Insulin sensitivity describes how sensitive the body is to the effects of insulin. Someone said to be "insulin sensitive" will require smaller amounts of insulin in order to produce a equivilent reduction in blood glucose levels than someone who has low sensitivity. Source
Insulin Insensitivity:
"Insulin resistance is defined clinically as the inability of a known quantity of exogenous or endogenous insulin to increase glucose uptake and utilization in an individual as much as it does in a normal population." (Lebovitz, 2001)
Ketosis:
"Athletes, clinicians, and practitioners are increasingly interested in the proposed performance and therapeutic benefits of nutritional ketosis (NK). NK is best operationally defined as a nutritionally induced metabolic state resulting in blood β-hydroxybutyrate concentrations of ≥0.5 mM. Most tissues readily metabolize ketone bodies (KBs), and KBs in turn regulate metabolism and signaling in both a systemic and tissue-specific manner. During fasting, starvation, or ketogenic diets, endogenous synthesis of KBs is amplified resulting in a state of NK." (Poff et al., 2020)
Ketone (body):
"Ketone bodies are produced by the liver and used peripherally as an energy source when glucose is not readily available. The two main ketone bodies are acetoacetate (AcAc) and 3-β-hydroxybutyrate (3HB), while acetone is the third, and least abundant, ketone body. Ketones are always present in the blood and their levels increase during fasting and prolonged exercise. They are also found in the blood of neonates and pregnant women." (Laffel, 2000)
Keto-Adaptation:
"However, being in ketosis does not necessarily mean the body is optimally using [ketone bodies] for fuel. To efficiently produce and use [ketone bodies] as an energy source, the body needs specific adaptations to accommodate the alteration in fuel preference from glucose to [ketone bodies]. This process has been termed keto-adaptation and is thought to require dietary carbohydrate restriction for a prolonged period of time to eventually reach metabolic adaptation, which manifests as sustained ketosis, increased rates of fat oxidation, and concomitantly a decreased rate of carbohydrate oxidation in energy-demanding tissues such as [skeletal muscle]." (Sherrier & Li, 2019)
Metabolic Flexibility:
"Metabolic flexibility describes the ability of an organism to respond or adapt according to changes in metabolic or energy demand as well as the prevailing conditions or activity... The more common concept of metabolic flexibility has been promulgated in the context of fuel selection in the transition from fasting to fed states, or fasting to insulin stimulation to explain insulin resistance." (Goodpaster & Sparks, 2017)