What Makes my Blood Sugar Go Up and Down?

By Timothy Bailey, MD, FACE, FACP

Let's say that two people each have an A1c level of 7.0%. But the first person's blood glucose tends to run in the “30’s” and “200’s”, while the second person stays in a much tighter range, between 80 and 180 mg/dL.

Does it matter? Well, clinical studies suggest that it might. Highly variable blood glucose may lead to diabetes complications, even if your 'average' A1c level looks good. That's because the A1c test is only an average number... it can cover up a lot of highs and lows.

In 1995, clinical investigators suggested that other factors besides a high A1c might lead to diabetes complications. They compared two groups of people who had similar A1c averages:

  • One group managed their diabetes intensively, with 3 or more insulin injections daily or with an insulin pump

  • The other group treated their diabetes with a more conventional regimen of two insulin injections per day

Even though the two groups had similar A1c averages, the people in the intensive treatment group had significantly fewer diabetes complications. Why? The investigators thought that lower blood glucose variability might be the cause of the better outcomes.

Researchers have been trying to figure out why and how big swings in blood glucose can lead to complications. The current prime suspect is something called 'oxidative stress'. This can be either an increase in unstable substances that can oxidize cells in the body, or a decrease in protective antioxidants. Future clinical trials will be needed to confirm this theory.

Clinical studies have led experts to recommend that people with diabetes keep their average blood glucose levels as close to “normal” as possible:

  • Pre-meal plasma glucose between 90-130 mg/dL
  • Post-meal plasma glucose less than 180 mg/dL
  • A1c level less than 7%

Blood glucose variability can differ based on the type of diabetes you have. People with type 2 diabetes may have less variability because they can still produce some insulin, which helps to bring their blood sugar down to normal throughout the day. On the other hand, people with type 1 diabetes produce no insulin, and can have greater blood glucose variability.

How Big Are Your Ups and Downs?

Here's a new phrase for your diabetes vocabulary: standard deviation.

It sounds complicated, but the idea is really simple. The more scattered, or variable, the blood glucose numbers are at any particular time of day, the bigger the standard deviation will be. For example, a big standard deviation before lunch means that you have large swings in your blood glucose before that meal. One goal of your diabetes care should be to keep blood glucose readings in a fairly narrow range. That way, the standard deviation will be as small as possible.

Blood glucose meters do not display the standard deviation, so in order to track it you will need to use the diabetes software that works with your meter. Software is useful because even if your doctor prefers to look at a logbook, the software can print out your logbook with the standard deviations already calculated.

You can use the standard deviation to look at your overall blood sugar control, or to check on specific times of day such as before breakfast. Just use the rule of thumb developed by Dr. Irl Hirsch, an endocrinologist at the University of Washington in Seattle. His guideline is that the standard deviation multiplied by two should be less than the average blood glucose number.

Let's say that a person has an average lunchtime blood glucose of 150, and a standard deviation of 50. 50 times 2 equals 100, which is less than 150, so the variability is acceptable. On the other hand, if the standard deviation is 90 it is considered too high, because 90 times 2 equals 180, which is greater than the average of 150.

How to reduce blood glucose variability

If your blood glucose levels tend to swing widely, here are some actions that you can discuss with your doctor:

  • Check your blood glucose more often. Imagine if you checked your blood glucose only once a day, in the morning. If you drew a line connecting those morning readings you might have a nice flat line making it appear that there is little variation. You know better! If you start checking before each meal and even throw in a few blood glucose readings after meals, you will get a much truer picture of your blood glucose range.

  • If you have type 2 diabetes, ask your doctor if a c-peptide test (a measure of how much insulin you produce) could help you in understanding your blood glucose variability.

  • New types of injected insulin have dramatically improved our ability to reduce glucose variability. Shorter-acting medications such as Humalog®, Novolog®, and Apidra® help reduce the spike after eating. “Starch blockers” such as acarbose or miglitol can also be helpful.

  • If you use an insulin pump, look into continuous blood glucose testing. In one Medtronic Minimed study, people who used continuous blood glucose testing voluntarily reduced their carbohydrate intake and also took more insulin to correct their highs.

  • Meals aren’t the only challenge to reducing blood glucose variability. Insulin works differently each time it is injected. This is especially true of the older insulin types, which many people with diabetes may have suspected. However, the newer long-acting insulins Lantus® and Levemir® are much more predictable, so that you may get fewer blood glucose surprises. That's a very good thing!

  • Insulin delivery devices are another help. Insulin pens make injecting much easier and many patients have been able to take more shots, leading to decreased variability. Remember, your pancreas would normally give you continuous insulin throughout the day. More frequent, smaller doses of insulin can better meet your needs. Think like a pancreas! Insulin pumps have also made intensive insulin therapy easier. In addition to giving more precise mealtime boluses, pumps let you set variable basal rates. For example, you can drop your insulin levels after going to bed so you have the option of not eating a bedtime snack.


    Blood glucose variability may be as important to your health as blood glucose averages and A1c levels. Your blood glucose meter and diabetes software give you the tools to identify the times you are out of range, so check your blood glucose levels often, and let your doctor know about any variability issues.

    Dr. Bailey is Board-Certified in Endocrinology, Metabolism, and Internal Medicine. He specializes in treating diabetes, osteoporosis, thyroid diseases, and lipid disorders at NorthCounty Endocrine, Escondido, CA. He is in full-time clinical practice and a clinical investigator active in conducting phase II to IV clinical trials. Optimally managing data from medical devices has been is a long-term interest of his. As a clinical Assistant Professor at the University of California at San Diego, he is active in the education of endocrinology fellows. He is a frequent speaker to both physician and patient groups.

    Insulin brands are trademarks of their respective holders.

    Important note: The content of this article is not intended to be a substitute for professional medical advice, diagnosis or treatment. Do not disregard your doctor's advice or delay in seeking it because of something you have read in this article.



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