How the Blood Sugar of Diabetes Affects the Body

Diabetes mellitus leads to persistently elevated blood sugar levels. Over time, high sugar levels damage the body and can lead to the multiple health problems associated with diabetes.

But why are high blood sugars so bad for you? How much sugar in the blood is too much? And what are good sugar levels, anyway? WebMD takes a look at how your sugar level affects diabetes and your health.

Diabetes and Normal Blood Sugar Levels

At present, the diagnosis of diabetes or prediabetes is based in an arbitrary cut-off point for a normal blood sugar level. A normal sugar level is currently considered to be less than 100 mg/dL when fasting and less than 140 mg/dL two hours after eating. But in most healthy people, sugar levels are even lower.

During the day, blood glucose 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. In some, 60 is normal; in others, 90. Again, anything less than 100 mg/dL while fasting is considered normal by today's standards.

What's a low sugar level? It varies widely, too. Many people's sugar levels won't ever fall below 60 mg/dL, even with prolonged fasting. When you diet or fast, the liver keeps sugar levels normal by turning fat and muscle into sugar. A few people's sugar levels may fall somewhat lower. Without taking diabetes medicine, though, or having uncommon medical problems, it's difficult to drop sugar levels to an unsafe point.

Sugar Levels, Diabetes, and Prediabetes

Sugar levels higher than normal mean either diabetes or pre-diabetes is present.

There are several ways diabetes is diagnosed.

• The first is known as fasting plasma glucose. A person is said to have diabetes if his or her fasting blood sugar level is higher than 126 mg/dL after not eating -- fasting -- for eight hours.
• The second method is with an oral glucose tolerance test. After fasting for eight hours, a person is given a special sugary drink. That person is said to have diabetes if two hours after the drink he or she has a sugar level higher than 200.
• The third way is with a randomly checked blood sugar level. If it is greater than 200, with symptoms of increased urination, thirst, and/or weight loss, that person is said to have diabetes. A fasting sugar level or oral glucose tolerance test will be needed to confirm the diagnosis.

But diabetes is not like a switch that gets turned on and off -- healthy one day, diabetic the next. Any sugar levels higher than normal are unhealthy. A blood sugar higher than normal, but not meeting the above criteria for full-blown diabetes, is called prediabetes.

Fifty seven million people in the U.S. have prediabetes. People with prediabetes are five to six times more likely to develop diabetes over time. Prediabetes also increases the risk for cardiovascular disease, although not as much as diabetes does. It's possible to prevent the progression of prediabetes to diabetes, with diet and exercise.

Sugar and Your Body

Why are high blood sugar levels bad for you? It turns out, your body doesn't have much of a sweet tooth. Glucose is precious fuel for all the cells in your body -- when it's present at normal levels. But persistently high sugar levels behave like a slow-acting poison.

1. High sugar levels slowly erode the ability of cells in the pancreas to make insulin. The pancreas overcompensates, though, and insulin levels remain overly high. Gradually, the pancreas is permanently damaged.
2. All the excess sugar is modified in the blood. It becomes a form that sticks to and coats bloodstream proteins, which are normally "sugar-free." Thanks to this sugary film, the proteins don't function well, can be deposited in blood vessels, and can cause damage to them.

Because high sugar levels are everywhere, the body can be damaged anywhere. Damage to blood vessels, in particular, means no area is safe from too much sugar. High sugar levels and damaged blood vessels cause the multitude of complications that can come with diabetes:

• kidney disease or kidney failure, requiring dialysis
• strokes
• heart attacks
• visual loss or blindness
• immune system suppression, with increased risk for infections
• erectile dysfunction
• nerve damage, called neuropathy, causing pain or decreased sensation in the feet, legs, and hands
• poor circulation to the legs and feet, with poor wound healing

In extreme cases, because of the poor wound healing, amputation is required.

Keeping sugar levels closer to normal can prevent many of the complications of diabetes. The American 

Diabetes Association's goals for glucose control in people with diabetes are sugar levels of 70 to 130 mg/dL before meals, and less than 180 mg/dL after meals.

Reviewed by John A. Seibel, MD on February 25, 2010

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