The Pasta Effect and the Gylcemic Index
Pasta is sometimes referred to as a low-glycemic-index (GI) or medium-GI food because the GI of many pastas are in the 40s or 50s, much lower than the GI of white bread or rice cakes.
By way of comparison, the GI of sucrose (table sugar) is about 60, the GI of white bread is about 70, and the GI of white rice is about 65.
The glycemic index is measured by giving volunteers enough of the test food to contain 50 g of carbohydrate and then measuring the area under the blood glucose (BG) curve for 2 hours. The higher the GI, the faster the food will raise your blood sugar.
Many of us prefer foods with GIs much lower than that of pasta, but others are able to tolerate reasonable amounts of such foods, so having an accurate GI is important for them.
I myself have given up pasta altogether, as it raises my BG too much, and when I feel a need for some type of noodle in a dish, I eat either tofu shirataki or bean thread noodle made from pea starch. The former contains almost zero digestible carbohydrate (it's mostly soluble fiber), and the latter, although it does contain a lot of carbohydrate, raises my BG levels much less than other pasta.
There's one aspect of regular pasta that is not commonly known, or commonly accepted, and that is the fact that pasta will often give you two peaks, one at the usual 1 or 2 hours (depending on how much fat you eat with the pasta) and the other much later, at 4 or 5 hours.
What this means is that you could measure your postprandial (PP) BG level at the usual time, 1 or 2 hours after eating some pasta, and find that the number was within your target ranges. You'd conclude that pasta was an OK food for you. But then, 4 or 5 hours later, your BG would go up and you wouldn't know it.
Furthermore, because the official GI measurements are stopped after 2 hours, the second peak would not have an effect on the official GI numbers. Something that actually raised your BG a lot might be listed as having a relatively low GI.
A friend once wrote to me with great excitement that she'd decided to splurge on an Italian dinner to celebrate a major wedding anniversary and when she measured her BG at about an hour, it was only up to 130, and back down to 85 at 2 hours, so she was thinking of adding pasta back into her life.
The next day, she wrote again, now disappointed. She'd kept measuring, and at 3 hours her BG went up to 180 and was still over 150 at 5 hours. She had a definite second peak, in this case higher than the first.
When I've measured pasta, I've often found the same thing, but no one really believed me. They suggested that I probably had gastroparesis. Or maybe the olive oil in the pasta was causing this effect. But if I have gastroparesis, why would it only have an effect with pasta and not other carbohydrate foods? Doesn't make any sense.
So I looked and looked and finally found a research study that showed the same thing: a second peak from wheat pasta that began at about 4 hours. The study was done in nondiabetics, who aren't apt to have gastroparesis, and the found that pasta gave two peaks.
They comment, "Many postprandial studies, especially glycemic studies, measure glucose and other plasma variables for only 2-3 h after a meal and thus miss changes that occur later than this." They attribute the second peak to the extrusion process used to make some pastas.
But again, no one seems to be paying much attention to this finding. Instead, they cite this article when writing about the benefits of fiber or whole grains, including pasta, in controlling diabetes.
For a year or so, I've used a continuous glucose monitor (CGM) on and off. It's expensive, and I'm too well controlled for my insurance to pay for the expensive sensors, but I'm willing to pay for them myself for occasional use. The big advantage of the CGMs is that it will show you things you don't expect, like delayed peaks.
Another advantage is that if you're testing a new food, it will indicate approximately where the peak is (I say "approximately" because the CGM can lag about 15 minutes behind the blood readings because the CGM measures interstitial fluid). That means you don't need to test every 5 or 10 or 15 minutes for fear of missing a peak.
In addition, if something unexpected comes up during a test, like a telephone call about an emergency that you really need to deal with, the test won't have been completely wasted. Even if you don't have time to test with fingersticks, at least you have the CGM results.
When I was wearing a CGM last year, I decided to try a pasta test. I ate 1 oz of regular pasta, cooked al dente, about 6 minutes. I had it with 1 teaspoon of olive oil and mashed garlic. I used the CGM readings to indicate when I should test with my meter.
Here's what I found. The blue lines are the CGM. The red symbols are the Ultra meter readings.
I started at about 8 a.m., with a BG level about 90 mg/dL. After about an hour and 45 minutes, I peaked at about 120, and at 3 hours I was down to 100. Didn't seem too bad. But then I started going up again, and the second peak occurred at about 4 hours. Neither of the peaks was tremendous, but the overall area under the curve was about twice what it would have been had I measured only the first peak.
More recently, I decided to try Dreamfields pasta again. Dreamfields is supposed to have been treated in a way that makes most of the starch not get digested, but the manufacturers say it doesn't work for everyone. Previous tests had shown that it raised my BG levels almost as much as regular pasta (with 2 oz of pasta cooked al dente, I peaked at 170 with regular and 160 with Dreamfields, big deal; both showed second peaks, smaller than the first), but I wanted to try again with the CGM. I was also curious to see if different batches of Dreamfields might have different effects.
To make sure I got a real peak, this time I ate 2 oz of the pasta, with 2 teaspoons of olive oil and a dab of tomato paste.
I started at around 1 p.m. with a BG level of about 100 mg/dL, and I didn't measure that much with the Ultra, relying on the CGM readings for the most part. According to the CGM, I peaked at 110 at about 1 hour and then started coming down. Sounded great! Maybe I could eat Dreamfields after all.
At 2 hours and 45 minutes, I was down to about 80, and still falling, and I was also hungry for something that wasn't pasta, so I had a handful of almonds. Almonds don't usually make my BG go up much, as they contain almost 50% fiber that isn't digested well. But according to the CGM, this time I went up 40 points, peaking at 120 at about 4 hours.
The problem is, I couldn't be certain that the second peak was from the pasta. Maybe it was from the nuts. I had to repeat the test the next day.
Again, I started at around 1 p.m. with a BG level of about 90 mg/dL.
I peaked at a little over 100 (I'm guessing at the number as I didn't measure with the Ultra until slightly after the peak) at about 1.5 hours and was back down to 80 or 90 at about 2.5 hours. [The very low numbers are clearly meter error.]
But then came the second peak. This time I was up to 126 at about 4 hours, and it took until 5.5 hours to return to baseline. At that point I had supper.
Now, I realize that none of these numbers are spectacularly high. Some people would be happy to see numbers that didn't go over 126 at the peak. But remember that this was 2 oz of pasta without much else. I didn't feel satisfied after eating that much food; I wanted more food. The 20 to 40 point increase from the pasta would be added to whatever increase occurred from the rest of the meal.
But the real point is that for some people, at least, pasta seems to have a second peak. Sometimes the second peak is higher than the first. Sometimes it's lower. And sometimes it's about the same. But it's there and often doubles the effect you'd expect. I also get a second peak with bean thread.
Another thing I learned from these tests is that I really don't like pasta all that well anymore. I'm sure I could learn to like it again if I had to, but why bother. The other foods I eat are so much tastier.
But if you're one of those people who can't live without pasta and you think it doesn't have much of an effect on your BG levels because they're reasonable at the 2-hour PP point, try measuring again at about 4 hours. They might be OK. Or you might be in for a big surprise.