Whey protein is rich in certain amino acids and low in fat. The key amino acids, the branched chain amino acids (BCAAs=leucine, valine and isoleucine) may help delay fatigue during endurance exercise. Another amino acid, cysteine, can be found in relatively high amounts in whey protein - compared to other protein sources such as soy or gelatin in which cysteine is lacking. Various protein groups found in whey protein have been cited as "immune stimulators".
Whey proteins can differ dramatically from one another depending on the processing method and the total protein content. For example, whey protein can exist as simple whey powder (30% or less total protein content), whey protein concentrate (30-85% protein) or whey protein isolate (90% or higher protein content). In the case of whey protein isolates (the most expensive type), two key processing methods, ion exchange filtration and cross-flow micro-filtration can remove different components of the total whey protein, resulting in end products with different taste, texture and functional properties. Whey proteins processed using the ion exchange methodology appear to retain the majority of the functional benefits associated with immune system maintenance. Enhanced resistance to infection and elevated glutathione levels (an antioxidant enzyme containing cysteine) have been noted in subjects consuming concentrated whey protein. Whey protein also contains lactoferrin, a protein that has been shown to possess bacteriostatic and bactericidal activity against microorganisms that can cause gastroenteric infections and food poisoning.
Whey protein has been used in a number of animal and human feeding studies, where it has shown benefits in promoting weight gain, elevating glutathione levels (an antioxidant), and preventing metabolic acidosis (although the same can be claimed for virtually any high-quality protein source). Whether or not the minor content differences between various whey proteins actually result in any appreciable differences in muscle gain in humans (their primary claim) has never been demonstrated.
There are no adverse side effects associated with whey protein.
Whey protein can be used as a general source of high quality, low fat protein in any diet. Those individuals who also want the supposed immune system benefits of whey protein, may want to consider the more expensive whey isolates produced by ion exchange filtration – be aware, however, that these claims are largely speculative and have not yet been adequately proven in human subjects. Individuals in this category may include athletes at risk for infection (during intense training or recovery) or anybody recovering from injury or illness. Those individuals simply looking for a high quality protein source to supplement their diet may want to consider one of the less expensive protein concentrates currently available, such as casein, egg, or soy.
Intake levels should be based on total caloric requirements, body weight and period of training. During intense training or recovery, you may want as much as 50% of your protein requirements to come from whey protein or other source of concentrated low fat protein (approximately 40 g/day for a 160lb man). As a general daily supplement, however, lower doses of whey, perhaps 10-20 grams per day, as part of an adequate intake combined with other protein sources, may be sufficient to deliver the biological benefits of whey. A useful combination strategy is to split protein intake evenly between high quality sources such as whey, egg, casein and soy proteins.