How Sugar Affects Diabetes

Medically Reviewed by Brunilda Nazario, MD on January 18, 2024
6 min read

High blood sugar (also called hyperglycemia or high blood glucose) is when there's too much glucose (sugar) in your bloodstream, typically because your body isn't using or producing insulin as well as it should. Insulin is a hormone that helps control blood sugar levels.

Normally, your pancreas releases insulin when your blood sugar, or blood glucose, gets high – after a meal, for example. This tells your body to absorb glucose until levels get back to normal.

But if you have diabetes, your body doesn’t make insulin (type 1 diabetes) or doesn’t respond to it normally (type 2 diabetes). That can leave your blood sugar too high for too long. Over time, this can damage nerves and blood vessels and lead to heart disease and other problems.

How much sugar in the blood is too much? And why is high blood glucose so bad for you? Here’s a look at how your levels affect your health.

Healthy blood sugar levels are less than 100 milligrams per deciliter (mg/dL) after not eating (fasting) for at least 8 hours. And they're less than 140 mg/dL 2 hours after eating.

During the day, levels tend to be at their lowest just before meals. For most people without diabetes, blood sugar levels before meals hover around 70 to 80 mg/dL. For some people, 60 is normal; for others, 90.

What's a low sugar level? It varies widely, too. Many people's glucose won't ever fall below 60, even when they fast for a long time. When you diet or fast, the liver keeps your levels normal by turning fat and muscle into sugar. A few people's levels may fall somewhat lower.

Normal blood sugar levels

  • While fasting, your blood sugar levels should be between 72 and 99 mg/dL
  • Two hours after eating, your levels should be up to 140 mg/dL

Symptoms of high blood sugar depend on whether you're in the early or later stages of the condition:

  • Early symptoms. You'll feel thirstier, pee more often, get headaches, and have blurred vision.
  • Long-term symptoms. If you've had hyperglycemia for a while, you may feel extremely tired (fatigued), lose weight, get skin or vaginal yeast infections, and have trouble healing from cuts or sores.

People with prediabetes, or a blood sugar level that's higher than normal, usually don't have any signs or symptoms of the illness.

When you have diabetes, certain things can cause your blood sugar to rise, including:

  • Being sick
  • Stress
  • Overeating foods with sugar or carbs
  • Being less active than normal
  • Missing doses of insulin
  • Your diabetes medicine isn't working well
  • Taking steroids or other medicines
  • Recovering from surgery

Doctors use these tests to find out if you have diabetes and prediabetes:

Fasting plasma glucose test. The doctor tests your blood sugar levels after fasting for 8 hours. A level higher than 126 mg/dL indicates you have diabetes, while 100 to 125 mg/dL means you may have prediabetes.

Oral glucose tolerance test. After fasting for 8 hours, you get a special sugary drink. Two hours later, if your sugar level is higher than 200, this could mean diabetes, and 140 to 199 mg/dL suggests prediabetes.

Random check. The doctor tests your blood sugar and it’s higher than 200, plus you’re peeing more, always thirsty, and you’ve gained or lost a lot of weight. They’ll then do a fasting sugar level test or an oral glucose tolerance test to confirm the diagnosis.

A1c test. This test averages your blood sugar level over a few months. A level below 5.7% is normal, while between 5.7% and 6.4% means you have prediabetes.

Any sugar levels higher than normal are unhealthy. Levels that are higher than normal, but not reaching the point of diabetes, are called prediabetes.

According to the American Diabetes Association, 96 million people in the U.S. have this condition, which can lead to diabetes if you don't make healthy lifestyle changes that your doctor suggests. It also raises the risk for heart disease, although not as much as diabetes does. It's possible to keep prediabetes from becoming diabetes with diet and exercise.

What are the dangers of high blood sugar? Glucose is precious fuel for all the cells in your body when it's present at normal levels. But it can behave like a slow-acting poison.

High sugar levels slowly make cells in your pancreas less able to make insulin. The organ overcorrects, and insulin levels stay too high. Over time, the pancreas is forever damaged.

High levels of blood sugar can cause changes that lead to a hardening of the blood vessels, what doctors call atherosclerosis.

Too much sugar can harm almost any part of your body. Damaged blood vessels cause problems such as:

  • Kidney disease or kidney failure, requiring dialysis
  • Strokes
  • Heart attacks
  • Vision loss or blindness
  • A weakened immune system, with a greater chance of infections
  • Erectile dysfunction
  • Nerve damage, also called neuropathy, that causes tingling, pain, or less sensation in your feet, legs, and hands
  • Poor blood flow to the legs and feet
  • Slow wound healing and the potential for amputation, in rare cases

Keep your blood sugar levels close to normal to avoid many of these health problems. The American Diabetes Association's goals for blood sugar control in people with diabetes are 70 to 130 mg/dL before meals, and less than 180 mg/dL after meals.

If your doctor has recently diagnosed you with diabetes, they may ask you to keep track of your blood sugar by testing it at home with a special device called a blood glucose monitor or home blood sugar meter. It takes a small sample of blood, usually from the tip of your finger, and measures the amount of glucose in it.

Follow your doctor’s instructions about the best way to use your device.

Your doctor will tell you when and how to test your blood sugar. Each time you do it, log it in a notebook, online tool, or an app. The time of day, recent activity, your last meal, and other things can all affect whether a reading will be of concern to your doctor. So try to log relevant information like:

  • What medication and dosage you took
  • What you ate, when you ate, or whether you were fasting
  • How much, how intense, and what kind of exercise you were doing, if any

That will help you and your doctor see how your treatment is working. If your doctor has diagnosed you with prediabetes, they'll probably want to check your blood sugar levels once a year or more often.

If you're living with diabetes, tracking your blood sugar is an important part of managing the disease. The target for people with diabetes is 70 to 130 mg/dL before meals and less than 180 mg/dL after meals. Be sure to track it at home with a blood glucose monitor, and watch out for the signs of high blood sugar.

What is the normal range for blood sugar?

If you have diabetes, in general, a normal blood sugar level is less than 180 mg/dL 1 to 2 hours after eating. But talk to your doctor about your personal blood sugar goals.

What is the most common diabetes diagnosis?

Type 2 diabetes makes up 90% to 95% of all diabetes diagnoses.