Final answer:
Ketogenesis is most active when glucagon is present, as it encourages the liver to release glucose and promotes the generation of ketones, especially during periods of low carbohydrate intake and reduced insulin levels.
Step-by-step explanation:
Ketogenesis is a metabolic process that is most active when the hormone glucagon is in circulation, not insulin or cortisol. Glucagon is a hormone produced by the pancreas that signals the liver to convert stored glycogen into glucose, which is released into the bloodstream to raise blood sugar levels. Conversely, insulin is released from the pancreas in response to elevated blood glucose levels and works to lower blood glucose by facilitating its uptake into cells. Cortisol, a glucocorticoid, also influences blood sugar levels by stimulating gluconeogenesis and the mobilization of fatty acids, but it does not directly activate ketogenesis. Therefore, ketogenesis is more closely associated with the presence of glucagon, especially when carbohydrate intake is low and insulin levels are decreased.