What are the earliest signs of insulin resistance?
Early signs of insulin resistance are measurable long before a diabetes diagnosis: elevated fasting insulin, abdominal fat, abnormal blood fats, and excess fat within the muscles. If you have risk factors, have your fasting insulin measured, because standard blood sugar tests respond too late.
Insulin resistance begins long before an official diabetes diagnosis. In people with completely normal blood sugar, fasting insulin levels are already elevated: the pancreas produces more insulin to compensate for the reduced effect. By the time the standard diabetes tests (fasting glucose, HbA1c) pick up a problem, early damage and complications have already begun. A simple measurement of fasting insulin in the blood can reveal this early stage.
The physical signs of early insulin resistance are subtle and not always noticeable. What is measurable: abdominal fat, higher blood pressure, and abnormal blood fats, particularly too many triglycerides and too little good (HDL) cholesterol. This cluster is known as metabolic syndrome and substantially raises the risk of diabetes and cardiovascular disease. You do not need high blood sugar for this to apply.
Muscles also give an early signal. In people with obesity but no diabetes yet, ultrasound can already reveal more fat and connective tissue within the muscle fibres. The more fat there is inside the muscle, the worse that muscle responds to insulin. Notably, this is not linked to body weight or BMI. Someone with a high body weight can still have healthy muscles, and vice versa. The study that demonstrated this was small (25 participants), so this is not yet a screening method.
In people at the prediabetes stage, at least four distinct underlying causes exist: muscle cells that respond poorly to insulin, a liver that keeps producing too much sugar, a pancreas that produces insufficient insulin, and disrupted gut hormones that normally stimulate insulin release. These subtypes cannot be distinguished from the outside, but each may require a different approach. The shape of the blood sugar curve during a glucose tolerance test can help identify them.
Insulin resistance can also be estimated using straightforward calculations based on routine blood values and body weight, such as the TyG index (triglycerides combined with blood sugar). Such a score can flag an elevated risk of fatty liver at an early stage. Other calculated scores predict the risk of stroke and cardiovascular disease. These are all associations from observational research, but they show that insulin resistance is an early, measurable phenomenon that does not have to be overlooked simply because blood sugar still looks normal.
Claims based on seven studies (PMID 28492735, 39715896, 40509678, 28639538, 41035026, 36120429, 40241176). Mix of observational cohort studies, a small ultrasound study (n=25), and mechanistic research. No large RCTs. Strength of evidence for the individual early signs is predominantly moderate.