“What exactly is blood glucose?” “What are the reference values?” “Does it rise only after eating something sweet?” “How is HbA1c different?” “What is a glucose spike?” “Is a lower GI always better?”
Blood glucose is a number many people see on health-check results, but understanding it properly means looking at how food is converted into glucose and how insulin, the liver and muscles help regulate it.
There are also several different ways of measuring glucose, including fasting blood glucose, post-meal glucose, random blood glucose and the 75 g oral glucose tolerance test (OGTT). HbA1c provides different information from a single blood glucose measurement.
This guide explains what blood glucose is, reference values, why it rises after meals, how insulin and the liver are involved, post-meal glucose, so-called glucose spikes, HbA1c, GI and GL, carbohydrates, dietary fibre, meal order, exercise and continuous glucose monitoring (CGM).
Contents
What is blood glucose?
Blood glucose refers to the concentration of glucose in the blood.
The word “blood sugar” can make it sound as though all types of sugar in the bloodstream are being measured, but the main sugar measured is glucose.
On laboratory reports, you may also see the term plasma glucose.
Did you know? Blood “sugar” mainly means glucose
The “sugar” in blood sugar does not simply mean table sugar.
The main sugar measured in the bloodstream is glucose.
Many carbohydrates from food are broken down during digestion into glucose and other simple sugars before being absorbed and used by the body.
Glucose is an important source of energy
Glucose is an important energy source for the human body.
Many tissues, including the brain and muscles, use glucose as part of their energy supply.
The body therefore has finely regulated systems that help keep blood glucose within an appropriate range.
Important parts of this system include:
- the pancreas
- insulin
- glucagon
- the liver
- muscle
For blood glucose, what matters is maintaining an appropriate range while supplying the body's tissues with the energy they need.
Why does blood glucose rise after eating?
When you eat carbohydrate-containing foods such as rice, bread, noodles, potatoes and fruit, digestible carbohydrates are broken down and absorbed through the small intestine.
As glucose enters the bloodstream, blood glucose rises after a meal.
This is a normal physiological response to eating.
As blood glucose rises, beta cells in the pancreas detect the change and release insulin.
Glucose can then be taken up by tissues such as muscle, or stored as glycogen in the liver and muscles for later use.
What does insulin do?
Insulin is a hormone produced by beta cells in the pancreas.
When blood glucose rises after a meal, insulin secretion increases and helps regulate glucose by:
- promoting glucose uptake into tissues such as muscle
- regulating glucose release from the liver
- supporting storage of glucose in forms such as glycogen
Did you know? Insulin has both basal and meal-related secretion
Insulin is not released only when you eat.
In healthy people, a small amount is released between meals as basal secretion, while additional insulin is released in response to rising glucose after food.
Together, these patterns help regulate blood glucose from fasting periods through to the post-meal period.
How blood glucose is maintained between meals
When you are not eating, the liver plays an important role in maintaining blood glucose.
The liver can release glucose stored as glycogen when the body needs it.
It can also produce new glucose from substances such as lactate, amino acids and glycerol through a process called gluconeogenesis.
During fasting, insulin levels fall and hormones such as glucagon help regulate glucose production and release.
Blood glucose is therefore influenced by both glucose entering from food and glucose supplied by the liver.
What is post-meal blood glucose?
Blood glucose measured after eating is often described as postprandial, or post-meal, blood glucose.
Its pattern can be influenced by:
- the amount of carbohydrate eaten
- the type of food
- dietary fibre
- fat and protein in the meal
- how quickly you eat
- how quickly food leaves the stomach
- insulin secretion
- insulin sensitivity
- physical activity after eating
Even when two people eat the same food, the glucose response may differ depending on portion size, the rest of the meal and individual metabolism.
What is a “glucose spike”?
The terms “glucose spike” and “blood glucose spike” have become common in health and wellness media.
They generally describe a relatively rapid rise in glucose after eating, followed by a subsequent change.
In medical assessment, terms and measurements such as the following are used depending on the purpose:
- post-meal glucose
- postprandial hyperglycaemia
- glycaemic variability
- fasting glucose
- HbA1c
Did you know? “Glucose spike” is a general descriptive term
Medical assessment of glucose metabolism uses established measurements such as fasting glucose, HbA1c and the 75 g oral glucose tolerance test (OGTT).
When looking at a health-check result, it helps to understand which test was performed and under what conditions.
Fasting glucose, random glucose and OGTT
The meaning of a blood glucose result depends partly on when and how it was measured.
| Test |
What is measured? |
What it helps assess |
| Fasting blood glucose |
Measured after a defined fasting period |
Glucose regulation in the fasting state |
| Random blood glucose |
Measured regardless of meal timing |
Blood glucose at that particular time |
| 75 g OGTT |
Glucose is measured before and after drinking 75 g of glucose |
The body's response to a glucose load |
Fasting glucose is commonly used in health checks, while HbA1c and OGTT may also be used when further assessment is needed.
What is HbA1c and how is it different from blood glucose?
HbA1c stands for haemoglobin A1c.
It measures the proportion of haemoglobin in red blood cells that has glucose attached to it.
A blood glucose measurement reflects the situation at the time of testing, while HbA1c is used as an indicator of average blood glucose over roughly the previous one to two months.
Did you know? HbA1c looks at glucose over a longer time frame
Blood glucose changes from meal to meal, but HbA1c is based on the interaction between glucose and red blood cells, so it does not change dramatically because of a single meal.
Looking at blood glucose together with HbA1c can therefore provide different perspectives on both the current situation and the longer-term pattern.
HbA1c can also be affected by red blood cell conditions
HbA1c is an important marker of glucose status, but changes in red blood cell lifespan or turnover can also affect the result.
Conditions such as iron deficiency, haemolysis, blood loss, transfusion and treatment for renal anaemia can sometimes cause HbA1c and average glucose levels to appear out of alignment.
For more about iron, see:
What Is Iron? Functions, Iron-Rich Foods, Daily Intake, Deficiency & Supplements.
How should blood glucose and HbA1c reference values be read?
According to the Japan Diabetes Society's Clinical Practice Guideline for Diabetes 2024, the following values are classified as being in the diabetic range.
| Test |
Diabetic-range value |
| Fasting plasma glucose |
126 mg/dL or higher |
| 75 g OGTT, 2-hour value |
200 mg/dL or higher |
| Random plasma glucose |
200 mg/dL or higher |
| HbA1c (NGSP) |
6.5% or higher |
These values are used as part of the medical assessment of glucose metabolism.
An actual diagnosis is made according to established diagnostic procedures, using blood glucose, HbA1c, symptoms and previous test results where appropriate.
Did you know? Fasting glucose of 100–109 mg/dL is classified as “high-normal” in Japan
The Japan Diabetes Society identifies fasting glucose of 100–109 mg/dL as a high-normal range within the normal category.
With fasting glucose of 110–125 mg/dL and in other relevant circumstances, a 75 g OGTT may be considered to evaluate glucose metabolism in more detail.
Did you know? Diagnostic criteria and treatment targets serve different purposes
The diagnostic criteria used to identify diabetic-range glucose and the glucose-management targets used after a person has been diagnosed with diabetes have different purposes.
Treatment targets may be individualised according to age, treatment method, risk of hypoglycaemia, complications and personal circumstances.
When reading numbers, it is therefore useful to check what the value is being used for.
How common are abnormal glucose findings in Japan?
In Japan's 2024 National Health and Nutrition Survey, the Ministry of Health, Labour and Welfare estimated:
- approximately 11 million people were “strongly suspected of having diabetes”
- approximately 7 million people were classified as “diabetes cannot be ruled out”
These are population-level survey categories used to assess public health trends.
Diagnosis in an individual person is based on medical testing such as blood glucose and HbA1c.
The survey also reported that the proportion of people strongly suspected of having diabetes tended to increase with age.
Blood glucose is affected by more than just sugar
When people think about blood glucose, they often focus on foods that taste sweet. However, carbohydrate is the nutrient most directly linked with post-meal glucose.
Starches found in foods such as rice, bread, noodles and potatoes are also broken down into glucose during digestion.
Carbohydrate includes:
- digestible carbohydrate
- dietary fibre
These components are handled differently in the digestive system, so they do not have identical effects simply because both are classified as carbohydrate.
Did you know? Starchy foods do not need to taste sweet to become glucose
White rice and bread do not taste strongly sweet, but they contain large amounts of starch.
Starch consists of many glucose units linked together and is broken down during digestion before being absorbed.
When thinking about blood glucose, it is therefore useful to look at the type and amount of carbohydrate, not only whether a food tastes sweet.
What is GI?
GI stands for Glycaemic Index.
It compares how much a carbohydrate-containing food raises blood glucose after eating relative to a reference food.
In general:
- higher-GI foods tend to produce a faster rise in post-meal glucose
- lower-GI foods tend to produce a more gradual rise
GI is best understood as one characteristic of a food.
In real meals, glucose responses can also vary according to portion size, cooking method, ripeness, dietary fibre, fat and protein consumed with the food.
The Japan Diabetes Society's public nutrition guidance also takes a cautious approach to interpreting the evidence for actively using low-GI foods as a diabetes-management strategy.
For this reason, it is practical to look at the overall amount, nutritional balance and composition of the meal rather than judging a food by GI alone.
What is GL, or glycaemic load?
Another useful concept related to GI is GL, or Glycaemic Load.
GI describes the glycaemic characteristics of the carbohydrate in a food, but the amount of carbohydrate actually eaten also matters.
GL is generally calculated as:
GI × available carbohydrate in one serving (g) ÷ 100
It therefore incorporates both GI and the quantity of available carbohydrate in the serving.
Did you know? The same GI can produce a different GL depending on portion size
Two servings of the same food can contain very different amounts of carbohydrate even though the food's GI is unchanged.
GL was developed to consider both the glycaemic characteristics of a food and the amount of carbohydrate actually eaten.
Like GI, GL is only one part of evaluating a meal and should be considered alongside dietary fibre, nutritional balance and total food intake.
Dietary fibre and the overall meal
When thinking about food and blood glucose, it is useful to look at the overall composition of the meal rather than focusing on a single food.
Foods that provide dietary fibre include:
- vegetables
- legumes
- seaweed
- mushrooms
- whole grains
- fruit
Eating carbohydrate alone can also produce a different digestive and post-meal glucose response from eating it as part of a mixed meal containing protein, fat and fibre.
A meal built from staple foods, protein-rich dishes and vegetables or other side dishes is generally more useful to consider than focusing on one ingredient in isolation.
Does the order in which you eat matter?
The order in which foods are eaten has also been studied in relation to post-meal glucose.
Public guidance from the Japan Diabetes Society describes eating meat, fish and vegetables before carbohydrate-rich staple foods such as rice or bread as one possible dietary strategy used in diabetes nutrition therapy.
Eating slowly and chewing well is also commonly included in advice for structuring meals.
Meal order is best considered together with:
- overall portion size
- the amount of carbohydrate
- dietary fibre
- protein
- fat
- meal timing
- eating speed
This makes it easier to look at the meal as a whole rather than relying on one technique alone.
Exercise and blood glucose
Muscle is one of the major tissues that uses glucose.
During physical activity, energy use in the muscles increases and glucose is used as part of that energy supply.
Regular exercise is also studied in relation to insulin action and glucose metabolism.
The Japan Diabetes Society explains that in diabetes exercise therapy, post-meal physical activity can increase glucose use by muscle and contribute to the regulation of post-meal glucose.
For people without diabetes, maintaining regular physical activity is also a basic part of general health.
For people receiving diabetes treatment, medications, insulin use and complications may affect the safest timing and type of exercise.
If you are being treated for diabetes, discuss the type, duration and timing of exercise with your doctor or another qualified healthcare professional.
What does CGM actually measure?
CGM stands for Continuous Glucose Monitoring.
Small sensors worn on the arm or another site can provide repeated measurements of glucose changes throughout the day.
Did you know? CGM measures glucose in interstitial fluid
CGM sensors measure glucose in the interstitial fluid beneath the skin.
Because changes in blood glucose take time to be reflected in interstitial fluid, there can be a time lag and some difference between a blood glucose measurement and a CGM reading.
The Japan Diabetes Society's 2026 guidance on the appropriate use of continuous glucose monitoring devices also describes CGM as measuring glucose in interstitial fluid.
Diabetes diagnosis uses established blood tests
Under Japan Diabetes Society diagnostic criteria, the glucose values used for diabetes diagnosis are based on venous plasma glucose measured in a medical setting.
CGM is a useful tool for observing glucose patterns and changes in everyday life.
How the data are interpreted depends on the reason for using the device and the person's treatment situation.
What to do if your health-check results concern you
If blood glucose or HbA1c on a health check catches your attention, first confirm:
- whether the glucose was measured fasting
- whether it was measured after a meal or at a random time
- what the HbA1c result was
- how the result compares with previous tests
Because blood glucose is influenced by meal timing and the conditions of measurement, looking at HbA1c and any recommended follow-up testing together can provide a clearer picture.
If a healthcare provider recommends repeat testing or further assessment, take your results with you and discuss the next steps with a doctor.
Summary: understand the system, not just one number
Blood glucose is continuously regulated through the combined effects of food, insulin, the liver, muscles and other metabolic processes.
The key points are:
- Blood glucose refers to the concentration of glucose in the blood.
- After eating, glucose from carbohydrate is absorbed and regulated by insulin and other metabolic processes.
- Between meals, the liver helps supply glucose so that an appropriate blood glucose level can be maintained.
- HbA1c reflects average glucose over roughly the previous one to two months rather than only the level at the time of testing.
- Diagnostic criteria and glucose-management targets during diabetes treatment serve different purposes.
- GI describes one characteristic of a food, while GL also incorporates the amount of available carbohydrate eaten.
- Meals are best considered in terms of carbohydrate, fibre, protein, fat, quantity and eating pattern together.
- CGM measures glucose in interstitial fluid, while diabetes diagnosis uses established blood tests in a medical setting.
When interpreting blood glucose, it is useful to look beyond whether a number is simply “high” or “low” and consider when it was measured, whether it was fasting or post-meal, what the HbA1c shows and how glucose is regulated in the body.
Related Articles
Main References
- Japan Diabetes Society. Clinical Practice Guideline for Diabetes 2024.
- Japan Diabetes Society. Guide to Diabetes Treatment 2026.
- Japan Diabetes Society. What Is Diabetes?
- Japan Diabetes Society. How Is Type 2 Diabetes Treated?
- Japan Diabetes Society. Healthy Eating Start Book.
- Japan Diabetes Society. Guidelines for the Appropriate Use of Continuous Glucose Monitoring Devices, 2026 revision.
- Ministry of Health, Labour and Welfare, Japan. National Health and Nutrition Survey 2024.
- Ministry of Health, Labour and Welfare, Japan. e-Health Net: Blood Glucose and HbA1c.
- Harvard T.H. Chan School of Public Health. Carbohydrates and Blood Sugar.
- National Library of Medicine MeSH. Glycemic Load.
Important Information
This article is intended to provide general educational information about blood glucose, HbA1c, post-meal glucose, GI and related nutrition concepts.
It is not intended to diagnose, treat, cure or prevent disease, and it does not claim that any particular food, health food or supplement lowers blood glucose or prevents or improves diabetes.
Diabetes and disorders of glucose tolerance are diagnosed in medical settings using blood glucose, HbA1c, OGTT, symptoms, medical history and other relevant information.
If a health check recommends repeat testing or medical assessment for blood glucose or HbA1c, or if you have symptoms such as excessive thirst, increased drinking, frequent urination or unexplained weight loss, consult a healthcare professional.
If you are receiving treatment for diabetes, major changes in food intake or exercise can interact with medicines or insulin and may increase the risk of hypoglycaemia. Discuss significant changes to diet, exercise or glucose monitoring with your doctor, dietitian or other qualified healthcare professional.
