Ask Joslin: Varying Blood Sugar Readings

Why do I get such different blood sugar readings even though I eat the same thing for breakfast every day?

Food intake, although a large component of blood glucose variability, is only one factor affecting your glycemic control. Exercise or physical activity is another. However, even when changes in physical activity and medications aren’t a part of the equation, there are many other factors that play into making your blood glucose a moving target.

One of these is the insulins we have available. NPH insulin is notorious for having large intra-individual variability (meaning you can get widely different results each time you use it), but even the newer insulins, Lantus® and Levemir®, are not anywhere near 100 percent reproducible every time. One study by Heise et al published in Diabetes found that the serum concentrations of insulin Lantus® and insulin Levemir® varied by 24 percent and 18 percent when participants received four same- quantity doses of the medication.

Other reasons you do not get reproducible blood glucose numbers are that the glucose output from your liver is not consistent every day and that your starting blood glucose readings will have an effect on the timing and action of your insulin.

Even your emotions and stress can play havoc with your numbers. Essentially the human body is not like a test tube: all variables can’t be controlled for

So, expect that you are never going to be able to reproduce the perfect lunch numbers you had today even if you choose to have a repeat of your meal tomorrow.


  1. Hi

    I am Type 2 diabetic.My blood sugar level now in the evening reaches 96 or 100 but in the morning fasting sugar test shows 250 to 280.
    I was just wondering is that harmful for a diabetic to dring at least 3 liters of beer,is this the cause of having raise inblood sugar early morning.Please advise.

    • Alcohol can lower blood glucose levels and put you at risk for hypoglycemia. Alcohol that also contains carbohydrate can both lower and raise glucose at different times. Whether or not you have diabetes 3 liters of beer is excessive. The alcohol in 8.5 beers can cause intoxication and make it difficult to appropriately manage self-care behaviors. The recommendations for alcohol is a maximum of one alcoholic beverage for women and 2 alcoholic beverages for men per day. One beverage is defined as once 12 ounce beverage, one 5 ounce glass of wine or 1.5 ounces of spirits.
      –Nora Saul, Manager of Nutrition Services at Joslin

  2. I also have fluctuations in my reading. Then recently I had a colonoscopy and after the 5 day prep with no food or medication for over 36 hours, the physician performing the exam found a pill in my bowel that was still in tact. Could this be part of the problem if my pills are not dissolving in my gut?

    • If it is Janumet, they do not dissolve. I asked my dr. about that and she said the meds come out, but the outer layer does not dissolve
      . I still don’t understand it at all.

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