FREE booklet: The GLP-1 Story by Dr. Sayantan Banerjee. Get your copy →

Fasting Blood Sugar of 100 to 125 mg/dL: What It Means and What to Do

A fasting glucose of 100 to 125 mg/dL is impaired fasting glucose, a form of prediabetes. Dr. Sayantan Banerjee explains the US and UK ranges, testing tips, and how to bring it down.

PREDIABETES REMISSION

Dr. Sayantan Banerjee

10/31/20268 min read

You checked your blood sugar first thing in the morning, or had a fasting blood test, and the number came back at 108, 115 or 122 mg/dL. Or, in UK units, 6.0, 6.4 or 6.8 mmol/L. It isn't high enough to be called diabetes, but it isn't normal either.

This range has a name: impaired fasting glucose. It is one of the ways prediabetes is diagnosed. This article explains what fasting glucose measures, how the US and UK define the ranges, why your morning reading might be higher than you expect, how to test properly at home, and how to bring the number down.

What fasting glucose measures

Fasting blood glucose is the amount of glucose in your blood after at least eight hours without food or drinks other than water. Usually it is measured first thing in the morning.

Overnight, when you aren't eating, your liver keeps your blood supplied with glucose by releasing it from storage and making new glucose. Insulin normally keeps this release in check. When the liver becomes resistant to insulin, often because of fat stored in the liver, it releases too much glucose overnight. That is the main reason fasting glucose rises in prediabetes and type 2 diabetes.

So a raised fasting glucose is mainly a sign that your liver is struggling to respond to insulin.

The ranges, in US and UK units

Different organisations use slightly different cut-offs.

Normal — ADA (US): below 100 mg/dL (5.6 mmol/L); NICE / NHS (UK): below 5.5 mmol/L; WHO: below 6.1 mmol/L (110 mg/dL)
Impaired fasting glucose (prediabetes) — ADA (US): 100 to 125 mg/dL (5.6 to 6.9 mmol/L); NICE / NHS (UK): 5.5 to 6.9 mmol/L; WHO: 6.1 to 6.9 mmol/L (110 to 125 mg/dL)
Diabetes — ADA (US): 126 mg/dL (7.0 mmol/L) or higher; NICE / NHS (UK): 7.0 mmol/L or higher; WHO: 7.0 mmol/L or higher

The ADA set its lower cut-off at 100 mg/dL in 2003, lowering it from 110, to catch people earlier. The WHO kept the higher threshold. That is why someone with a fasting glucose of 105 mg/dL (5.8 mmol/L) may be told they have prediabetes in the US but may not be given that label elsewhere.

A diagnosis of diabetes needs either two abnormal results or one abnormal result plus symptoms. A single reading of 126 or more isn't enough on its own unless you have clear symptoms.

Converting between units

To convert mg/dL to mmol/L, divide by 18. To convert mmol/L to mg/dL, multiply by 18.

90 — mmol/L: 5.0
100 — mmol/L: 5.6
105 — mmol/L: 5.8
110 — mmol/L: 6.1
115 — mmol/L: 6.4
120 — mmol/L: 6.7
125 — mmol/L: 6.9
126 — mmol/L: 7.0

Impaired fasting glucose versus impaired glucose tolerance

Prediabetes comes in two main forms, and many people have both.

Impaired fasting glucose (IFG) means the morning reading is raised. It mainly reflects insulin resistance in the liver.

Impaired glucose tolerance (IGT) means glucose is too high two hours after drinking a standard glucose drink. It mainly reflects insulin resistance in muscle and a slower insulin response to meals.

Some people have a normal fasting glucose but raised glucose after meals, and vice versa. That is why different tests sometimes give different answers. A large 2020 review in The BMJ found that impaired glucose tolerance carried a somewhat higher risk of heart disease and death than impaired fasting glucose alone, although both were linked to higher risk than normal glucose.

If your fasting glucose is raised, your doctor may also check your A1C or arrange a glucose tolerance test to get a fuller picture.

Why your morning number might be higher than expected

Many people are surprised that their fasting glucose is higher than their readings later in the day. There are several reasons.

The dawn phenomenon. In the early morning, the body releases hormones such as cortisol and growth hormone that help you wake up. These hormones raise glucose. In people with insulin resistance, the body can't fully counter this rise, so morning glucose is higher.

Late or large evening meals. A big meal, especially one high in refined carbohydrates, late in the evening can keep glucose higher overnight.

Alcohol. Alcohol can lower glucose at first and then cause rebounds. It also adds to liver fat over time.

Poor sleep. Short or disrupted sleep raises cortisol and reduces insulin sensitivity the next morning.

Illness and stress. Infections, pain and stress all raise glucose.

Not truly fasting. Milky coffee, juice or a snack before the test will raise the result.

How to test fasting glucose properly at home

If you use a home meter, a few steps give more reliable results:

Test first thing in the morning, before eating or drinking anything except water.
Make sure it has been at least eight hours since your last food.
Wash and dry your hands first, since sugar on the skin can give a falsely high reading.
Use the side of your fingertip and the first good drop of blood.
Check that your strips are in date and stored properly.
Test on several mornings and look at the average, not a single number.

Home meters are useful for tracking trends, but they are less accurate than a laboratory test. International standards allow results within about 15% of the true value for most readings. So a home reading of 104 mg/dL could reflect a true value anywhere from the high 80s to about 120. A diagnosis should be based on a laboratory test.

What a reading in this range means for your future

Impaired fasting glucose is a sign of raised risk, not a diagnosis of diabetes. Many people with it never develop diabetes, especially if they make changes. But the risk is real. A review in The Lancet estimated that about 5% to 10% of people with prediabetes develop type 2 diabetes each year. Higher readings, closer to 125 mg/dL (6.9 mmol/L), carry more risk than readings just above 100.

The good news comes from the US Diabetes Prevention Program, where lifestyle changes cut the risk of type 2 diabetes by 58%, and returning to normal glucose even once was linked with a 56% lower risk later on.

How to bring your fasting glucose down

Because fasting glucose mainly reflects the liver, the most effective changes are those that reduce liver fat and improve insulin sensitivity.

Lose weight if you need to. Liver fat falls quickly with weight loss, often within weeks. Many people see their morning readings fall early in a weight-loss programme.

Cut sugary drinks and refined carbohydrates. These feed fat production in the liver.

Move more, every day. Regular activity improves insulin sensitivity. A walk after dinner can lower overnight glucose.

Eat dinner earlier, and keep it moderate. Finishing your evening meal two to three hours before bed, with plenty of vegetables and protein and a modest portion of starchy food, may help morning readings.

Limit alcohol, especially in the evening.

Improve sleep. Aim for at least seven hours, at regular times.

Build muscle. Strength training two or three times a week increases the body's capacity to handle glucose.

Ask about medication if needed. Metformin works mainly by reducing glucose release from the liver, so it particularly lowers fasting glucose. It may be considered for some people with prediabetes, especially those with a high BMI.

What to do this month

Confirm the result with a laboratory test if it was a home reading.
Ask for an A1C and a cholesterol test, including triglycerides.
Measure your waist and calculate your waist-to-height ratio.
Start a 10 to 15 minute walk after your evening meal.
Move your evening meal earlier if you can, and cut late snacks.
Swap sugary drinks for water, tea or coffee.
Ask about a prevention programme.
Retest in three to six months.

Evening habits that help morning glucose

Because fasting glucose reflects what your liver does overnight, the evening is a good time to focus on. Here is an example of an evening routine that many people find helps their morning readings.

6 pm to 7 pm: dinner built on vegetables and protein, such as grilled fish, chicken, tofu or lentils with a large portion of vegetables, and a modest portion of starchy food, such as a fist-sized serving of brown rice or new potatoes.
Straight after dinner: a 10 to 15 minute walk.
Evening: no sugary drinks, sweets or large snacks. If you are hungry, a small portion of plain yogurt, a few nuts or some vegetables with hummus.
Alcohol: none, or a small amount with food, well within guidelines.
10 pm to 11 pm: lights out, aiming for at least seven hours of sleep.

Try this pattern for two weeks and compare your average morning readings with the two weeks before. Changes to the evening often show up in the morning numbers within days.

The dawn phenomenon and the Somogyi effect

You may read about two causes of high morning glucose. The dawn phenomenon, described above, is a natural early-morning rise in glucose driven by hormones such as cortisol. It is common and well established.

The Somogyi effect is a theory that a low glucose in the night causes a rebound high in the morning. It was described in people using insulin. More recent studies using continuous glucose monitors suggest it is uncommon, and in most people a high morning reading reflects the dawn phenomenon or high glucose through the night rather than a rebound. If you take insulin or a sulfonylurea and have high morning readings, discuss them with your diabetes team, who may suggest checking your glucose overnight.

When to repeat the test

If you had a single fasting glucose in the prediabetes range, your doctor may repeat it or check your A1C to confirm. If both are in the prediabetes range, the diagnosis is clearer.

If your result was in the diabetes range, 126 mg/dL (7.0 mmol/L) or above, it should be repeated on another day, unless you have clear symptoms, to confirm the diagnosis.

Once you start making changes, a repeat test after three to six months shows whether they are working.

How my book can help

The Prediabetes Exit Strategy explains insulin resistance in plain language and how changes to eating, movement, sleep and daily routine can bring fasting glucose back towards normal. More on the book page.

Frequently asked questions

Is a fasting blood sugar of 110 bad? It is in the prediabetes range by US standards (6.1 mmol/L). It is a signal to make changes, not a diagnosis of diabetes.

What is a normal fasting blood sugar? Below 100 mg/dL (5.6 mmol/L) by ADA standards, or below 5.5 mmol/L by NHS standards.

Why is my fasting glucose high but my A1C normal? The tests measure different things. Fasting glucose reflects overnight liver output, while A1C reflects average glucose over three months. Discordant results are common.

Can coffee raise fasting glucose? Black coffee usually has a small effect. Milk, sugar or syrups will raise it, so test before your morning coffee.

How quickly can fasting glucose come down? Many people see improvements within a few weeks of weight loss, more activity and earlier, lighter evening meals.

Should I test every day? Not necessarily. Testing on a few mornings each week, or for a week every month, is enough to see trends.

Does exercise in the morning help fasting glucose? Any regular exercise helps over time. Some people see lower readings the morning after an evening walk.

Can dehydration affect a fasting glucose reading? Severe dehydration can raise glucose a little. Drink water normally before a fasting test.

Is 99 mg/dL normal? Yes, by ADA standards it is just below the prediabetes range, although risk rises gradually across the normal range too.