Investigating the role of ghrelin and hormones involved in energy metabolism on energy balance and body composition when varying diets and caloric contents are consumed could provide insight into the etiology of obesity and related diseases. In this dissertation, I investigated the effect of Atkins and AHA diets in normal weight and overweight women and men in both the fed and fasted states on the change in circulating concentrations of ghrelin (acylated and total), adiponectin, glucagon, growth hormone, insulin, and leptin before and after consuming lunch and over waking hours (07:00 to 22:00). Eight subjects, ages 20-32, participated in each study: four normal subjects with a normal BMI and four overweight subjects with an overweight BMI. Each subject received both diets by a crossover design. Two normal and two overweight subjects were assigned to each diet. The Atkins’ diet contained 10% of energy as carbohydrate, 45% of energy as protein, and 45% of energy as lipid, and the AHA diet contained 63% of energy as carbohydrate, 12% of energy as protein, and 25% of energy as lipid. Diets were isocaloric. Each diet was fed for 14 days, and then subjects switched to the other diet. Blood was taken every hour from 7 am to 9 pm on days 13 and 27 of the study. Plasma samples were analyzed for active and total ghrelin, leptin, insulin, glucagon, growth hormone, and adiponectin. Lunch results: in women, acylated ghrelin concentrations were influenced by the interaction of diet and weight. Men consuming the AHA diet had an increase in acylated ghrelin concentrations over the lunch period, whereas, there was no change in acylated ghrelin concentrations in men consuming the Atkins diet. Women and men consuming the Atkins diet had higher glucagon concentrations before lunch. When consuming the AHA diet, overweight women had a greater increase in insulin that of normal weight women. In overweight women, the change in insulin concentrations was greater when the AHA diet was consumed in comparison to the Atkins diet. Result for 14-hour study: acylated ghrelin did not have preprandial increases before breakfast and lunch with varied responses before dinner in overweight and normal weight women fed either the Atkins or AHA diets in both sequences. Plasma glucagon was greater after lunch through the end of the 14-hour period in women consuming the Atkins diet. Insulin concentrations were higher and had greater responses to meal ingestion in women consuming the AHA diet in comparison to the Atkins diet. Men consuming the AHA diet in period 2 had increased acylated ghrelin and total ghrelin concentrations in comparison to when the men consumed the Atkins diet. Insulin concentrations were suppressed in men consuming the Atkins diet in comparison to men consuming the AHA diet. Glucagon concentrations were higher in men consuming the Atkins diet in comparison to the AHA diet after lunch until the end of the study period (hours 7-14). Weight class affected some hormone concentrations different and thus women and men in overweight and normal weight classes were analyzed separately. Each class of macronutrients influences circulating hormone concentrations. Experimental design is critical in obtaining information on the relationship of hormones related to appetite and body composition. Our results indicate that current recommendations may not be appropriate for all individuals as men and women both normal weight and overweight have different hormone responses to the macronutrient content of the diet.