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Blood glucose levels: what they mean

by Josephine Andrews
Published: Last Updated on 451 views

Blood sugar levels indicate the sugar content of the blood . It changes throughout the day depending on food intake. Blood sugar levels rise after eating and are lowest in the morning after getting up. In some diseases, the regulation of the blood sugar level is disturbed. Read everything you need to know about blood sugar levels.

What are the blood sugar levels?

The energy supply of the cells is ensured by the blood sugar. Carbohydrates taken in with food reach the intestines , where they are broken down into smaller sugar molecules and absorbed into the blood . A hormone produced by the pancreas , insulin , ensures that sugar is absorbed from the blood into the cells. It is the most important supplier of energy for the cells, their “fuel” so to speak.

People whose pancreas does not produce enough insulin suffer from what is known as diabetes mellitus . Your blood sugar level is usually too high. Over time, this can damage blood vessels and organs. At the same time, there is a lack of energy in the cells because the sugar cannot be channeled into the cells.

When do you measure blood sugar levels?

The blood sugar value is determined for various questions:

  • to diagnose diabetes (diabetes mellitus).
  • to control blood sugar levels in known diabetes mellitus
  • to monitor and adjust therapy in diabetics
  • annually in people who are at higher risk for diabetes
  • for symptoms that indicate diabetes (weight loss, thirst, frequent urination, drop in performance)
  • in pregnant women and newborns
  • if you suspect hypoglycemia or hyperglycemia
  • in case of unconsciousness without an explicable cause
  • in malnutrition or malnutrition
  • with known disorders of carbohydrate metabolism (e.g. due to enzyme deficiency)

How are blood sugar levels determined?

The blood sugar level is usually determined in the morning before the patient has eaten ( fasting blood sugar ). A drop of blood is obtained by pricking the finger or the lobe of the ear and applied to a small stick. The stick, in turn, is inserted into a blood glucose meter. After about half a minute, the device then displays the sugar content in the blood. However, the blood sugar value can also be determined as part of a normal blood test.

Oral Glucose Tolerance Test (OGTT)

If abnormal fasting blood sugar values ​​(and/or other factors such as an increased urge to urinate or severe thirst) give rise to the suspicion of diabetes (diabetes mellitus), the oral glucose tolerance test (OGTT) can bring clarity:

First, the fasting blood sugar is determined. Then the patient drinks a precisely defined amount of a sugar solution. After two hours, it is measured how high the blood sugar level has risen and how quickly it falls again.

Clearly elevated values ​​in the sugar test indicate diabetes. Then there are people who do not yet have diabetes but already have abnormal sugar levels. The sugar metabolism is already disturbed in them (precursor to diabetes).

oGTT in pregnant women

Pregnant women are routinely offered an oral glucose tolerance test ( OGTT ). The reason is that a small proportion of women develop diabetes during pregnancy. This gestational diabetes can cause serious complications for both mother and child. Therefore, as a precaution, all pregnant women should take the sugar load test between the 24th and 28th week of pregnancy ( 50g-oGTT as a preliminary test, 75g-oGTT ). In this way, developing diabetes can be recognized and treated at an early stage.

Blood Glucose Chart

Normal fasting blood sugar levels depend on age:

Fasting blood sugar: normal values
up to 1 day 34 – 99 mg/dl
2 days 46 – 81 mg/dl
3 days to 17 years 60 – 99 mg/dl
from 18 years 74 – 99mg/dL

Normal values ​​in the oral glucose tolerance test

In the following table you can read when the blood sugar values ​​after eating and during the OGTT (2 hours after drinking the sugar solution) are abnormally high, borderline high or normal:

fasting blood glucose level Casual blood glucose levels (blood glucose levels after eating) oGTT

(2h value)

diabetes mellitus ≥ 126 mg/dl

≥ 7 mmol/L

≥ 180mg/dl

≥ 10 mmol/L

≥ 200mg/dl

≥ 11.1 mmol/L

Abnormal fasting sugar 100 – 125 mg/dl

5.6 – 6.9 mmol/L


7.8 – 11.0 mmol/L

Blood sugar levels normal < 100 mg/dL

< 5.6 mmol/L


< 7.2 mmol/L



Attention : Depending on the measurement method, the technical literature sometimes contains deviating standard values.

When are blood sugar levels too low?

The blood sugar levels are too low in the following cases:

  • Insulin overdose during diabetes therapy
  • Overproduction of insulin in tumors of the pancreas (e.g. insulinoma)
  • Hormone imbalances caused by hypofunction of the pituitary gland, the thyroid gland or the adrenal cortex
  • after excessive physical work without sufficient food intake
  • Malnutrition, for example in alcoholics, after excessive fasting or in refusal to eat (anorexia nervosa)
  • severe liver damage, for example liver cirrhosis
  • after drinking alcohol on an empty stomach

Reduced blood sugar initially leads to hunger, dizziness , tiredness and sweating. Failure to continue to consume sugar can lead to seizures and circulatory collapse, shock or death.

People who have had diabetes for a long time often have only a few or no typical symptoms of low blood sugar and are therefore unable to take countermeasures in good time. They must therefore check their blood sugar levels regularly, especially if they are more physically active (e.g. during sports).

When are blood sugar levels too high?

Elevated blood sugar levels can be caused by:

  • diabetes mellitus
  • gestational diabetes
  • Hormone imbalances caused by tumors in the adrenal medulla or tumors in the pituitary gland
  • Diseases of the pancreas
  • rare hereditary diseases
  • as a side effect of certain medications

Elevated blood sugar levels often lead to a feeling of thirst, blurred vision and an increased urge to urinate. There is also a risk of a life-threatening coma . Elevated blood sugar damages the blood vessels in the long term. The consequences can be arteriosclerosis , kidney failure, stroke and loss of vision. Severe tissue damage is also possible, for example in the lower legs and feet (“diabetic foot “).

What to do if blood sugar levels change?

It is not uncommon for blood sugar to occasionally be slightly below or above the normal value. This is then related to food intake, physical activity or possible infections. Nevertheless, a blood sugar value that falls outside of the norm should always be checked again. It may be necessary to determine other values ​​that can confirm the suspicion of diabetes mellitus. In such a case, a diabetologist (internal medicine specialist specializing in diabetes) should be consulted. In addition, the family doctor will closely monitor the blood sugar levels in the event of a disturbed sugar metabolism. 

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