremely high insulin levels or the presence of hyperaminoacidemia). For young, healthy individuals, insulin alone does not seem to significantly affect MPS.
These findings provide important insights into the limitations and potential applications of exogenous insulin in muscle-building strategies.
Background:
Insulin is widely known for its role in regulating muscle protein metabolism. However, the effectiveness of exogenous insulin in boosting muscle protein synthesis (MPS) has been debated, with inconsistent results in human studies. Some research suggests that insulin may be less effective when administered outside of a specific metabolic context, while others propose that it could be beneficial under certain conditions.
Objective:
The goal of this systematic review was to determine whether exogenous insulin administration increases MPS rates in both young and older adults.
Study Design:
This was a systematic review of clinical trials that assessed the impact of exogenous insulin on muscle protein synthesis. The analysis was divided into different models that excluded certain factors to better isolate insulin’s effect on MPS:
- Model 1: Trials with concurrent hyperaminoacidemia (high amino acid levels).
- Model 2: Trials with insulin-induced hypoaminoacidemia (low amino acid levels).
- Model 3: Trials with supraphysiological insulin concentrations (insulin levels much higher than normal).
- Model 4: Trials involving older, more insulin-resistant subjects.
Key Findings:
1. Amino Acids and Insulin Together: When both exogenous insulin and amino acids were administered, muscle protein synthesis was effectively increased. However, this effect was primarily attributed to the elevated amino acid levels, not insulin alone.
2. Hypoaminoacidemia and Insulin: Exogenous insulin administration without adequate amino acids led to reduced amino acid levels, which inhibited any potential insulin-stimulatory effect on MPS.
3. Supraphysiological Insulin Levels: At insulin concentrations exceeding 50,000 pmol/L, exogenous insulin did appear to enhance muscle protein synthesis. This suggests that insulin may have an effect when administered at extremely high levels.
4. Age-Related Factors: In older adults, the effectiveness of exogenous insulin in promoting MPS appeared to be diminished due to anabolic resistance, a common issue in aging individuals that limits the body's ability to build muscle.
5. Young, Healthy Adults: For healthy, young adults, the review concluded that exogenous insulin administered systemically does not increase muscle protein synthesis under typical conditions.
Conclusion:
The data suggest that while exogenous insulin can increase muscle protein synthesis when combined with amino acid supplementation, it does not independently promote muscle growth unless in specific contexts (e.g., extremely high insulin levels or the presence of hyperaminoacidemia). For young, healthy individuals, insulin alone does not seem to significantly affect MPS.
These findings provide important insights into the limitations and potential applications of exogenous insulin in muscle-building strategies.