U.S. Olympic Swimmer Michael Phelps told ESPN that he eats between 8,000 and 10,000 calories a day. This is 4 times the amount of calories an average adult needs. Phelps must have a calorie-dense diet because of the large amount of calories he burns during high levels of physical activity he participates in when training and competing. University of Pittsburgh Director of Sports Nutrition Leslie Bonci, MPH, RD, estimated that, given his 6’4’’ height at 190lb build he would burn about 1,000 calories per hour when training. In comparison, the average adult does not do as much physical activity and therefore does not burn, or need to consume, as many calories. One’s lifestyle and body composition can inform us on what we should eat. A more specific term for this concept is personalized nutrition. Here, we investigate potential benefits and controversies regarding the use of personalized nutrition in athletic performance as well as overall health.