Back at the beginning of WHEE I made an offer to write a diary on glycemic index and load. Life intervened for a while but just after offering to write for tonight I ran into Doc's diary on high fructose corn syrup. Clearly today was a good day to write this diary!
In this diary I'll give an introduction to carbohydrates, what the glycemic index of a food is, what glycemic load means, and who (if anyone) should care, and why. Join me over the fold ---
What WHEE is:
WHEE (Weight, Health, Eating and Exercise) is a community support diary for Kossacks who are currently or planning to start losing, gaining or maintaining their weight through diet and exercise or fitness. Any supportive comments, suggestions or positive distractions are appreciated. If you are working on your weight or fitness, please -- join us! You can also click the WHEE tag to view all diary posts.
An introduction to carbohydrates
Carbohydrates (together with fats and proteins) are one of the main nutritional components of food, or sometimes called a macronutrient. All carbohydrates share the chemical formula (1 carbon: 2 hydrogen: 1 oxygen) - think "hydrated carbon." They can be any length from three to seven carbons long but the main dietary carbohydrates are six carbons long (the five carbon ones ribose and deoxyribose are key to DNA and RNA).
Although in cells there's a whole slew of carbohydrates, and even a surprising variety of six carbon ones (called hexoses) the sugar people worry about is glucose, this particular carbohydrate or sugar:
Glucose is the main sugar in food (though Doc's diary mentioned above will educate you about fructose, the other major one). Glucose is the sugar that makes up starch and cellulose, and half of table sugar (sucrose). Most importantly, our bodies can hardly use glucose without the help of the hormone insulin, both to take it up into cells and to control its metabolism. People whose bodies can't make insulin have I diabetes; people whose bodies don't respond to insulin have type II diabetes. Both forms lead to high blood sugar and often heart disease and nerve damage, and can lead to loss of limbs or eye sight. You don't want to have diabetes. Trust me: my mother did.
For all of us, blood sugar levels will rise and fall as we eat, exercise, sleep and relax. But for diabetics, those blood sugar levels are far more extreme and need to be controlled if possible. Before insulin was discovered, that could only be done by a high protein diet and even that was not very effective. Insulin was an immense advance and saved the lives of thousands of diabetics. One shot of insulin and blood sugar levels would come down with surprising efficacy.
Even so there were puzzles. People would eat two meals that apparently had exactly the same amount of glucose in them, and yet their blood sugar levels would vary widely. This was a problem because the amount of insulin needed was affected by how high the blood sugar went. In 1980–1981, David Jenkins and his colleagues articulated the idea of the glycemic index in their research to find out which foods were best for people with diabetes.
What is the glycemic index of a food?
After we eat a meal, especially one with carbohydrates, our blood sugar rises and then falls. But we need to know more: how high does it rise? how soon does it fall? If we could find a number that would capture at least in part those two aspects of our blood sugar response to foods, that might make it easier for diabetics (and the rest of us) to maintain an even keel. Thus was born the idea of "glycemic index."
These are two slightly idealized graphs of how our blood sugar responds to different foods. The left hand graph emphasizes one key to the glycemic index: the test meal has the same amount of glucose as the standard meal, typically 50 gm. The standard meal can be either glucose or white bread, which is slightly confusing but there are advantages to both standards (come on! how often do you eat glucose? white bread?) The important thing to notice is that the area under the curve is less for the lentils, which have a glycemic index or GI of 40, compared to the standard meal.
The right hand graph, apparently for a fantasy food, does capture an important point. A high GI food can often cause such a spike in blood sugar, with a spike in insulin, that our bodies overshoot, and by two or three hours, blood sugar may actually be below the fasting level. Not good: how do you think you feel when your blood sugar is lower than fasting?
Why do foods differ in their glycemic index? Well, the structure of the carbohydrate may be different. Look at these two molecules:
The amylopectin has lots of free ends while the amylose has one. The enzyme that breaks down starch works from the end. So the branched one breaks down quickly and the other breaks down slowly. Other things that can affect GI include how finely broken up the food is: oat flour has a higher GI than do steel cut oats.
So how do foods vary in their glycemic loads? Here are a couple of tables that give you a sense of the variation:
There are also lots of sites with glycemic index: here, and here, and here (pdf with lots of international foods).
Well and good. Eating foods with a lower GI should raise blood sugar less. But there is one more dimension to this. We defined the glycemic index by a meal that had the same amount of glucose as the standard (glucose or white bread). But does that represent our meals? Do you know how many carrots you need to eat to get 50 gm of glucose? And thus the glycemic load was born.
What is glycemic load?
The glycemic load takes the glycemic index (the inherent tendency of a food to raise blood sugar) and corrects it for serving size. Thus the glycemic load is more representative of how a food might actually affect our blood sugar in a meal. But does it actually make a difference? It can: take a look at this table:
Popcorn and watermelon had a high glycemic index, but a typical serving doesn't have that much glucose so they have a low glycemic load. Sadly the spaghetti that had a low GI, based on 50 gm of glucose in a test meal, turns out to have a high glycemic load because most of us eat more spaghetti than that in a typical serving.
Who (if anyone) should care, and why?
If you're diabetic, you should definitely care. It is now the standard recommendation for diabetics to eat lower glycemic meals, and knowing the values for foods can help with that.
If you have family who are diabetic, or you are obese, you probably should care. People can have (I do) the trait that predisposes you to type II diabetes even when they still have normal blood glucose. I like to say the way I keep from becoming diabetic is by acting as though I already were.
If you hate the soggy boggy feeling that hits an hour or two after a sweet meal, you may benefit from knowing this. When your blood glucose sags below the fasting level it's hard to think.
How regular your meals are can also affect how your body uses its food:
Regular eating was associated with lower EI [energy efficiency] (P < 0.01), greater postprandial thermogenesis (P <0.01), and lower fasting total [cholesterol] (4.16 compared with 4.30 mmol/L; P < 0.01) and LDL (2.46 compared with 2.60 mmol/L; P < 0.02) cholesterol. Fasting glucose and insulin values were not affected by meal pattern, but peak insulin concentrations and area under the curve of insulin responses to the test meal were lower after the regular than after the irregular meal pattern (P < 0.01 and 0.02, respectively).</p>
Conclusion: Regular eating has beneficial effects on fasting lipid and postprandial insulin profiles and thermogenesis.
There's lots more that could be said, but I think I will post now and add more as we go along. Here's to happy blood sugar!
For future episodes of this series, we are grateful to:
Aug 4
Tues AM - Kascade Kat
Tues PM - Brimi
Aug 5
Wed AM - Edward Spurlock
Wed PM - rdluu
Aug 6
Thurs AM Darthstar
Thurs PM Albatross
Aug 7
Fri AM ???
Fri PM ???
Aug 8
Sat AM ???
Sat PM ???
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