When To Test? A New Study Pinpoints Timing

One of the most common questions people email me is when exactly they should start measuring the "hour after eating" at which I suggest they should test their blood sugar. Does that hour start after the first bite or at the end of the meal?

Luckily for us, an obscure paper published last year give us a definitive answer. Luckily for me, that answer is identical to the advice I've been giving people who have asked me this question for the last five years. (I based my answer on a previous study and the reports of people posting about when they tested on online discussion groups.)

The study used used data collected from people with both Type 1 and Type 2 Diabetes, using insulin, who were wearing continuous glucose monitors. You can read it here:

Peak-time determination of post-meal glucose excursions in insulin-treated diabetic patients. Daenen S, et al. Diabetes Metab. 2010 Apr;36(2):165-9. Epub 2010 Mar 11.

For insight into why this study is so useful you have to keep in mind that the whole point of testing at one hour is to find the highest blood sugar reading after the meal.

This study found that the average blood sugar peak after breakfast was found at 72 minutes after the start of the meal, with most people's values falling between 49 minutes and 95 minutes.

However, one person in five saw a peak after 90 minutes from the start of the meal. The researchers observe that "Peak time correlated with the amplitude of postprandial excursions, but not with the peak glucose value." I.e. A rise of 100 mg/dl to 170 mg/dl from a starting value of 70 mg/dl would take longer than a rise of 30 mg/dl from 140 g/dl to that same 170 mg/dl, which makes sense.

Since many of us spend about 15 minutes eating a meal, this explains why many people will do just fine if they test hour after finishing their meal.

But not everyone gulps down their meals, so how fast we eat along with several other factors, including how fast our digestion works and what kinds of foods we eat, will also influence when that blood sugar peak occurs.

For example, some people find that meals heavy in fat digest more slowly than those that are made up mostly of starches and sugars. Large meals of any composition may produce a slightly delayed spike. And meals heavy in protein may, under some conditions, produce a rise in blood sugar at the next meal because dietary protein can be converted into blood glucose over a period of six hours.

So what we can take from this is that, as is so often true with anything to do with blood sugar, the only way we can know for certain when our own blood sugar is likely to peak is by testing at various times after eating the exact same meal and discovering when we see the highest reading for that meal.

If you always see the peak a lot earlier or later than the average person would, adjust your testing schedule. But don't make yourself crazy about it. A rough approximation of an hour after the end of the meal or an hour and fifteen minutes after the first bite will give you a reading that for most people will be informative enough.

Given the poor accuracy of meters, a reading in the middle 100 mg/dl range could easily be 15 mg/dl higher or lower strictly due to meter variation so once you've determined that your highest readings aren't occurring significantly later than average--two hours after you start your meal, for example, don't fret about exact timing.

The other important piece of information we look for when we test after eating is how fast our blood sugar is coming down after that peak because the longer blood sugars stay over 140 mg/dl, the more damage they do. (Details on what researchers have found about wht blood sugar levels cause damage can be found HERE.)

When you take a second reading is up to you and depends on how many strips you have and what previous tests have taught you about how your blood sugar works. Most of us will find it informative to test an hour after the peak occurs to see how fast our blood sugar is dropping from its peak.

In this particular study people's blood sugar dropped on average 0.82 mg/dL per minute or 49.2 mg/dl per hour. But the actual range of how fast their blood sugar dropped was very large, with the range in which most readings clustered extending from 7 mg/dl per hour to 91 mg/dl per hour and some outliers dropping not at all or even faster.

If you see only a very small drop in your own blood sugar an hour after its peak, or a rise, you should check in another hour. If the usual pattern you see is for your blood sugar to stay high for two hours or more after peaking, it's time to cut back on the carbohydrate in your meals, since carbohydrates are what raise blood sugar.

If cutting carbohydrates doesn't get your blood sugars rising less and dropping faster, it's time for a visit to the doctor to discuss adding a safe medication would be advisable. (Metformin and insulin are by far the safest choices. You can read about all the drugs doctors prescribe to drop blood sugar HERE.)

If you start using insulin at meal times you should also test your blood sugar towards the end of the insulin's period of activity. How long the insulin stays active varies both with the kind of insulin you use and the dose.

Regular Human Insulin (R insulin) which is sold as Humulin or Novolin, is active for a period that can extend as long as 6 hours, so testing at 4 or 5 hours can warn you if you are in danger of a hypo.

For Humalog, Novolog (Novorapid), and Apidra, which have shorter times of duration, testing at 3 or 4 hours is wise until you determine you aren't in danger of a hypo.

If you see a low value at any time when your insulin still has more time left to work, take some glucose to raise your blood sugar and adjust your dose the next time you eat that meal or a one with similar amount of carbohydrate.

If your doctor hasn't taught you how to adjust your dose of fast acting insulin to match your carbohydrate intake, ask for that kind of training. If you can't get it, educate yourself by reading John Walsh's book, Using Insulin or Dr. Bernstein's Diabetes Solution by Dr. Richard K. Bernstein.