The investigators hypothesize that the low carbohydrate diet will suppress plasma active ghrelin and increase appetite-suppressing hormones to a greater degree and for longer duration than the low fat diet and will thereby reduce hyperphagia and increase satiety. The investigators also hypothesize that the low carb diet will improve hormonal and metabolic markers (fatty acids, amino acids and organic acids) of insulin sensitivity and inflammatory cytokine profiles of children with PWS.