March 1, 2012

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.


Alan said...

Thank you for the link Jenny. It is good to see that they are finally starting to do research in this field. Maybe it will cause some doctors to look at the peak post-prandial timing rather than the traditional and pretty useless standard of two hours for post-prandial testing.

Now to see if they extend that to finding out the consequences of the earlier spikes and the benefits of using them to modify menus for smaller spikes.

Cheers, Alan

Jenny said...

Nice to see you posting here, Alan.

Unfortunately, there is so much bad research "proving" that testing doesn't improve glycemic control that most doctors don't care about it at all.

That research is done by telling people to test fasting and eat lots of bananas, pasta and oatmeal. Needless to say, when they follow directions, testing doesn't have any positive impact on their blood sugars, because the more bananas and oatmeal they eat the worse it gets.

Helen Howes said...

Re: Oatmeal
Reply #4 - Feb 22nd, 2012 at 9:38am Quote Modify
Converstion with the D nurse
Nurse: "You should have porridge for breakfast"
Me: "I did that but my blood sugars went off the top of the meter in less than 1/2 hour, then crashed to hypo after 2 hours"
Nurse: "Oh, you don't want to test after eating it, you'll frighten yourself.."
Me: Stunned silence

Jenny said...

Helen. As the kids would say, OMG. It would be funny if it weren't so typical.

jackie long said...

I have been diagnosed with high blood sugar and was given metoforman to help control it,yet my va doctor says I don't have dabetes because of the test they do called the a1c average.Some times my sugar is as low as 50 and runs from 140 to over 200.They say my avg is ok.Doe's this test work while taking metoformin?I am confused because I have to watch what I eat and test every day.Is this Type 11 diabetes?

Jenny said...

Jackie, The A1c is a poor test for diagnosing diabetes but doctors use it because it is cheap, and because few of them really understand diabetes.

You can read about what blood sugar levels damage your organs HERE and HERE.

Then you can learn how to lower your blood sugar to a safe level HERE.

Metformin is a good, safe drug and should help but it works a lot better if you also adjust your diet as explained above.

Unknown said...

Interesting. I was told to test 2 hours after the beginning of the meal--which works out to about 90 minutes after I finish the meal. Most of the time, it's pretty low by then, but it's been wonky lately.

Natalie said...

Thank you for the link. You are absolutely right that the right time to test varies according to many different factors, and the more you test, the more you know your own body and can adjust peak-testing time accordingly. Also, I'm glad you mentioned end-of-insulin testing. Especially for those of us who have delayed stomach emptying, the food can stay in our stomachs WELL after the first bolus of insulin stops working. I have had meals that didn't completely empty out for 12 hours, and that means testing every 3 hours (which is how long Novolog lasts for me) for that period. If the meal happens to be dinner, it's a royal PITA.

Also, it appears that A1c is likely to become the standard for diagnosis, poor as it may be, because it's easier for the DOCS. They completely ignore the fact that different people glycate differently, and some people can be running diabetic BGs with A1cs in the 4's and 5's. I did (4.8). Seems to me that anyone who presents with an A1c between 5 and 6.5 (the likely cutoff) should at least have a couple of fasting tests, and in utopia, a glucose tolerance test. And the A1c should be a routine test for ANYONE who has ANY risk factors for T2. But those of us on the edges of the bell curve have no right to medical care that fits US, because average reigns!

Jenny said...

I'm hearing scary stories from people running seriously diabetic blood sugars whose doctors refused to treat them because their A1cs were in the high 6s.

Anemia and some other genetic conditions can make the A1c completely useless.

Don't blame doctors for this one, though, this is the American Diabetes Association working to save money for insurance companies who has "educated" them to use the A1c. The ADA serves the needs of everyone who make money of people with diabetes, not those of us who have it.

Piazzi said...

many Many thanks for Many many good posts

I stumbled upon your blog by sheer chance of web browsing and got hooked into reading post after post for hours

One thing I have found that has helped me deciding what to eat and what not is the basic knowledge how one gram of carb raises BG level

in 200 lb person, 1 gram of carb raised 3 mg/dl, in a 150 lb person it raises 4

now, if before meal, BG is 100 and I don't want it to raise above 150 at the peak and I am 200 lb, then I have 50/3= 16 grams of carb if I eat pure carb and nothing else

I can use this guide line and choose what I cab eat

Since this may lead to small portions of stuff, I may need to eat more often (say every 4 hours or so) but at less portions since I have a 150 cap on the peak

30, 60, 90, 120 min tests of BG can then verify of I am doing as I want and over time, I will have a catalogue of what i have had and what effects I have seen as a guide for food selection

Jenny said...


That amount of carb would raise your blood sugar that much if you had no insulin working. But if you are making insulin, and if your blood sugar goes over about 120 mg/dl you'll secrete a bunch of it in response. That insulin will take care of some amount of carbs. That's why you have to test meals that contain known amounts of carb to figure out how much you can tolerate.

The 2 gram cure works because people don't secrete insulin when their blood sugar is low.

Kim Graves said...

Hi Jenny,

I'm a new fan of yours: I've read your book and am pleased with the progress I'm making. My sugar never goes over 120 even after meals, but here's my question: I'm taking three readings: pre-meal; 1 hour; 2 hour for both breakfast and dinner. Here are the average values since I started the new diet. Breakfast:103/113/115; Dinner: 96/102/104. These are clearly good, but my question is why are the values not dropping after an hour?

Thanks for your help, Kim

Jenny said...

Kim, It's possible that your blood sugar is rising for another half hour and then heading downward, but with such excellent, normal readings I wouldn't worry about it.

Howard said...

I was writing an article about one of your posts from last year, and while in the process of adding you to my blogroll on my new blog, I decided it had been too long since I visited here. I see you are still producing quality stuff (and no doubt I will occasionally cover some of the same stuff). I have thought about putting my own two cent's worth about the Red Meat Menace, but I think it has been adequately covered already.

You seemed shocked that the "study" seemed to have been heavily influenced by the funding source.

I'm not particularly shocked over that -- the hallmark of an observational study is that it always manages to arrive at the conclusion required by its funding source. I have not yet encountered an exception.

Unknown said...


in one of your previous posts, you said that normal non-diabetics will peak at 30-45 minutes, then quickly take care of that spike. You said that quick spike in normal non diabetics could be as high as, but no higher than 160. Obviously everyone different.

I am a non diabetic, fasting at 85, A1C at 5.5

I have a family history, so I keep an eye out, also I thought I was feeling the effects of sugar spikes over the summer. So in started to test. My 45 min test after some shredded wheat and yogurt is at 154, but my 60 is 129. This article says you want to find your highest value? Doesn't everyone peak and then come down? So you wouldn't want your absolute peak, because that stimulates the insulin. Confused for sure. Also is it normal to have your post meal (3hrs or so be slightly lower than your fasting?)

Jenny said...

Your readings look like they are still well within the normal range. The 1 hour reading is the one you want to look at.

That said, the fact that your blood sugars is lower than fasting at 3 hours might indicate the beginning of reactive hypoglycemia which is the very first stage of the process that, unheeded, leads to Type 2 over many years. It can signal that you are secreting a bit more insulin than is needed which may point to insulin resistance.

This early on the best thing to do is to avoid fast carbs where possible and check every 6 months or so to see how your blood sugar control is holding up.

Milly said...

I'm a new fan; have read, enjoyed and implemented both of your books. It was so welcome to get truthful accurate information. I'm 5'2'began the program and
went from 162 lbs to 133lbs and then plateaued.
I've been reluctent not started and meds so far but am considering it as I am experiencing morning spikes which are bringing readings of 150 and up for several hours each morning. I cut back on BP med and that helped for a while but the spikes are back and contining.
Once the morning spikes come back down my 1 hr post prandial is <140, 2h<120.
Anyway I'm thinking of going on metformin to help get the last 10 or 12 lbs off and manage the spikes. Any thoughts?


Jenny said...


If changing your diet and cutting carbs don't normalize blood sugar it is very wise to try metformin, as it is the safest of all the diabetes drugs and has decades of safety record behind it. Metformin will stop the liver from dumping glucose when you are fasting, and that may help with those high fasting readings.

Milly said...

Thanks Jenny. I was in this am to discuss and am getting push back about the metformin and my decision to try it. They did an insulin test and are going to review that to see if they can find a reason to break with their protocol which says I'm in good control.
I may very well need to find a different practce - and am so
appreciate of the knowledge I gained form your books so that I can advocate for myself!
I live in NH so should be able to easily find an MD who's more collaborative and up to date.

Anonymous said...


I posted above for Ryan

After a bowl of Cheerios in the morning.

Fasting = 83, 45 min=130, 60=115, 120=83, 150=69, 180=77

You've said this is reactive hypo, and points to insulin resistance. I don't want to progress any worse. It doesn't dip like this for other meals, just usually breakfast. It seems like a bowl of Cheerios shouldn't do this. It would seems like it would need to be more carbs. When I add more carbs, it peaks a little higher, then will dip to the same, then level off. I just lost 30lbs in the last few moths (bmi from 27 to now 23) and completely changed my diet from high carbs and refined sugars to more lean protein and slow carbs. Could my body still be adjusting? Could my glucogen store be low? My blood pressure was high for a few years, but now its perfect. I'm also excersisng and lifting more than ever. How can I avoid these lows, and reverse any damage I may have done. Thoughts?

29M, previous A1C was 5.5 (when i had the extra 30 lbs and bad diet. Haven't had one since losing the weight and eating better)

In using a Relion Micro from Walmart, and I understand the error, but I think I'm feeling the effects of hypo sometimes.

Also, when I did your bagel challenge it was at 150 at 60 minutes, then 130 ten minutes later, then under 100 at 120 minutes. This points to IR as well with the high 1hr value, but do I need to worry if it is under 140 nearly right after that. Thoughts?

Jenny said...

Anonymous, This is not an appropriate place to discuss people's individual health problems.

My focus here and on the blog is on helping people with diabetes avoid developing diabetic complications. Keeping blood sugar under 140 mg/dl at all times should achieve that.

Your blood sugars are well within the normal range, so given the inaccuracy of the Relion meter ther is little point in obsessing about the values you are seeing.

Cheerios are actually quite high in carbs, especially if eaten with milk which is also high in carbs. If you want to limit reactive lows, try eating a protein breakfast instead of cereal. Most people are most insulin resistant at breakfast, so that is a good time not to start out with a high carb meal.

kandi said...

if i start out at 140 in the a.m. and i end 165 two hrs later how do i determine my range

Jenny said...

Kandi, I'm not sure what you mean by determine your range. Ideally you would like to get both those numbers down another 25 mg/dl or more if possible as many studies show that keeping your post-meal numbers under 140 mg/dl and your fasting blood sugar well under 125 mg/dl will prevent damage to your heart, eyes, kidneys etc.