B. HYPOTHESIS The investigators hypothesize that overfeeding a high fat as compared to an isocaloric high carbohydrate diet influences the source of intrahepatocellular triglycerides. During a high fat diet, relatively more of intrahepatocellular triglycerides originate from peripheral lipolysis and less from DNL than during a high carbohydrate diet in the face of a similar increase in liver fat. It is also possible given the lack of previous overfeeding data comparing 2 different overfeeding diets that the high fat diet induces a smaller increase in liver fat than a high carbohydrate diet even in the face of an identical increase in caloric intake because a greater fraction of ingested fat is channeled to adipose tissue than the liver. The investigators also hypothesize that liver fat may increase more in carriers than non-carriers of the I148M variant in PNPLA3 during a high carbohydrate than a high fat diet.