Carb loading prior to Glucose Tolerance Test?
May 6, 2006 9:46 PM   Subscribe

PregnancyFilter: On Monday, I am scheduled for a 3 Hour Glucose Tolerance Test to rule out (I hope) Gestational diabetes. Can anyone explain why I've been advised by my midwife to increase my carb intake for the three days before the test?

She even gave me a specific list (one from column A, one from column B) of foods that I should eat each day for the three days before the test.

Adding carbs to my diet seems, at best, counterintuative to me, since its very clear now that its carbs - and in particular carbs with a high glycemic index - that will increase or spike blood sugar. I'm puzzled why I'm not just asked to "eat normally".

Also, as I've been researching this on the web, a number of sources recommend "a normal diet consisting of >150g carbs per day" prior to the test, and of course there is early-morning test to get the fasting glucose levels, but I can't find any recommendation (other than the suggestions from my midwife) of actually increasing carb intake prior to the test.

Any thoughts?
posted by anastasiav to Health & Fitness (11 answers total) 1 user marked this as a favorite
Added note: Yes "why don't you ask your midwife" is an appropriate answer to this question, but I already have. The answer she gave me ("because it will improve the accuracy of the test") wasn't quite satisfactory enough. I figured I'd just be able to google it on my own (I expect her to know a lot about childbirth, not as much about blood chemestry) but have come up blank.
posted by anastasiav at 10:04 PM on May 6, 2006

I'd actually want to stay away from that. Go eat your regular diet -- for sure. BUT, for goodness sakes -- I can't see why you'd want to stack up on carbs.

You're glucose challenge test will be just that: "a challenge" of sugar to your body. If your body can handle that pretty significant challenge just fine then you won't go to the next aspects of the test -- as I've cited and quoted below......

Given that's its only a couple days, the midwive's advice is probably benign. But, I'm curious as to her rationale for it.

The test is designed to see how you do fasting and not-fasting. While I can't understand why you'd consciously prime it (and risk bombarding your insulin receptors more than usual) -- it does seem that carbs over 150g = carbo loading (as my cite declares). So, perhaps that's what the midwife was getting at then.

Here's what the protocoal is going to be: Just as a note (OGTT = oral glucose tolerance test).

Good luck to you and your baby!

OK -- here's something pretty close to what you can expect from emedicine:

Screening for gestational diabetes
GDM only occurs during pregnancy. The diagnosis is established by glucose tolerance testing. Risk factors for gestational diabetes include advanced maternal age, ethnicity, obesity, obstetrical history of diabetes or macrosomia, and strong family history of diabetes. The best method for diagnosing GDM continues to be controversial. The 2-step system is currently recommended in the United States. A 50-g, 1-hour screening test is administered to all pregnant women at 26-28 weeks', followed by a 100-g, 3-hour OGTT for those with an abnormal screening result. Alternatively, a 1-step, 75-g, 2-hour test can be administered. Other measurements (eg, maternal HbA1C, random postprandial or fasting blood sugar level, fructosamine level) are not recommended because of low sensitivity.
OGTT prerequisites for gestational diabetes are as follows:

One-hour, 50-g glucose challenge result greater than 135 mg/dL

Overnight fast of 8-14 hours

Carbohydrate loading for 3 days (>150 g carbohydrates)

Seated and not smoking during the test

Two or more values met or exceeded

Either a 2-hour (75 g of glucose) or 3-hour (100 g of glucose) test
Plasma glucose criteria for gestational diabetes are as follows:

Fasting test

With glucose load of 100 g, result of 95 mg/dL (5.3 mmol/L)

With glucose load of 75 g, result of 95 mg/dL (5.3 mmol/L)

One-hour test

With glucose load of 100 g, result of 180 mg/dL (10 mmol/L)

With glucose load of 75 g, result of 180 mg/dL (10 mmol/L)

Two-hour test

With glucose load of 100 g, result of 155 mg/dL (8.6 mmol/L)

With glucose load of 75 g, result of 155 mg/dL (8.6 mmol/L)

Three-hour test - With glucose load of 100 g, result of 140 mg/dL (7.8 mmol/L)
Screening for GDM during pregnancy is recommended because less than 20% of women with significant glucose intolerance during pregnancy exhibit glucosuria or other symptoms during pregnancy. However, whether universal screening of all pregnant women or targeted screening of patients with risk factors is most efficacious continues to be controversial. At present, both methods (universal and selective screening) are used in reputable centers. In areas in which the prevalence of insulin resistance is 5% or higher (eg, the southwestern and southeastern United States), universal screening is recommended.

posted by skepticallypleased at 10:24 PM on May 6, 2006

Skepticallypleased, are you aware that you stated that you wouldn't carb-load, then pasted medical information on the protocol that said that carb loading for three days was part of the test?
posted by Cricket at 10:45 PM on May 6, 2006

First let me assure you that in most centres, a high-carb diet before a glucose tolerance test (GTT) is standard procedure, particularly in pregnant women. So by following the midwife's instructions, you aren't doing any harm and you may well be doing some good.

This article suggests why a high-carb diet is recommended. They gave some pregnant women a low-carb diet and some a high-carb diet the night before a GTT for gestational diabetes. The ones with a low-carb diet had significantly more false-positives. The conclusion that they came to is that having a low-carb diet:

1. Decreases insulin levels: Decreased insulin levels in this case mimic decreased insulin levels in gestational diabetes (i.e. --> false positive test result).
2. Activates the glucose-fatty acid cycle: Low blood glucose (one cause of which is a low-carb diet) decreases insulin levels, and this decreases inhibition on non-esterified fatty acids (NEFA). So you have higher NEFA levels. Higher NEFA levels decrease the body's uptake of glucose. Obviously for a GTT you don't want this situation occurring, because a decreased glucose uptake will cause a false-positive test result even if the mechanisms for insulin and glucose control are actually working as they should be.

If you have a normal or high-carb diet, carrying on that way before the test shouldn't be problematic. However for women who usually go for low-carb options, recommending that they "eat normally" might skew the test results.

On the other hand, there are studies that suggest that the effect of preparatory carb-loading on GTT accuracy is negligible. The article described above may have been an anomaly, or the articles that suggest the opposite might be flawed in some way. So why do they recommend it despite unclear evidence? High-carbs doesn't decrease the accuracy of the test, whereas low-carbs might. It doesn't cost anything from a public health perspective to tell women to eat a high-carb diet, so why not err on the side of caution?
posted by teem at 11:27 PM on May 6, 2006

The "glucola" test is a screening, not a final diagnostic tool. Don't try to cheat it by altering your body's metabolism or tolerance for carbohydrates, it only sidesteps the intended purpose of the test: To ensure you and your child's health.

Your midwife is probably trying to avoid getting a positive (false or otherwise) to keep your doctor headaches to a minimum. That, however, can only serve one purpose: To blind your doctor to potential issues during your pregnancy. Listen to your gut (since your gut made you question the advice and post it here), and don't mess around with it comes to your baby!
posted by Merdryn at 5:38 AM on May 7, 2006

Cricket -- I think what I wanted to state was that I didn't know 150g of glucose/day was carbo loading. If that's whats recommended then I would also do it.
Sorry for the confusion.
posted by skepticallypleased at 5:48 AM on May 7, 2006

Merdryn, if you're saying what I think you're saying then that is the worst advice I have ever read on the green. There is no midwife conspiracy to save doctors hassle by sabotaging test results. If anything, midwives have both selfish (no one wants to be the one responsible for a malpractice lawsuit) and altruistic (many midwives, believe it or not, care about the wellbeing of babies and their mothers) interests when it comes to instructing expecting parents about what they should be doing.

Anastasiav, if you would rather eat a normal-carb diet before your GTT, you're welcome to do it knowing that there are clinical trials asserting that the effect on your test result with be negligible. If you want to eat a low-carb diet, do it knowing that there are studies concluding that it will decrease the accuracy of your test result. If you like, seek out the articles in question and review their methodology if you disagree with their findings.

However don't listen to Merdryn and blindly follow your gut. Sorry if my tone is unnecessarily serious given that we're just talking about a GTT here, but falling into Merdryn's pattern of "follow your gut" thinking can be dangerous.
posted by teem at 6:16 AM on May 7, 2006 [1 favorite]

i am not a doctor. my wife is an endocrinologist though.

this is called a "tolerance" test for a reason. you increase your carb intake and then see how the body reacts in response to the load. some short-term tests are done simply by drinking a soda or something like a snapple (which has a LOT of carbs) and then measuring your blood glucose levels at set intervals after the carb intake to see how fast your body gets rid of the glucose.

i'll ask her if the 3-day "carb loading" for the 3-hour tests is the right thing to do.
posted by joeblough at 9:02 AM on May 7, 2006

yeah. she said that for the 3 hour test you are supposed to eat a high-carb diet for 3 days prior to make sure your body is primed and ready to make enough insulin during the test.

Merdryn: sorry, wrong.
posted by joeblough at 9:08 AM on May 7, 2006

teem is right, but there's a simple way to look at it:

The glucose tolerance test is like a scientific experiment. The idea is to carry out a test under controlled conditions and obtain a result.

Interpreting the result requires knowledge; in particular, to interpret the test requires knowing what results are likely to be obtained in the cases of a) healthy people and b) people with gestational diabetes.

Your result will be compared to the "norms" obtained under controlled conditions. If you don't replicate the conditions under which the test norms were obtained, you may invalidate the results of your test. Since your goal is to obtain a valid result, you should stick to the protocol.
posted by ikkyu2 at 7:34 PM on May 8, 2006

So I'm the asshole for suggesting that she not try to mess with the intended purpose of the GTT?

Man, more and more, MeFites are starting to get full of themselves.

My wife's OB made it clear that she was NOT to change ANYTHING about her diet and exercise before any such screenings, as the screening metrics are based on NORMAL dietary intake. Altering that intake messes with the intended purpose of the test.

Then, you fine folks say it's irresponsible to suggest that she listen to her gut; the gut that's telling her "this is suspicious, I should ask someone", and that gut instinct is wrong? I think you're insulting to the poster to suggest that her instinct to research the matter and not to try to "cheat" the test was incorrect and dangerous. I applaud the poster for seeking more information (and following that gut instinct) and worrying that her midwife's suggestion may not be correct.

I think it's irresponsible for anyone to suggest to a pregnant woman that it's okay to mess with tests without speaking to the doctor. Granted, I should've suggested that she talk to her doctor, but I didn't see anyone else making the same suggestion. Shame on you. There's a baby at stake, for crying out loud.

anastasiav, I hope your test went well. You're doing a wonderful thing, bringing new life to this world; if you have questions about the process, your doctor and midwife (together! Leave the chemistry to the doctor, though!) will be a great source of help and strength. Good luck with your upcoming bundle of joy!
posted by Merdryn at 3:59 PM on May 13, 2006

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