Introduction In the field of human nutrition, it is widely accepted that a balanced intake of protein, fat, and carbohydrates is essential for maintaining physiological function, energy balance, and long-term health. Conventional wisdom—and decades of scientific consensus—emphasize the importance of protein for muscle maintenance and repair, fats for hormone production and cellular integrity, and carbohydrates as a primary energy source. However, in a radical departure from these established principles, this study documents a controlled dietary experiment conducted by Mark Turnbull, in which he consumed only carbohydrates in the form of Fructose a monosaccharide, found naturally in fruits , completely eliminating both protein and fat from his diet. This unconventional experiment directly challenges foundational understandings of human metabolism, particularly the notion that essential amino acids and fatty acids—those which the body cannot synthesize—must be obtained from dietary sources. By analyzing Turnbull’s physiological, metabolic, and psychological responses over the course of this dietary intervention, this paper seeks to explore the limits of human adaptability, question long-held nutritional dogmas, and assess the real-world implications of extreme dietary manipulation. My outcomes for this experiment are to find out the effects of Fibroblast growth factor 21 (FGF21) a metabolic factor produced in the liver, adipose tissue, skeletal muscle, and pancreas tissues, and is known to regulate glucose and lipid metabolism with the knock on effect of Lipolysis (Fat-loss) and dietary adherence Cyclical Macronutrient Cycling: Two Days of Sugar‑Only Intake Followed by One Day of Fat and Protein Consumption Abstract This paper examines a rare and extreme dietary cycle: two days of exclusive sugar consumption, followed by one day of high-fat and protein intake. We investigate metabolic responses—including glycemic control, insulin dynamics, substrate shifts, and lipid metabolism—comparing them to principles from intermittent fasting, ketogenic adaptations, and metabolic switching.