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What Do Your Fasting Blood Glucose and Hemoglobin A1c Levels Mean?

By Diana Licalzi, MS, RD, December 13, 2021

fasting blood glucose hemoglobin a1c testBlood glucose (aka “blood sugar”) serves as the primary energy source for our brain and body. Healthy blood glucose levels are therefore essential for maintaining overall health and longevity. Unfortunately, several factors including the Standard American Diet and a sedentary lifestyle can increase glucose beyond normal levels, and over time, result in prediabetes and type 2 diabetes. If undetected, consistently high glucose can lead to long-term health complications, including nerve damage, cardiovascular disease, and kidney failure. This article explains the two most common blood tests (fasting blood glucose and hemoglobin A1c) used to detect abnormal glucose levels and ways to properly regulate them.


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What is fasting blood glucose?

Glucose normally fluctuates throughout the day, particularly after meals. On a typical diet, we consume 45-60% of our calories from carbohydrates, which the body converts into glucose for fuel. When glucose enters our bloodstream, the pancreas receives a signal to release a hormone called insulin. As you can see below, insulin acts as a "key,” allowing glucose to funnel out of the bloodstream and into our cells. As this process continues, blood glucose levels drop back down to a normal range.


insulin action-1We can measure glucose levels with a fasting blood glucose (FBG) test. An FBG measures blood glucose concentration at a single point in time and provides a glimpse into your body’s efficiency at regulating your blood sugar. An FBG test requires fasting for at least 8 to 12 hours beforehand for an accurate depiction. Routine blood tests ordered by your physician, such as basic and comprehensive metabolic panels, include an FBG test.  

Fasting Blood Glucose Ranges [1]

  • Normal range: 65-99 mg/dL
  • Prediabetes: 100-125 mg/dL
  • Diabetes: 126mg/dL or higher on two separate tests

Healthy glucose levels are associated with better weight control, increased energy, and improved mood and cognition. High FBG after eight to 12 hours without eating indicates an issue with glucose regulation, and should be discussed with a physician.

What is Hemoglobin A1c?

When FBG levels are high, physicians typically order a hemoglobin A1c, or HbA1c, test to gain more insight into someone’s overall health. Unlike FBG tests, which are only meant to look at short-term health, your HbA1c value reflects your average blood glucose concentration over the previous three to four months. Here's why. 

As glucose builds up in the blood, it binds to hemoglobin, a protein found in red blood cells. Since glucose remains attached to hemoglobin throughout its life (90-120 days), HbA1c provides insight into your long term glucose exposure. The higher the glucose in your bloodstream, the more glucose will attach to hemoglobin, thus, the higher your HbA1c. HbA1c is reported as a percentage: the greater the percentage, the greater your blood glucose level. 

Hemoglobin A1c Ranges [1]

  • Normal range: below 5.7%
  • Prediabetes: 5.7-6.4%
  • Diabetes: over 6.4% 

As a rule of thumb, every 1% change in HbA1c results in an approximate 30 mg/dL change in glucose.[2] 


What increases HbA1c?

As previously mentioned, insulin is essential for properly regulated glucose levels. But several factors, including some outlined below, interfere with insulin’s effect, either by weakening its signal or making cells less responsive to it. 

Excess body weight

Excess body fat is a major contributor to insulin resistance.[3] The accumulation of fat, especially visceral fat (the kind that surrounds our internal organs), directly interferes with insulin signaling. This results in raised blood glucose and HbA1c levels, and therefore poses a risk for type 2 diabetes. Maintaining a healthy weight is imperative to well-controlled glucose levels. Even a 5-10% reduction in body weight can significantly improve glucose levels in overweight individuals.[4] 

Meat intake

Several studies consistently show a link between the consumption of meat and the incidence of type 2 diabetes. A 2015 meta-analysis—a study that reviews multiple studies at once—examined the meat intake of over 50,000 people. They found that the consumption of both processed and unprocessed meat was associated with higher fasting glucose levels.[5] One explanation for this outcome may be due to red meat's high content of saturated fat. When examining saturated fat specifically, its high intake also increases one’s risk of developing type 2 diabetes.[6] It could also be due to the high content of iron in red meat, which interferes with insulin signaling.[19]  

Low fiber & high sugar intake

The majority of Americans do not reach the recommended intake of dietary fiber. Fiber is a nutrient found in all plant-foods, but humans do not contain the necessary enzymes to break it down for digestion. As a result, fiber goes undigested in our body, and foods high in fiber tend to keep us full for longer. Multiple studies show that a fiber-rich diet leads to an overall decrease in calorie intake and weight loss, and reduces your risk of developing diabetes. You should aim for at least 25-38 grams of fiber per day - that’s around 2 cups of black beans or 3 cups of raspberries. Low fiber intake is often coupled with high sugar intake. Sugar is not only found in juice, soda, and candy but it’s also added to salad dressings, nut butters, yogurt, and even bread. Several studies indicate that excess sugar intake can result in increased fat, especially in the stomach region and liver.[21] Excess fat, especially in these areas, can increase one’s HbA1c and risk for diabetes. 


What if my HbA1c is normal but my glucose is above optimized?

Because an FBG test measures your glucose levels at a single point in time, it is more susceptible to your daily or weekly actions. If your HbA1c is normal but your blood glucose is above optimized, it could be due to one of the reasons below.  

Sleep deprivation

There’s evidence that poor sleep patterns impair glucose metabolism. People on restricted sleep demonstrate worsened insulin response to food, resulting in higher blood glucose spikes. In one study, sleep-deprived people had higher glucose and insulin levels after consuming breakfast compared to those who slept a healthy eight hours.[7] Other studies confirm this finding—insulin resistance is more common in people deprived of sleep.[8-9] 


The body releases a steroid hormone called cortisol to activate our "flight-or-fight" responses to counteract acutely stressful situations. Cortisol stimulates the release of blood glucose by breaking down its storage form, glycogen. If you’ve experienced a stressful week, good chances are your blood sugars may be slightly higher than normal. 


Because glucose fluctuates after meals, we recommend fasting for a full 12 hours prior to getting a blood test for an accurate reading. This means water only, not even black coffee. People react differently to coffee so we suggest to err on the side of caution and stick to just water.

A1c vs glucose

How to lower HbA1c and blood glucose levels

Maintaining a healthy weight, limiting meat intake, and getting a good night's sleep all contribute to better glucose control. However, several other lifestyle changes can help, too. 

Adopt a more vegetarian diet, including beans & nuts

Vegetarian diets are inversely associated with the risk of developing type 2 diabetes.[10] In a meta analysis comparing nine different experiments, researchers found that vegetarian diets consistently improved fasting blood glucose and HbA1c.[11] Why? They generally emphasize a diet rich in whole grains, fruits, vegetables, beans, and nuts, and lower levels of saturated and trans fat. Plus, beans often replace meat in these diets by acting as a protein source. Thanks to their high fiber content and low glycemic index (i.e. they don’t cause a blood sugar spike), beans stabilize blood glucose and insulin levels.[12] So individuals with type 2 diabetes may notice significantly lower A1c levels if they increase their bean intake to at least one cup per day.[13] Similar to beans, nuts are low glycemic and help regulate blood sugar levels. In a recent experiment, people who replaced some of their food with a serving of nuts saw improved glycemic control and HbA1c levels.[15] On a grander scale, eating nuts has an inverse association with type 2 diabetes—that is, people who include nuts in their daily lives tend to have a lower risk of developing diabetes.[16-17] 


Exercise is a crucial component in managing glucose and insulin levels. It directly improves blood glucose control, increases insulin sensitivity, and helps maintain a healthy weight.[14] Exercise is a very effective tool in preventing the development of type 2 diabetes. 

Practice mindfulness

Incorporating mindfulness into your daily routine can improve both glucose and cortisol levels. [20] Start by practicing mindfulness for 5 minutes at a time and gradually increase this time to 30 minutes or more per day. 

Prep more meals at home

Takeout and restaurant food are typically higher in calories and saturated fat, both of which we know can increase blood glucose levels. Cooking meals at home allows you to control what goes into your food and generally leads to healthier glucose and insulin levels.[18] 

Consider supplements

Lastly, certain supplements may help lower your blood glucose as well. You can find them here

Your physician will routinely measure your glucose at annual check-ups, but it usually stops there. InsideTracker tests your blood, pulls data from thousands of studies, and combines this information with your unique characteristics to deliver personalized recommendations for you to incorporate into your daily life. 


A summary for maintaining or lowering blood glucose and HbA1c levels:

  • Maintain a healthy weight
  • Limit red meat and sugar intake
  • Get 8 hours of sleep each night
  • Try a vegetarian diet (at least some days)
  • Maintain an exercise regimen
  • Incorporate nuts and beans into your diet
  • Make more meals at home
  • Practice mindfulness
  • Consider adding the right supplements for you


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Diana Licalzi, MS, RD 
Diana is a Content Strategist and Team Nutritionist at InsideTracker. She is a Registered Dietitian that specializes in plant-based nutrition and type 2 diabetes. You'll often find Diana whipping up plant-based recipes, sipping on a mocktail, or hiking up a mountain. You can follow her on Instagram at @dietitian.diana.