In one sentence
How much insulin your pancreas has to keep in the blood to hold your fasting glucose where it is; a high number beside a glucose that sits inside its range means the cells are answering the hormone poorly and the pancreas is making up the difference.
Why it matters
Insulin resistance is quiet for years because the pancreas compensates: glucose stays in range while the insulin needed to keep it there climbs, which is why the fasting insulin can move long before HbA1c or fasting glucose does. The homeostasis model that turned the two fasting values into an index was validated against the clamp, the reference method: fasting insulin alone correlated with clamp-measured insulin resistance at 0.81, and the model's estimate at 0.88 (Matthews 1985). In 258 overweight adults without diabetes, a fasting insulin above 109 pmol/L identified the insulin-resistant third with 57 percent sensitivity and 85 percent specificity, about as well as the metabolic syndrome criteria did, and the authors noted that the usefulness of a cut-point depends on the assay, because insulin assays were not standardized (McLaughlin 2003); an American Diabetes Association work group set out the case for standardizing them (Staten 2010). For VT the test earns its place because it changes what can be said about the glucose area of the record, and because it is rarely ordered in New Zealand practice.
What moves it
Up: body fat, above all around the waist; inactivity; a short night; a large carbohydrate load the evening before; stress; the second half of the menstrual cycle; pregnancy; corticosteroid medicines. Down: weight loss, exercise (for a day or two after a session, and over months with training), a lower-carbohydrate diet, and a longer fast, which is why the length of the fast before the draw has to be the same every time.
How to measure it well
Morning, after a 10- to 12-hour fast with water only, not longer; no hard training or alcohol for 48 hours; an ordinary meal the night before. Note the unit the laboratory prints (pmol/L or mU/L) and the laboratory, because assays and their conversion factors differ by more than the changes you are looking for, and compare only within one laboratory. In New Zealand it is usually a self-pay addition to the same draw.
How Vitality Tracker uses it
Stored as reported in the unit the laboratory printed, with the laboratory's range and flag, never overridden; pmol/L and mU/L are kept as separate series unless the laboratory states its factor, which VT then applies. From fasting glucose on the same draw VT computes HOMA-IR (its own entry). The typical laboratory reference varies by assay; VT's working optimal (v0.1) is below 60 pmol/L (about 8.6 mU/L at the standard factor), shown beneath in smaller type and marked working. Re-test every three months with HbA1c while you are changing something, otherwise yearly; a month on a glucose sensor answers the day-to-day question better than either.
How far to trust the number
Laboratory, but the least standardized test on the panel: the same blood can read differently on two makers' assays, and the day-to-day variation within a person is large as well. Read it as a trend within one laboratory, never as a single figure against a table.
The ranges
The laboratory’s range and flag are shown with your result and never overridden; the working optimal stays beneath, in smaller type, until the clinical advisor has reviewed it.
Sources
- Matthews DR, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 1985;28:412–9. doi:10.1007/BF00280883
- McLaughlin T, et al. Use of metabolic markers to identify overweight individuals who are insulin resistant. Ann Intern Med 2003;139:802–9. doi:10.7326/0003-4819-139-10-200311180-00007
- Staten MA, et al. Insulin assay standardization: leading to measures of insulin sensitivity and secretion for practical clinical care. Diabetes Care 2010;33:205–6. doi:10.2337/dc09-1206
- Vitality Tracker, Recommended baseline lab panel v0.1; Lab results data model (assay-dependent units); optimal set v0.1 (working).