The trouble is that most people never hear the alarm. In the United States, where the numbers are tracked most closely, about 115 million adults, more than two in five, have prediabetes, and roughly eight in ten of them do not know it.[@cdc2026] Globally, the International Diabetes Federation estimated that in 2024 around 635 million adults had impaired glucose tolerance and 488 million had impaired fasting glucose, the two laboratory forms of prediabetes.[@duncan2026] The two groups overlap, so the numbers cannot simply be added together, but the message is clear: this is one of the most common health conditions in the world, and one of the least noticed.

This chapter is about learning to notice it. We will define exactly what prediabetes is, look at the quiet signs that can give it away, and then walk through the risk factors that raise your odds, wherever in the world you live. The hopeful thread runs through all of it: prediabetes is the stage at which the most can be changed.

2.1 Defining Prediabetes

A dial, not a switch

Blood glucose is not either "fine" or "diabetic". It is a continuum. At one end are people whose glucose is comfortably normal. At the other end are people whose glucose is high enough to meet the diagnostic definition of diabetes. In between lies a wide territory in which glucose is higher than normal, but not yet high enough to be called diabetes. Doctors call this territory prediabetes, or, in the World Health Organization's language, intermediate hyperglycaemia.[@who2006]

As we saw in Chapter 1, this stage is not a sudden event. Studies that followed people for years before their diagnosis found that changes in insulin sensitivity and in insulin secretion begin long before glucose reaches the diabetes range.[@tabak2009] Prediabetes is simply the point on that long road where the glucose number finally starts to reveal what has been happening inside.

The two states that make up prediabetes are:

• Impaired fasting glucose (IFG): glucose is raised after an overnight fast, when the liver should be releasing only a small, steady trickle of glucose into the blood.
• Impaired glucose tolerance (IGT): glucose is raised two hours after a standard 75-gram glucose drink, meaning the body clears sugar from the blood more slowly than it should.

A person can have one, the other, or both. A third route, the HbA1c test, measures average glucose over roughly the previous two to three months and is used by the American Diabetes Association (ADA) to define prediabetes as well. We will look at how each test works in Chapter 4.

The numbers

Two organisations dominate the world's guidelines, and they do not draw the line in exactly the same place. The ADA sets the lower boundary of prediabetes for fasting glucose at 100 mg/dL (5.6 mmol/L). The WHO sets it higher, at 110 mg/dL (6.1 mmol/L).[@ada2026,who2006] For the two-hour test and for the upper boundary, where diabetes begins, the two agree. The table below shows both, side by side.

Note: WHO's 2006 definition of intermediate hyperglycaemia is based on the glucose values. HbA1c ranges for prediabetes differ between organisations, so ask your laboratory or doctor which one they use. Diagnosis of diabetes normally requires confirmation with a second test unless the person has clear symptoms with a very high glucose.[@ada2026]

Do not worry if the difference between the two systems seems confusing. It matters mainly for where the alarm sounds: someone with a fasting glucose of 105 mg/dL has prediabetes under the ADA's rules and a normal reading under the WHO's. Either way, it is a reading worth paying attention to, because risk rises steadily as glucose rises. There is no cliff edge at any single number.

Science Corner: Two kinds of prediabetes Impaired fasting glucose and impaired glucose tolerance are not the same condition wearing two names. Researchers have linked IFG more closely to insulin resistance in the liver, which keeps releasing glucose overnight, and IGT more closely to insulin resistance in muscle, which is slow to take up glucose after a meal.[@tabak2012] Some people have both and, on average, are at the highest risk. This is one reason a single fasting test can miss people whose main problem shows up only after eating.

What happens next: three roads

If prediabetes were a one-way street to diabetes, the news would be grim. It is not. Long-term studies show something much more interesting. Each year, roughly 5 to 10 percent of people with prediabetes progress to type 2 diabetes, and a similar proportion return to normal glucose levels.[@tabak2012] Most of the rest stay where they are for the time being. The exact numbers depend heavily on how prediabetes is defined and who is studied, but the broad picture is a fork in the road with three branches: progress, stay, or return.

{{fig:F2-A}}

The word "return" deserves a pause. In the follow-up of the Diabetes Prevention Program, people who returned to normal glucose regulation at least once, even temporarily, had a 56 percent lower risk of developing diabetes over the following decade than those whose glucose stayed in the prediabetes range.[@perreault2012] The study was observational, so it cannot prove that returning to normal caused the protection. But it fits everything we know about the biology: the less time the body spends under high glucose and high insulin, the less strain it puts on the beta cells.

It is also important to be honest about what prediabetes does to those beta cells. By the time glucose enters the prediabetes range, a substantial share of beta-cell function may already have been lost. You will sometimes see a figure of "about half" quoted. The truth is that estimates vary widely from study to study and person to person, and no single number should be treated as fact for any one individual.[@tabak2009,tabak2012] What matters for you is the direction: the earlier the alarm is heard, the more function there is left to protect.

Finally, prediabetes is not only about future diabetes. The International Diabetes Federation notes that impaired glucose tolerance and impaired fasting glucose signal a higher risk of type 2 diabetes and an already heightened risk of cardiovascular disease.[@idf2021] That is one more reason to treat the alarm seriously, rather than waiting for a diabetes diagnosis.

The proof that it can be changed

The strongest evidence that prediabetes can be turned around comes from a landmark trial called the Diabetes Prevention Program (DPP). In the late 1990s, researchers in the United States enrolled 3,234 adults with overweight and raised glucose, and assigned them to one of three groups: a placebo, the diabetes medicine metformin, or an intensive lifestyle programme.[@knowler2002]

The lifestyle programme had two main goals: lose about 7 percent of body weight, and do at least 150 minutes of moderate physical activity per week, roughly a brisk half-hour walk five days a week. There was no exotic diet and no gym membership required. Participants met with a lifestyle coach and worked on eating a little less fat and fewer calories.

After an average of just under three years, the results were striking:

• The lifestyle programme reduced the risk of developing diabetes by 58 percent compared with placebo.
• Metformin reduced it by 31 percent.
• Lifestyle change was more effective than metformin, and its benefit was seen in men and women and across the racial and ethnic groups studied.[@knowler2002]

To put that in everyday terms: in the placebo group, about 29 in 100 people developed diabetes over three years. In the lifestyle group, about 14 in 100 did.[@knowler2002] These figures are from one trial, in a particular group of people, over a limited period, so please do not read them as a guarantee for any individual. But few areas of medicine can point to a result this consistent from something as ordinary as walking and eating a little better.

Science Corner: What "58 percent" actually means The 58 percent is a relative risk reduction, a comparison between groups. It does not mean that 58 percent of people avoided diabetes, or that lifestyle change works for only 58 percent of people. It means that the rate of new diabetes in the lifestyle group was 58 percent lower than the rate in the placebo group. The absolute difference in the share of people who developed diabetes over about three years was roughly 15 percentage points (about 29 versus 14 in every 100). If you divide one by the other you will get about 50 percent, not 58: the trial's 58 percent compares the rate of new cases per person-year of follow-up, which takes into account that people were followed for different lengths of time. Both ways of describing the result are true; it is worth being aware of which one you are reading.

2.2 The Silent Symptoms and Warning Signs

Why you probably feel fine

Here is the uncomfortable fact behind the "eight in ten don't know" statistic: prediabetes usually produces no symptoms at all.[@ada2026] Blood glucose in this range is not high enough to make you thirsty, tired, or unwell in an obvious way. You may feel completely normal, even energetic. As one of the people quoted at the start of this book put it: "I didn't have any symptoms and I didn't really take it seriously." For most people, the first sign is a number on a lab report, if they are tested at all.

That is why this section is not really a list of symptoms. It is a list of clues, most of them subtle, several of them shared with many other conditions, and none of them a diagnosis. Their value is that they tell you when to ask for a test.

Clues your body may give

A larger waistline. As Chapter 1 explained, fat deep in the abdomen is the most metabolically active kind. A tape measure is one of the cheapest and most useful tools you have. We will explain how to use it properly in section 2.3.

Dark, velvety patches of skin. A skin change called acanthosis nigricans appears as darker, thicker, velvety skin, most often at the back and sides of the neck, in the armpits, or in the groin. It is a visible sign that insulin levels have been high for some time. In a study of 320 middle-aged adults, it was found across the whole range from normal glucose to diabetes, but it was quite specific for insulin resistance: when it was present, insulin resistance was very likely.[@alvarez2020] It is not a sign of poor hygiene and it does not wash off. If you notice it, mention it to your doctor.

Tiredness after meals, strong hunger, or cravings. Many people with insulin resistance report feeling sleepy or hungry a couple of hours after a carbohydrate-rich meal. This can be a clue, but it is also a very common experience in people whose glucose is perfectly normal, and it has many other causes: poor sleep, stress, iron deficiency, thyroid problems. Treat it as a prompt for curiosity, not as proof.

Other markers of metabolic strain. High blood pressure, raised triglycerides, and low HDL ("good") cholesterol tend to cluster with insulin resistance. If your doctor mentions any of these, it is reasonable to ask about a glucose test too.[@ada2026]

When prediabetes has become diabetes

The signs of a truly raised glucose are different and more obvious: increased thirst, frequent urination (especially at night), unexplained weight loss, blurred vision, and unusual fatigue.[@ada2026] Recurrent infections and slow-healing cuts can also occur. These are not symptoms of prediabetes. If you have them, you should see a doctor promptly rather than waiting for a scheduled check-up, because they suggest that glucose may already be well into the diabetes range.

The absence of symptoms should never be read as reassurance. Many people with type 2 diabetes have no symptoms for years and are diagnosed only when a complication appears or a routine test happens to catch it. Testing, not symptoms, is the only reliable way to know.

Who should be tested, and when

Because the alarm is silent, guidelines recommend testing people who have no symptoms at all. The ADA recommends that screening for prediabetes and type 2 diabetes begin at age 35 for all adults, and earlier for adults with overweight or obesity who have additional risk factors.[@ada2026] The United States Preventive Services Task Force recommends screening adults aged 35 to 70 who have overweight or obesity, and gives that recommendation a grade of B.[@uspstf2021] If your results are normal, guidelines generally suggest repeating the test at least every three years, and more often if your risk is high. Ask your own doctor what interval suits you.

2.3 Global Risk Factors

Risk is a pattern, not a verdict

A risk factor is something that raises the odds of a condition without guaranteeing it. Some risk factors you can change, and some you cannot. Both kinds are worth knowing about, because the ones you cannot change tell you how closely to watch, and the ones you can change tell you where to act.

Risks you cannot change.

• Family history. Having a parent, brother, or sister with type 2 diabetes raises your risk.[@ada2026] Genes and shared family habits both play a part, and it is usually impossible to separate them.
• Age. Risk rises with age, which is why screening guidelines set a starting age.
• Ancestry. In many countries, people of South Asian, Hispanic or Latino, African, Native American, Pacific Islander and Asian descent are diagnosed with type 2 diabetes at higher rates and lower body weights than people of European descent.[@ada2026] This does not mean any individual is doomed. It means that if you belong to one of these groups, the tape measure and the blood test matter even more, and that they should be applied earlier.
• A history of gestational diabetes or polycystic ovary syndrome (PCOS). Both are recognised markers of higher long-term risk, and both are worth mentioning to your doctor even if they occurred many years ago.[@ada2026]

Risks you can change.

• Excess body fat, especially around the abdomen. This is the single strongest modifiable driver, and the reason the DPP focused on a 7 percent weight loss target.[@knowler2002]
• Physical inactivity. As Chapter 1 showed, muscle is a large glucose reservoir, and contracting muscle can take up glucose even without insulin.[@richter2013] When muscle is inactive, that back door stays closed.
• Blood pressure and blood fat abnormalities, which often move together with insulin resistance and can improve as it does.[@ada2026]

The full picture of sleep, stress, and diet quality as risk factors deserves chapters of their own, and they get them: sleep and stress in Chapter 10, and food in Part 2. For now, the important point is that most of the changeable risk factors respond to the same small set of daily habits.

Measuring what matters: your waist

Body Mass Index (BMI) is the most common measure of body size, but it has limits. It cannot tell muscle from fat, and it cannot tell where fat is stored. Two people with the same BMI can have very different amounts of abdominal fat, and therefore very different levels of metabolic risk. For that reason, the WHO recommends looking at waist measurements alongside BMI.[@who2011whr]

How to measure. Stand relaxed and breathe out normally. Find the lowest rib and the top of your hip bone on one side, and place a soft tape measure at the midpoint between them, then go all the way around, keeping it level and snug but not pressing into the skin. Read the number at the end of a normal breath out. (In Chapter 1 we suggested the level of the navel as a quick first attempt. The rib-to-hip midpoint is the standard method and is what the guidance below refers to.)

{{fig:F2-B}}

What the numbers mean. In the WHO's framework, risk of metabolic complications starts to rise when waist circumference exceeds about 94 cm (37 inches) in men and 80 cm (31.5 inches) in women, and becomes substantially raised at about 102 cm (40 inches) and 88 cm (35 inches), respectively.[@who2011whr] Another measure, the waist-to-hip ratio (waist divided by hip circumference), points to increased risk at values of roughly 0.90 and above in men and 0.85 and above in women.[@who2011whr] These are population-level guides, not diagnoses, and they were developed mostly in people of European descent.

Science Corner: Why the same number does not mean the same risk everywhere In people of Asian descent, metabolic risk tends to appear at a lower BMI and a smaller waist than in people of European descent. A WHO expert consultation on this question concluded that the standard BMI thresholds can under-estimate risk in Asian populations. It described BMI values of 23 and above as an increased-risk range and 27.5 and above as a high-risk range for public health action, rather than the usual 25 and 30.[@who2004bmi] If you are of Asian descent, ask your doctor whether testing should begin earlier than the general guidance.

The global picture

Prediabetes and diabetes are not confined to wealthy countries. The IDF estimates that about 589 million adults aged 20 to 79 were living with diabetes in 2024, and that around 252 million of them did not know it.[@duncan2026] By 2050 the number is projected to rise to about 853 million, with the fastest proportional growth expected in Africa (a rise of about 142 percent) and in the Middle East and North Africa (about 95 percent).[@duncan2026] Wherever the health system has the fewest resources to test and treat, the silent stage of the condition is most likely to go unnoticed.

This is why this book uses examples from many kitchens and many cultures. The biology of insulin does not change from Jakarta to Johannesburg to Jalisco. What changes is the food, the way people move through the day, and how easy it is to get a simple blood test. The rest of the book takes up those differences, one by one.

Key Takeaways

• Prediabetes means glucose is above normal but below the diabetes range. The ADA and the WHO draw the lower fasting-glucose line at slightly different places (100 versus 110 mg/dL); both agree on the two-hour and diabetes thresholds.
• Each year, roughly 5 to 10 percent of people with prediabetes progress to diabetes, and a similar proportion return to normal. It is a fork in the road, not a one-way street.
• In the Diabetes Prevention Program, an intensive lifestyle programme (about 7 percent weight loss and 150 minutes of activity a week) cut the risk of developing diabetes by 58 percent over about three years; metformin cut it by 31 percent.
• Prediabetes usually has no symptoms, which is why most people who have it do not know. Testing, not symptoms, is how it is found.
• Risk goes up with family history, age, certain ancestries, a history of gestational diabetes or PCOS, excess abdominal fat, and inactivity. Some of these you can change, and others tell you when to start testing.
• Waist measurements add information that BMI alone cannot, and thresholds may need to be lower for people of Asian descent.

Action Points

1. Check whether you are due for a test. If you are 35 or older, or younger with overweight and one or more risk factors from this chapter, and you have not had a glucose test in the last three years, book one. Ask for a fasting glucose, an HbA1c, or both.
2. Measure your waist properly. Use the rib-to-hip midpoint method described above, and write down the number and the date. It is your baseline.
3. Map your family. Ask your relatives who has had diabetes, gestational diabetes, or heart disease. Write it down and share it with your doctor.
4. If you already know you have prediabetes, treat the next three months as an experiment: aim for one brisk 10-to-15-minute walk after your main meal on most days, and choose one meal a day to swap a refined starch for a whole or high-fibre version. Then retest, and see what the number says.
5. Ask for the numbers, not just the label. If a report says "borderline" or "slightly raised", ask for the actual glucose and HbA1c values, and which threshold system the lab is using.

This book is intended for education and does not replace personal medical advice. If you have diabetes or take glucose-lowering medication, please consult your healthcare team before changing your diet, exercise, or treatment.