Glucose Testing: After Meals?
Should diabetics test their blood sugars after eating, too? The debate continues.
Reviewed By Charlotte Grayson
Every diabetic is familiar with fasting blood glucose tests. You don't eat for at least eight hours, you check your blood sugar, and by that, you establish what your baseline glucose is.
This was at least the traditional idea of blood sugar testing. But given that most of us spend a lot of the day -- maybe even most of the day -- having eaten something within the last few hours, can we really call the glucose levels we have after eating "abnormal"? In fact, isn't it more abnormal to go eight hours during the day without eating or drinking anything?
In short, this is the idea behind postprandial -- or after-meal -- glucose testing. Since we spend so much time in a postprandial state, the argument goes, it's important to monitor blood glucose levels during that time, too. While it may make intuitive sense, postprandial testing is one of the most hotly debated subjects in diabetes care. Is it an important new way of testing blood glucose that will reshape diabetes treatment, or is it merely a distraction from what's really important?
Setting the Limits
Paul Jellinger, MD, past president of the American Association of Clinical Endocrinologists (AACE), is a firm believer in the importance of postprandial testing. In 2001, when Jellinger was president of AACE, the organization issued a consensus paper on diabetes treatment that discussed postprandial testing.
"What we came up with was a number of new guidelines for postprandial testing," Jellinger tells WebMD. "We made a recommendation that a person's postprandial blood sugar, taken two hours after eating, should not exceed 140 mg/dL."
How did they arrive at that limit? Given the ways in which we all eat, postprandial testing would seem inherently imprecise; for instance, the glucose level of someone who just ate a salad for lunch might be pretty different from someone who just finished a Thanksgiving dinner. To resolve this issue, the AACE set a cutoff based on a comparison of the typical glucose levels of diabetic and non-diabetic people after eating.
"We know that in a normal person without diabetes, it's very rare for his or her blood sugar to exceed 140 mg/dL two hours after a meal," says Jellinger. "It can happen, but not often."
However, Jellinger is quick to concede that the cutoff number has been contentious. Just because people without diabetes don't usually reach a blood-glucose level of more than 140 mg/dL does not necessarily mean that anything above that particular number increases the risks of diabetic complications.
"I think postprandial testing is very important," says Om Ganda, MD, director of the lipid clinic at the Joslin Diabetes Center and associate clinical professor at Harvard Medical School. "In order to control diabetes well, you need to control the blood sugars 24 hours a day, and not just when you're fasting. People can have a pretty normal fasting blood sugar but still have a high postprandial level."
While there haven't been a great number of studies of postprandial hyperglycemia, there have been a number of suggestive studies of something called post-challenge hyperglycemia. Post-challenge glucose tests are administered after a person takes a set amount of glucose, usually 75 mg in a liquid form. The precise relationship between post-challenge hyperglycemia and postprandial hyperglycemia has not been firmly established, but proponents cite a few studies that indicate a good correlation.
"There's a lot of epidemiological evidence coming from all over the world suggesting that post-challenge blood sugars and thus postprandial blood sugars have their own independent risks or at least greatly augment understood risks," says Jellinger. "We thought that it was in our patients' best interest to bring this issue to light."
Experts have been focusing an increasing amount of attention on the cardiovascular risks of diabetes -- such as heart attack and stroke -- and some epidemiological studies have suggested that postprandial hyperglycemia is directly related to cardiovascular complications.
In addition, some suggest that postprandial testing may detect people with impaired glucose tolerance (IGT) or impaired fasting glucose (IFG) -- so-called "pre-diabetes" -- who might be missed by fasting tests.
"Epidemiological evidence suggests that there are a lot of people out there who don't have diabetes and who don't have pre-diabetes but have abnormal postprandial glucose levels," Ganda tells WebMD. "And based on a number of studies in the U.S. and Europe, they may be at an increased risk of cardiovascular disease."
However, not everyone agrees that postprandial testing is so important. No one debates that people with diabetes are more likely to have postprandial glucose spikes -- or excursions -- than those without diabetes. What is debated, and debated fiercely, is whether these spikes require any specific treatment separate from typical care for diabetes.
"I think that the recent attention to postprandial glycemia is a distraction," says David M. Nathan, MD, director of the diabetes center at Massachusetts General Hospital and a professor of medicine at Harvard Medical School. "In general, postprandial glycemia, fasting glycemia, and chronic glycemia, as measured by the A1c test are highly correlated. Attention should just be focused on overall lower glycemia."
Nathan points to the biggest problem with the postprandial hyperglycemia hypothesis. While epidemiological research has suggested a connection between postprandial hyperglycemia and diabetic complications, this sort of research looks at a large number of variables and isn't designed to test postprandial hyperglycemia specifically. There isn't yet a method to single out the particular effects of postprandial hyperglycemia from other common risk factors like hyperglycemia, obesity, and hypertension. As a result, there's no way to know if postprandial glucose levels really matter on their own.
"I just don't think postprandial testing is worth putting energy into now, because if you look for other risk factors of diabetes, you will find all of these people anyway," says David E. Goldstein, MD, principal investigator from the health sciences center at the University of Missouri School of Medicine. "There just isn't evidence that postprandial glucose levels matter independent of other factors, like A1c."
"But it's a hot topic now, though," Goldstein says wearily. "It's a snake pit, or a bottomless pit. I don't know what to call it."
Detecting and Treating Postprandial Hyperglycemia
While the significance of postprandial testing and the standards for doing it haven't been firmly established, proponents like Goldstein and Ganda ask their patients to keep a log of blood sugars before and after a certain meal each day for a few weeks before a doctor's visit; that way, they can see whether there are any worrying spikes in the glucose levels above 140 mg/dL.
Treatment for postprandial hyperglycemia can include behavioral techniques such as exercise and weight loss, and medication. One ongoing study, the European Study to Prevent Non-Insulin Dependent Diabetes (STOP-NIDDM), seems to show that treatment of people with IGT -- based on postprandial blood sugars -- with the drug acarbose (Precose or Prandase) helped prevent the onset of type 2 diabetes and reduced the risks of cardiovascular problems. In his practice, Goldstein has had success using sulfonylureas and fast-acting insulins in reducing postprandial blood sugar levels.
And while the general significance of postprandial hyperglycemia is debated, there is some consensus that it is important for certain groups of people. In 2001, a panel chaired by Nathan released an ADA position paper on postprandial testing. One of the few issues that was agreed upon was the benefit of postprandial testing in pregnant women who develop gestational diabetes. Women whose postprandial glucose is monitored and reduced have fewer complications during pregnancy and their risks of having a cesarean delivery are reduced.
Nathan, who is dubious of the general uses of postprandial testing, believes it may have other advantages.
"The importance of postprandial glycemia is greater in the pre-diabetic state, where it can be a more sensitive indicator of abnormal metabolism," he tells WebMD. This may be especially true for the elderly, who may exhibit postprandial hyperglycemia that isn't detected by fasting tests.
Deciding What To Do
Obviously, there isn't any scientific consensus on whether postprandial glucose testing is an important part of general diabetes care. While proponents like Jellinger concede that we don't yet have absolute proof of the importance of postprandial hyperglycemia, he believes that the epidemiological studies are evidence enough. "I don't think that we're doing our patients any favors if we wait for a study that may never happen," he says.
However, critics like Goldstein argue that given the impending and potentially catastrophic epidemic of type 2 diabetes, "the last thing we need to put all of our energy into is postprandial hyperglycemia," he says. "We should be working on the bread and butter things."
So what should you do? The best advice is to talk to your doctor and see whether he or she suggests postprandial testing in your particular case. While treatment focused specifically on your postprandial glucose levels may not be necessary, it might be useful to know what they are.
"The study of postprandial glucose testing is just in its infancy," says Fran Kaufman, President of the American Diabetes Association. Whether it will emerge as a significant tool of diabetes control just isn't clear yet.
Originally published March 17, 2003.
Medically updated June 18, 2004.
SOURCES: Thomas Buchanan, MD, director of the clinical research center at the Keck School of Medicine; professor of medicine at the University of Southern California; leader of the Troglitazone in Prevention of Diabetes (TRIPOD) study. Om Ganda, MD, associate clinical professor of medicine at Harvard Medical School; senior physician and director of the lipid clinic at the Joslin Diabetes Center; attending physician at Beth Israel-Deaconess Medical Center, Boston. David E. Goldstein, MD, chair of the NGSP Steering Committee; professor of child health and principal investigator at the Health Sciences Center at the University of Missouri School of Medicine. Paul Jellinger, MD, past president of the American Association of Clinical Endocrinologists (AACE); clinical professor in the department of medicine at the University of Miami School of Medicine. Fran Kaufman, MD, president, American Diabetes Association (ADA); head of the division of endocrinology and metabolism and director of the comprehensive childhood diabetes center at the Children's Hospital in Los Angeles; professor of pediatrics at the Keck School of Medicine. David M. Nathan, MD, director of the general clinical research center and of the diabetes center at Massachusetts General Hospital; professor of medicine at Harvard Medical School; chairman of the Diabetes Prevention Project (DPP).
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