Protein, creatine, etc.
--Whey Protein Powder
Most protein powders on the market feature whey protein concentrate or whey protein isolate as primary ingredients. Whey is a "fast-acting" protein, so most people are able to digest it easily. That makes it a prime choice for post-workout protein shakes as well.
After assessing a wide collection of research related to protein and exercise, the International Society of Sports Nutrition published an article in 2007 stating that having protein shortly after a workout is associated with fewer medical visits, decreased muscular soreness and better immune response to infections. Subjects who received a placebo after their workouts got sick more often and logged more total doctors' visits.
Supplementing a healthy diet and regular exercise with protein shakes has proven advantages, but it isn't the best option for everyone. The International Society of Sports Nutrition recommends choosing whole and natural foods over supplements like protein powders and shakes whenever possible, as whole foods provide greater overall nutrition and contain protective substances that can't be reproduced in supplements.