What is ketosis?

Ketosis is a metabolic state in which the body shifts its primary source of energy from carbohydrates to fats. This transition typically occurs when carbohydrate intake is significantly reduced, leading to decreased availability of glucose. The body then begins to metabolize fatty acids, producing ketones as a byproduct. These ketones become an alternative energy source for many tissues, including the brain, which usually relies heavily on glucose.

In scenarios like fasting, prolonged exercise, or adhering to a ketogenic diet, the reduction in carbohydrate intake prompts this metabolic adaptation. Unlike the body's initial reliance on glucose, ketosis supports energy production through fat oxidation, illustrating a significant shift in nutritional metabolism.

The other options rest on different metabolic processes or phases. For instance, the first choice describes the condition of using carbohydrates for fuel, which occurs in its opposite state rather than ketosis. The third choice pertains to catabolism and protein synthesis, while the fourth relates to the digestive process rather than metabolic fuel utilization. Accordingly, the unique defining characteristic of ketosis lies in the burning of fat due to low carbohydrate availability, making the selected answer accurate.