Knowing when to test and what to look for can help keep people with, or at risk of, diabetes healthy.
Contents of this article:
What are fasting blood sugar levels?
Bread and sweetened snacks may cause large blood sugar spikes.
Following a meal, blood sugar levels rise, usually peaking about an hour after eating.
How much blood sugar rises by and the precise timing of the peak depends on diet. Large meals tend to trigger larger blood sugar rises. High-sugar carbohydrates, such as bread and sweetened snacks, also cause more significant blood sugar swings.
Normally, as blood sugar rises, the pancreas releases insulin. Insulin lowers blood sugar, breaking it down so that the body can use it for energy or store it for later.
However, people who have diabetes have difficulties with insulin in the following ways:
- People with type 1 diabetes do not produce enough insulin because the body attacks insulin-producing cells.
- People with type 2 diabetes do not respond well to insulin and, later, may not make enough insulin.
In both cases, the result is the same: elevated blood sugar levels and difficulties using sugar.
This means that fasting blood sugar depends on three factors:
- the contents of the last meal
- the size of the last meal
- the body's ability to produce and respond to insulin
Blood sugar levels in between meals offer a window into how the body manages sugar. High levels of fasting blood sugar suggest that the body has been unable to lower the levels of sugar in the blood. This points to either insulin resistance or inadequate insulin production, and in some cases, both.
When blood sugar is abnormally low, diabetes medications may be lowering blood sugar too much.
How to test fasting blood sugar
There are two tests for fasting blood sugar: the traditional blood sugar test and a new test, glycosylated hemoglobin (HbA1c). This test measures how the body is managing blood sugar over a period of time.
The HbA1c test is used to check how a person's sugars have been controlled over a certain period of time. The HbA1c levels do not fluctuate much and can give a good indication of a person's sugar levels for several months ahead. This may mean that people who use certain diabetes medications and whose blood sugar is well-controlled, may not need to do traditional daily monitoring.
However, in many cases a doctor will still ask people with diabetes to use the traditional system and check their levels daily.
In most cases, doctors ask people to measure fasting blood sugar immediately upon waking, before they have had anything to eat or drink. It may also be appropriate to test blood sugar before eating or sometimes 2 hours after a meal, when blood sugar has returned to normal levels.
The right time to test depends on treatment goals and other factors. For example, most people with diabetes do not need to test between meals unless they are on a diabetes drug that can lower blood sugar. Other people may test between meals if they feel their sugars are low.
Since they do not make any insulin, some people with type 1 diabetes also test multiple times a day. They may do this because they need to check their levels regularly in order to adjust their insulin dosing.
A healthcare professional may recommend using a glucometer to test daily levels of fasting blood sugar.
To test, people should do the following:
- Prepare the testing strip and glucose monitor so that they are accessible and ready to receive a sample.
- Cleanse the testing area - usually the side of a fingertip - with an alcohol swab.
- Lance the testing area. Bracing it against a firm surface can help with the impulse to pull away.
- Squeeze the testing area around the wound to maximize blood flow, and squeeze a drop of blood onto the test strip.
- Put the strip into the monitor.
- Record the time, blood sugar reading, and recent food intake in a log.
Blood sugar levels vary throughout the day and with food intake, so no single blood sugar reading can reveal the full picture of how someone is processing sugar.
There is also no single blood sugar reading that is ideal in all contexts. For most people, HbA1C levels should be less than 7, but the target number changes based on a variety of personal factors.
Target blood sugar numbers are as follows in milligrams per decilitre (mg/dL):
- Fasting (morning testing before food or water): 70-100 mg/dL for people without diabetes; 70-130 mg/dL for people with diabetes.
- Two hours following a meal: less than 140 mg/dL for people without diabetes; below 180 mg/dL for people with diabetes.
How to keep fasting blood sugar healthy
To keep fasting blood sugar from rising too high, it is vital to follow a healthful diet. Some strategies include:
- limiting sodium intake
- reducing intake of sweetened snacks
- choosing whole-grain breads and pastas instead of high-calorie, low-nutrient white breads and pastas
- eating foods rich in fiber, which helps the body lower blood glucose
- eating high-protein foods, which can support feelings of fullness
- choosing non-starchy vegetables, which are less likely to trigger blood glucose spikes
People taking diabetes drugs who are at risk of dangerous blood sugar dips should follow a similar diet. They also need to take proactive steps to prevent blood sugar from dropping. Those include:
- eating regular meals
- increasing food intake and snacking frequency during times of greater physical activity
- avoiding or limiting alcohol
- consulting a doctor if vomiting or diarrhea make it difficult to control blood sugar
Symptoms of unhealthy fasting blood sugar
Symptoms of unhealthy fasting blood sugar may include low energy, tiredness, and headaches.
Blood sugar that is too low can cause symptoms such as:
- feeling shaky, jittery, or sweaty
- difficulty concentrating
- low energy
- pale skin
- feeling sleepy or tired
- headaches or muscle aches
- fast or irregular heartbeat
- feelings of weakness
- lack of coordination
In extreme cases, low blood sugar can trigger seizures, loss of consciousness, confusion or the inability to drink or eat.
Abnormally high blood sugar can cause the following symptoms:
- increased hunger or thirst
- excessive urination
- blurred vision
Like low blood sugar, high blood sugar may cause loss of consciousness or seizures if left untreated.
When to see a doctor
Any significant change in blood sugar patterns warrants a visit to a doctor. People with diabetes and those at risk of diabetes should also consult a doctor if:
- blood sugar levels become unusually high or low
- well-controlled blood sugar levels are suddenly uncontrolled
- people have new or worsening symptoms of diabetes
- they start or stop taking a new medication
- they experience abnormally high blood pressure
- they develop an infection or sore that won't heal
Diabetes requires ongoing monitoring, and treatment can change over time. Information about diet and exercise is vital to outlining a proper treatment plan.
People with diabetes can help their doctors to help them by keeping detailed logs and being honest about dietary or lifestyle changes.