· Lee Chee Cheow, M.Sc. Dip CMP · Metabolic Health · 5 min read
Understanding Prediabetes and How It Progresses
Prediabetes doesn't appear overnight — it's the visible part of a process that can start over a decade before diagnosis. Here's how the numbers are defined, and what's actually happening in the body during that long lead-up.
Prediabetes is often described as a warning sign, but that undersells what’s actually going on. By the time blood sugar numbers cross into the prediabetes range, the underlying process — a slow erosion of the pancreas’s ability to keep up with insulin demand — has typically been under way for years. Here’s how the diagnosis is actually made, and what the longer timeline looks like.
How prediabetes is diagnosed
Doctors use three main tests to assess blood sugar status, and under current ADA and WHO guidelines, each has its own threshold for normal, prediabetes, and diabetes:
| Category | Fasting plasma glucose | 2-hour plasma glucose (OGTT) | HbA1c |
|---|---|---|---|
| Diabetes | ≥126 mg/dL | ≥200 mg/dL | ≥6.5% |
| Prediabetes | 100-125 mg/dL | 140-199 mg/dL | 5.7-6.4% |
| Normal | <100 mg/dL | <140 mg/dL | <5.7% |
Fasting plasma glucose is the simplest of the three — a blood draw after not eating for at least eight hours. The oral glucose tolerance test (OGTT) measures how the body handles a standard glucose load over two hours, and tends to catch problems that a fasting test alone can miss. HbA1c reflects average blood sugar over roughly the preceding three months, since it measures how much glucose has attached to red blood cells, which live for about that long. Because the three tests measure slightly different things, it’s possible to fall into different categories on different tests — which is one reason doctors often use more than one.
Prediabetes, by any of these three measures, means blood sugar is elevated above normal but hasn’t yet reached the threshold used to diagnose diabetes.
The longer timeline: what happens before diagnosis
Diagnostic thresholds capture a single moment, but the disease process behind them unfolds over a much longer arc — often 15 years or more before a diabetes diagnosis, and continuing well beyond it if unmanaged.
Two things move in opposite directions over that period. Insulin resistance — the body’s cells becoming less responsive to insulin’s signal to absorb glucose — rises steadily, starting from a baseline and increasing to a peak of roughly 110-120% of normal in the years immediately before diagnosis (roughly year -5 to year 0), then plateauing at that elevated level.
Beta-cell function — the pancreas’s capacity to produce insulin in response — follows the opposite path. It initially rises to compensate for growing insulin resistance, peaking at around 100-110% of normal roughly 5 to 10 years before diagnosis. But that compensation isn’t sustainable: beta-cell function then declines steadily, falling to as little as 15-20% of normal by around 25 years into the disease process. This decline — not the insulin resistance itself — is what ultimately tips someone from compensated prediabetes into overt diabetes, and it’s why the condition is often described as progressive rather than static.
Blood glucose reflects this shift with a lag. Postmeal (postprandial) glucose climbs gradually across the same timeline, while fasting glucose rises somewhat later and more gradually still — which is part of why fasting glucose alone can under-detect early-stage problems that an OGTT would catch sooner.
The broader progression pattern
Alongside the glucose and insulin mechanics, there’s a commonly observed sequence of related changes: weight gain tends to precede lipid disorder, which tends to precede hypertension, which tends to precede prediabetes, which — without intervention — tends to precede diabetes itself. This isn’t a fixed or inevitable chain for every individual, but it reflects how these conditions typically cluster and build on one another over time, with kidney and cardiovascular complications developing progressively as the underlying process continues unaddressed.
Why this timeline matters
The practical implication of a 15-to-25-year disease arc is that prediabetes is not the starting point — it’s a checkpoint partway through a process that began earlier and is still very much reversible at that stage. Beta-cell decline is the part of the process that’s hardest to reverse once advanced, which is one reason catching things at the prediabetes stage, while insulin resistance is still the dominant issue and beta-cell function is still relatively preserved, matters more than catching things later.
Frequently asked questions
How long before a diabetes diagnosis does the underlying process typically start? The natural history data suggests measurable changes in insulin resistance and beta-cell function can be under way 15 years or more before diagnosis, with complications continuing to develop for years afterward if unmanaged.
Why do fasting glucose, OGTT and HbA1c sometimes give different results for the same person? Each measures something slightly different — a single fasting reading, glucose handling after a standardised load, and a three-month average, respectively — so someone whose blood sugar spikes after meals but is well-controlled fasting might show up as prediabetic on an OGTT before it appears on a fasting test.
Is prediabetes reversible? The mechanics described here — insulin resistance rising while beta-cell function is still largely intact — suggest the prediabetes window is generally a more responsive stage for lifestyle intervention than later, once beta-cell decline is more advanced. This is general information, not a guarantee for any individual; discuss your specific numbers with your doctor.
This article is for general information only and does not constitute medical advice. If you have been diagnosed with prediabetes or diabetes, consult your doctor for guidance specific to your situation.