Whey protein

Whey protein is a milk protein produced as a by-product of cheese making; it accounts for 20% of the protein in milk.^{1International Sports Sciences Association, Strength and Conditioning, p. 474.} Its three main supplement forms are concentrate, isolate and hydrolysate, with digestion becoming faster in that order.^{1International Sports Sciences Association, Strength and Conditioning, p. 474.} Concentrate can cause irritation in clients sensitive to lactose.^{1International Sports Sciences Association, Strength and Conditioning, p. 474.}

In short-term studies, whey stimulated muscle protein synthesis more than slower proteins such as casein and soy, thought to reflect its rapid delivery of leucine.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 225.} High-quality, fast-digesting proteins strongly stimulate synthesis during the first 3 hours after eating.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 225.}

In practice

• In younger people, 20 to 25 g of a high-quality protein that digests quickly and supplies plenty of leucine maximally stimulates muscle protein synthesis after lifting; older adults may need 40 g or more.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 228.}
• IPTA advises getting protein from whole foods first, citing a lack of well-controlled evidence that supplements directly improve athletic performance.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 545.} ISSA presents powder as an option for athletes who can’t meet their needs through food.^{1International Sports Sciences Association, Strength and Conditioning, p. 474.}
• IPTA notes that supplements aren’t FDA-approved for safety or effectiveness and cites a report finding heavy metals and other contaminants in many powders.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 545.} It advises reading labels and consulting a physician or sports dietitian.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 545.}
• Prescribing supplements is outside a trainer’s scope of practice.^{5International Sports Sciences Association, Fast Track Study Guide: Nutrition, p. 1.}

Serving size evidence

One trial found that 20 g servings of whey outperformed 40 g servings over 12 hours; a later whole-body training study found about 20% greater synthesis after 40 g, suggesting larger doses help when more muscle is worked.^{6Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 233.}