This study investigates the hormonal and metabolic responses during a 1-week fast in six normal subjects and two patients with maturity-onset diabetes. The findings suggest that the glucose-insulin feedback mechanism is the primary control process regulating the release of peripheral fuels to provide energy for metabolism during fasting. Key points include: 1. **Insulin and Glucose**: Insulin levels remained low but stable throughout the fast, with a strong correlation between insulin and glucose levels. Glucose turnover studies indicate that nearly all glucose produced is utilized by tissues not sensitive to insulin, such as the brain. 2. **Growth Hormone**: Levels of growth hormone varied significantly among subjects, with no clear correlation with other parameters. This suggests that growth hormone may not play a significant role in the fasting process. 3. **Metabolic Balances and Glucose Turnover**: Studies of metabolic balance and glucose turnover demonstrate the predominance of lipid as fuel and the reduction in glucose metabolism, which spares nitrogen stores by decreasing gluconeogenesis. 4. **Discussion**: The data highlight the importance of insulin in regulating the release of peripheral fuels during fasting. The findings also emphasize the need for maximal efficiency in reactions utilizing nitrogenous materials, suggesting that hormonal and metabolite-modulated control mechanisms are crucial for survival during prolonged fasting. 5. **Summary**: The study provides insights into the hormonal and metabolic responses during fasting, highlighting the role of insulin in regulating fuel release and the diminished glucose metabolism, which spares nitrogen stores.This study investigates the hormonal and metabolic responses during a 1-week fast in six normal subjects and two patients with maturity-onset diabetes. The findings suggest that the glucose-insulin feedback mechanism is the primary control process regulating the release of peripheral fuels to provide energy for metabolism during fasting. Key points include: 1. **Insulin and Glucose**: Insulin levels remained low but stable throughout the fast, with a strong correlation between insulin and glucose levels. Glucose turnover studies indicate that nearly all glucose produced is utilized by tissues not sensitive to insulin, such as the brain. 2. **Growth Hormone**: Levels of growth hormone varied significantly among subjects, with no clear correlation with other parameters. This suggests that growth hormone may not play a significant role in the fasting process. 3. **Metabolic Balances and Glucose Turnover**: Studies of metabolic balance and glucose turnover demonstrate the predominance of lipid as fuel and the reduction in glucose metabolism, which spares nitrogen stores by decreasing gluconeogenesis. 4. **Discussion**: The data highlight the importance of insulin in regulating the release of peripheral fuels during fasting. The findings also emphasize the need for maximal efficiency in reactions utilizing nitrogenous materials, suggesting that hormonal and metabolite-modulated control mechanisms are crucial for survival during prolonged fasting. 5. **Summary**: The study provides insights into the hormonal and metabolic responses during fasting, highlighting the role of insulin in regulating fuel release and the diminished glucose metabolism, which spares nitrogen stores.