Prediabetes: What It Means, Why It Matters and How to Act on It

What is prediabetes?

Prediabetes is the name for blood glucose that sits between normal and the threshold for type 2 diabetes. It is not a mild form of diabetes; it is the stage before it, when the body's glucose control is under strain but has not yet failed.

It is extremely common. The CDC estimates that more than one in three American adults has prediabetes, and the large majority of them do not know. That matters because prediabetes is the point at which the largest gains can be made with the smallest interventions.

Doctors sometimes use two more specific terms. Impaired fasting glucose means the fasting test is raised. Impaired glucose tolerance means glucose after a standard sugar drink is raised. A person can have one, the other or both, and they reflect slightly different problems: raised fasting glucose points to the liver releasing too much glucose overnight, while impaired tolerance points to muscle not clearing glucose quickly enough after meals.

What happens inside the body

The story of prediabetes is the story of insulin resistance meeting a pancreas that is still coping.

In a healthy person, insulin released after a meal prompts muscle, liver and fat cells to take up glucose, and tells the liver to stop releasing its stored glucose. When cells become resistant to insulin — most often because of excess abdominal fat and inactivity — they need more insulin to do the same job. The pancreas responds by producing more, and for a long time that works: glucose stays close to normal at the cost of high insulin levels.

Prediabetes is the point where compensation starts to slip. The pancreas is still producing large amounts of insulin, but not quite enough to hold glucose in the normal range. Fasting glucose drifts up because the liver is no longer fully suppressed overnight. After-meal glucose rises higher and stays up longer because muscle uptake is slower. The beta cells that make insulin are working at full stretch, and — this is the important part — they have already lost a meaningful share of their function by the time prediabetes is diagnosed.

That is why prediabetes is a warning rather than a diagnosis to file away. The trajectory is towards further beta-cell decline and diabetes, but the trajectory is not fixed.

What causes it

The same factors that cause type 2 diabetes cause prediabetes, because it is the same process at an earlier point:

• Excess weight, especially around the waist. Visceral fat releases fatty acids and inflammatory signals that directly interfere with insulin action.
• Low physical activity. Skeletal muscle is where most glucose goes after a meal; inactive muscle is insulin-resistant muscle.
• Genetic susceptibility. Family history and certain ancestries raise risk at any given weight.
• Age. Insulin sensitivity falls with age, partly through loss of muscle.
• Poor sleep and sleep apnea, which raise cortisol and reduce insulin sensitivity.
• A diet high in refined carbohydrates and sugary drinks, which repeatedly demands large insulin surges.
• Previous gestational diabetes or polycystic ovary syndrome, both of which involve insulin resistance.

Who is at greater risk

The risk factors mirror those for type 2 diabetes: age over 45, overweight or obesity, a large waist, inactivity, a first-degree relative with diabetes, high blood pressure or abnormal cholesterol, a history of gestational diabetes, PCOS, and African American, Hispanic or Latino, American Indian, Alaska Native, Asian American or Pacific Islander heritage. Screening is recommended from age 35 for everyone, and earlier for those who are overweight with any additional risk factor.

Symptoms

Prediabetes almost never causes symptoms. Glucose is not high enough to spill into the urine, so the thirst and frequent urination of diabetes do not occur. Occasionally people notice fatigue, or a darkening and thickening of skin in the neck folds or armpits (acanthosis nigricans), which is a visible sign of high insulin levels. Skin tags in the same areas are associated with insulin resistance.

Because symptoms are absent, prediabetes is found by testing, not by noticing.

How doctors diagnose it

Any of the three standard glucose tests can identify prediabetes:

A1C is the most convenient because it needs no fasting. The glucose tolerance test is the most sensitive for early after-meal abnormalities but takes two hours and is used less often. Different tests can disagree in the same person, and a result at the edge of a range is usually repeated.

Risk within the prediabetes range is not uniform. An A1C of 6.3% carries a considerably higher chance of progression than 5.7%, and someone with both a raised fasting glucose and impaired tolerance is at higher risk than someone with only one.

Why prediabetes matters beyond diabetes

The name draws attention to diabetes, but the damage has already begun before glucose crosses that line. People with prediabetes have a higher risk of heart attack and stroke than people with normal glucose, and a proportion already show early signs of nerve, kidney or eye changes at diagnosis. Prediabetes also usually keeps company with high blood pressure, high triglycerides, low HDL cholesterol and a large waist — the cluster known as metabolic syndrome — and it is the combination that drives cardiovascular risk.

This is why a prediabetes diagnosis is a reason to check blood pressure and cholesterol, not just glucose.

What to do about it

Prediabetes is unusual among medical findings in having a treatment with very strong evidence, minimal side effects and no prescription.

The evidence

The Diabetes Prevention Program randomly assigned more than 3,000 adults with prediabetes to intensive lifestyle change, metformin, or placebo. Over about three years, the lifestyle group — who aimed for 7% weight loss and 150 minutes of activity a week — reduced their risk of progressing to diabetes by 58%. Metformin reduced it by 31%. Fifteen years later, the lifestyle group's advantage persisted. Trials in Finland, China and India found the same pattern.

Weight

The single most effective change. Losing 5–7% of body weight — 10 to 14 pounds for someone who weighs 200 — is enough to produce most of the benefit. The loss does not need to be fast, and it does not need to reach a "normal" weight to count.

Activity

At least 150 minutes a week of moderate activity such as brisk walking, cycling or swimming, spread over most days. Resistance training adds benefit because muscle is the body's main glucose sink. Breaking up long periods of sitting with short walks lowers after-meal glucose on its own.

Eating pattern

There is no single required diet. The features that consistently help are more fibre (vegetables, legumes, whole grains, fruit), fewer refined carbohydrates and sugar-sweetened drinks, protein at each meal, and attention to portion size. Mediterranean-style and plant-forward patterns have the most supporting evidence. See foods that affect blood sugar.

Sleep

Consistently short sleep worsens insulin resistance within days. Aim for seven or more hours with regular timing, and ask about sleep apnea if you snore heavily or wake unrefreshed.

Structured programmes

In the United States, the CDC-recognised National Diabetes Prevention Program delivers the lifestyle intervention from the trial through in-person and online providers, and many insurers cover it. Similar programmes exist through the NHS in England and through provincial health services in Canada.

Medication

Metformin is the only medication with substantial trial evidence in prediabetes. Guidelines suggest considering it for people at particularly high risk — younger adults with a BMI of 35 or more, women with prior gestational diabetes, or those whose A1C rises despite lifestyle change — but it is not routine, and it was less effective than lifestyle change in the trial.

When to contact a doctor

If you have risk factors and have never been tested, ask for an A1C or fasting glucose. If you have been told you have prediabetes, arrange yearly re-testing and ask for blood pressure and cholesterol to be checked too. Contact your doctor sooner if you develop increased thirst, frequent urination, unexplained weight loss or blurred vision, which can indicate progression to diabetes.

Can supplements play a supportive role?

Some people also explore dietary supplements as part of a broader wellness routine. Supplements are not a substitute for weight management, activity, sleep or medical monitoring, and no supplement has been shown in trials to prevent progression from prediabetes to diabetes in the way lifestyle change has.

Some ingredients — berberine, chromium, cinnamon, magnesium and others — have been studied for effects on glucose, with mixed and mostly modest results. Can supplements lower blood sugar? looks at each in turn. Anyone considering one should mention it to their doctor or pharmacist.

Frequently asked questions

References