Improving the antioxidant capacity in an animal by increasing the level of glutathione in the blood or tissue of the animal comprising providing a composition containing beta-casein to the animal for consumption, where the beta-casein comprises at least 75% by weight of one or more beta-caseins not capable of producing beta-casomorphin-7 on enzymatic digestion. Uses include as an oxidant, for treating or preventing the symptoms of cancer, inflammation, kwashiorkor (protein deficiency), seizure, autism, Down's syndrome, chronic fatigue syndrome, Alzheimer's disease, Parkinson's disease, sickle cell anaemia, liver disease, cystic fibrosis, HIV, AIDS, infection, heart attack, stroke, and diabetes, avoiding or reducing the effects of aging, promoting the recovery of tissue following physical exercise, and promoting fertility.