PURPOSE OF REVIEW: Quality or quantity of food has been at the heart of the diet debate for decades and will seemingly continue for many to come unless tightly controlled studies are conducted. To our knowledge, there has never been an overfeeding study comparing the effects of multiple diets. RECENT FINDINGS: This study reports a case study of an individual who ate 5800 Calories per day of 3 different diets for 21 days at a time. The 3 different diets were low-carb, low-fat, and very-low-fat vegan. The weight gain over 21 days was 1.3 kg for low-carb, 7.1 kg for low-fat, and 4.7 kg for very-low-fat vegan. SUMMARY: In this n-of-1 study, consuming 5800 Calories/day of 3 different diets for 21 days did not lead to the same amount of weight gain. Further research should be conducted on how the human body gains weight with an emphasis on how different foods affect physiology. If these findings are replicated, there would be many ramifications for obesity treatment and healthcare guidelines.