Diabetes and Metabolic Health: Types, Tests and Complications
A plain-language guide to how blood sugar is regulated, how diabetes and prediabetes are identified, and which parts of the body the condition can affect.
A high blood sugar reading, a letter mentioning "prediabetes", or a relative's diagnosis can all send someone looking for answers. The terms come quickly: A1C, insulin resistance, metabolic syndrome. This guide lays out what those words mean, how diabetes is identified, what complications are described by major health bodies (including effects on nerves and bones), and what monitoring generally involves. It is educational only. It does not interpret anyone's results, and it does not recommend treatment.
How glucose and insulin work
Glucose is the body's main source of energy. Some is made by the body and some comes from food. Insulin, a hormone made by the pancreas, helps glucose move out of the blood and into cells. The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) explains the problem simply: when the body makes too little insulin or uses it poorly, "glucose then stays in your blood and doesn't reach your cells."
Over time, persistently high glucose can damage blood vessels and nerves. That, rather than a single high reading, is why the condition is taken seriously.
The main types of diabetes
Type 1 diabetes is an autoimmune condition. The NIDDK says the immune system destroys the insulin-making cells of the pancreas, so the body makes little or no insulin. Type 2 diabetes is by far the more common form; the NIDDK puts it at about 90% to 95% of cases, and the World Health Organization (WHO) at more than 95%. In type 2, the NIDDK says, cells do not use insulin properly.
Gestational diabetes develops during pregnancy and usually resolves after birth, but the NIDDK notes it raises later type 2 risk. A few forms are less typical: monogenic diabetes, caused by a change in a single gene, and diabetes that follows pancreatic surgery or conditions that damage the pancreas, such as cystic fibrosis or pancreatitis. Prediabetes sits between normal and diabetes and is covered below.
The WHO describes type 1 as characterised by deficient insulin production that requires daily insulin. Its cause and prevention are not known. Because the two types arise differently, advice written for one does not automatically apply to the other.
How common diabetes is
The numbers are large. The WHO reports that the number of people living with diabetes rose from 200 million in 1990 to 830 million in 2022, and that about 59% of adults aged 30 and older with diabetes were not taking medication for it in 2022. In the United States, the Centers for Disease Control and Prevention (CDC) reports 40.1 million people with diagnosed or undiagnosed diabetes in 2023, about 12.0% of the population. Of adults with diabetes, 27.6% (around 11.0 million people) had not been diagnosed.
That last figure matters. Type 2 diabetes can develop quietly, which is why testing exists.
Tests used to identify diabetes
Three blood tests are in common use. According to the NIDDK's diabetes tests page, the A1C test provides your average blood glucose over the last 3 months and can be done without fasting. The fasting plasma glucose test measures blood glucose at a single point in time, usually in the morning after fasting for at least 8 hours. The oral glucose tolerance test (OGTT) also requires an 8-hour fast beforehand and is used to detect type 2 diabetes, prediabetes and gestational diabetes.
Thresholds depend on who is publishing them. The NIDDK clinician page, citing the American Diabetes Association (ADA) Standards of Medical Care in Diabetes 2020, lists these values.
| Test | Prediabetes range | Diabetes range |
|---|---|---|
| A1C | 5.7% to 6.4% | 6.5% or higher |
| Fasting plasma glucose | 100 to 125 mg/dL | 126 mg/dL or higher |
| OGTT, 2-hour value | 140 to 199 mg/dL | 200 mg/dL or higher |
These are one body's published criteria at one point in time. Guidance is revised, other organisations and countries publish their own recommendations, and so the numbers above should not be applied to a result from another source or country without checking local guidance. The WHO fact sheet itself gives no numeric thresholds.
The NIDDK page also stresses that one abnormal value is not usually enough. Diagnosis generally requires two abnormal results, either from the same sample or from separate ones. It notes that an A1C test and a blood glucose test can disagree, that laboratory results are ranges rather than exact numbers, and that A1C may be unreliable at very low or very high values or when it conflicts with other tests. Disagreement between tests can simply mean glucose is in the early stages of rising.
Prediabetes
Prediabetes means glucose is higher than normal but not high enough for a diabetes diagnosis. It is common. The CDC reports 115.2 million US adults had prediabetes, and 31.3 million people aged 65 and older, or 52.1% of that age group. The NIDDK also states that prediabetes carries a higher risk of heart disease.
Prediabetes does not always progress. The WHO describes impaired glucose tolerance and impaired fasting glycaemia as intermediate states with a high risk of progressing to type 2 diabetes, which is a risk and not a certainty. The NIDDK clinician page suggests that people whose results are close to the diabetes range may be observed and retested in 3 to 6 months.
Metabolic syndrome
Metabolic syndrome is a label for a cluster of risk factors rather than a disease of its own. The US National Heart, Lung, and Blood Institute (NHLBI) describes it as a group of conditions that together raise the risk of coronary heart disease, diabetes and stroke. You may have it if you have three or more of five conditions: a large waistline, high blood pressure, high blood sugar, high blood triglycerides and low HDL ("good") cholesterol. The NHLBI states that about 1 in 3 US adults have it.
The NHLBI overview gives no numeric cut-offs for the five components, and this page does not list any, since a clinician applies any definition to the whole picture. The practical point is the clustering. Each factor alone raises risk, and together they raise it more.
Complications of diabetes
The NIDDK lists heart disease and stroke, kidney disease, eye disease, foot problems, nerve damage and low blood glucose among the possible complications. Gum disease and bladder or sexual problems also appear on its list. The WHO says diabetes can cause permanent vision loss by damaging blood vessels in the eyes, that it can lead to kidney failure, and that people with diabetes face a higher risk of heart attack and stroke.
Nerve damage deserves its own mention because it is so common. The National Institute of Neurological Disorders and Stroke (NINDS) calls diabetes the leading cause of peripheral neuropathy in the United States, stating that about two thirds of people with diabetes have mild to severe nerve problems. Estimates of how common nerve damage is vary between sources and depend on how it is defined, so treat any single figure as approximate. Our guide to diabetic neuropathy basics goes through the types, symptoms and how it is evaluated. The thyroid can also affect nerves and muscles; see thyroid problems and nerve symptoms for how the two are sometimes confused.
Not every person develops complications, and they are not inevitable. The risk depends on factors such as how long someone has had diabetes, blood glucose over time, blood pressure and other conditions.
Diabetes and bone
Bones are rarely the first thing people associate with diabetes, but fracture risk is increased. A 2016 review in the journal Diabetes by Sellmeyer and colleagues concludes that fracture risk is significantly increased in both type 1 and type 2 diabetes, and that people with diabetes have worse fracture outcomes than those without it. The authors report that people with type 1 diabetes have double the risk of any fracture and four to five times the hip fracture risk, while among older adults with type 2 diabetes the risk of hip fracture is increased 40% to 70%.
The reasons differ by type. Type 1 is characterised by modest deficits in bone mineral density that explain some but not all of the extra risk. Type 2 is associated with higher bone density, yet the review says older adults with type 2 diabetes fracture at a higher bone density than those without diabetes, so a standard bone density T-score tends to underestimate their risk. This is why a DXA result alone may not tell the whole story; see the bone density test explained. For wider context, read osteoporosis and bone health and fall prevention at home for the practical side of avoiding fractures.
Lifestyle and monitoring in general terms
Day-to-day care for diabetes is individualised, and it is the work of the person and their health team. At a general level, the WHO lists measures for preventing or delaying type 2 diabetes: reaching and keeping a healthy body weight, staying physically active with at least 150 minutes of moderate exercise each week, eating a healthy diet and avoiding tobacco.
Monitoring typically means periodic A1C tests, sometimes home glucose checks, and regular screening for complications, including eye, kidney and foot examinations. Targets are personal. Targets are personal and are set by a clinician, so this page does not quote any. Medicines, insulin, and device decisions are for the treating team, not for general guides.
Individual care varies, and your own doctor or specialist has the final word. If you are preparing for an appointment, questions to ask an endocrinologist can help. For another common hormone-related condition that is sometimes tested at the same visit, see thyroid health basics.
Frequently asked questions
What is the difference between prediabetes and diabetes?
In prediabetes, glucose is above normal but below the diabetes range. Under the ADA criteria listed by the NIDDK, an A1C of 5.7% to 6.4% falls in the prediabetes range and 6.5% or higher in the diabetes range. Different organisations may draw lines differently.
Can A1C and fasting glucose give different answers?
Yes. The NIDDK acknowledges that a blood glucose test may suggest diabetes when an A1C does not, and the reverse. Diagnosis generally relies on two abnormal results, and the clinician decides how to interpret disagreement.
Is metabolic syndrome the same as diabetes?
No. The NHLBI describes metabolic syndrome as a cluster of risk factors that raises the chance of diabetes, heart disease and stroke. A person can have it without having diabetes, and high blood sugar is just one of its five components.
Why would diabetes affect fracture risk if bone density can be normal?
According to Sellmeyer and colleagues, the quality and strength of bone can be compromised in type 2 diabetes even when density is preserved, so density tests can underestimate risk. Other contributors they note include complications of the blood vessels.
The short version
Diabetes is a disorder of glucose regulation, identified through A1C, fasting glucose or glucose tolerance tests whose thresholds vary by organisation and country. Prediabetes and metabolic syndrome are earlier or broader warning categories, and complications can reach nerves, eyes, kidneys, the heart and the skeleton. Because several of these develop quietly, testing and regular follow-up carry more weight than any one reading.