Insulin

Insulin is a peptide hormone that regulates cell membrane permeability and helps move glucose and other substances into cells.^{1Rippetoe et al., Practical Programming for Strength Training, 3rd ed., p. 98.} It supports recovery by helping replace depleted glucose and amino acids.^{1Rippetoe et al., Practical Programming for Strength Training, 3rd ed., p. 98.} Its role in muscle protein synthesis is permissive rather than stimulatory, with its main effect on lean mass coming from reduced muscle breakdown.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 236.}

After exercise, insulin stimulates greater glucose uptake, so eating carbohydrate soon afterward replenishes glycogen faster.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 237.} Delaying carbohydrate by 2 hours can reduce the resynthesis rate by as much as 50%.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 237.}

In practice

• Schoenfeld says spiking insulin with post-workout carbohydrate isn’t necessary for muscle growth unless the client trained fasted; a mixed meal keeps insulin elevated for 3 to 6 hours or more.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 237.}
• Helms and colleagues note that an appropriate amount of protein raises insulin to the same level as carbohydrate.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 132.} A 45 g dose of whey isolate raises it enough to maximize net muscle protein balance.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 237.}
• Hypoglycemia is the principal exercise risk for clients with diabetes because exercise increases both injected insulin absorption and muscle glucose uptake.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 552.} ACSM says the risk is highest during and immediately after exercise but hypoglycemia can occur 12 hours or more afterward, requiring food or medication adjustments mostly in insulin users.^{6Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 665.}
• Advice on insulin dosing or meal timing belongs to the client’s physician.^{7Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 553.}