GI had no idea there was so much diversity

by Jeremy Cherfas on August 1, 2007

We know hardly anything about the differences among varieties of the same crop. Oh sure, we know what different varieties look like; that’s easy. But detailed differences in composition are hard to find. There are the classics, of course, like wetet be gunche sorghum in Ethiopia, whose name translates as “milk in my mouth”. It contains almost a third more protein than other sorghum varieties and, even more important, about double the level of lysine, a vital amino acid for human nutrition. And there are the red and black varieties of rice, which are known to be high in iron and other minerals and vitamins and which are traditionally used to treat anaemia, especially in pregnant women. (I have been unable to discover whether this treatment is effective, in a Western sense, but it seems entirely reasonable, and a bit churlish to deny it.) But in general, we know next to nothing about the nutritional qualities of varieties, as opposed to species.

All this was prompted by our post a while ago about a low glycaemic rice. It turned out that this particular product was the result of a process that pushed the glycaemic index of a rice variety low enough that it could legitimately boast the label “low GI”. It isn’t clear how low the GI of the rice is before processing. Anyway, that got us to thinking how useful it would be to know which varieties do actually have a lower GI than others, so off we headed into the maze of internets.

Glycemic First, though, a word of explanation. Glycaemic (or glycemic) index is a measure of how quickly the carbohydrates (starches and sugars) in a food are digested and enter the blood stream. It is usually defined as the area under the 2-hour blood-glucose curve after eating a standard portion of the food concerned compared to some standard (usually pure glucose or white bread), which is adjusted to contain the same total carbohydrate as the sample under scrutiny. Measuring the GI is by no means straightforward. For one thing, the standard itself can vary widely among different populations of people and from sample to sample. (Just freezing white bread prior to toasting it considerably lowers its GI.) Nevertheless, GI has become a widely touted number, easy for ordinary people to understand, because low GI foods are generally believed to be good for you. The slower release of sugar into the blood stream is associated with lower insulin demand (good for diabetics, and the rest of us), better long-term control of blood glucose, better control of blood lipids and more besides. That is why a low GI rice is worth having, and why low GI foods in general are being promoted.

Anyway, it turns out that it is not really that easy to find useful information about specific varieties. Search the mother-lode database, for example, and you find rice with a GI of 112 and rice with a GI of 42 and both are described as “White rice, type not specified”.1 A baked Russet Burbank potato — no fat — weighs in at a GI of 111 while an unspecified cooked potato in Kenya scores only 24.2

So we did a bit more digging and found a few tasty morsels. People have looked at the GI of commercial varieties of potato in England and in the US, but for very few varieties. In Australia, a look at three varieties of potato found no significant differences among them. A Sri Lankan study looked at white and red rice, and found that while there were differences in GI among varieties, there was no simple relationship between the colour of the variety and its GI. And just this week, Mexican scientists reported that blue corn tortillas had a lower GI than white corn tortillas.

Slim pickings indeed. Of course I am sure I have missed some important research results. Someone, somewhere may even have compiled the kind of information we’ve been looking for, although if they have it is less than useless if it can’t be found. As ever, we would welcome more information.

Good measurements of characteristics such as GI for specific, named and recognizable varieties, whether the products of modern breeding or traditional farmer varieties, would be really valuable for lots of reasons, not least to add substance to claims that diversity of diet in and of itself is good for one. So here’s a modest suggestion. The Global Crop Diversity Trust has started handing crates of Gates cash to genebanks, in part so they can rejuvenate their holdings prior to giving them to the Arctic Seed Monkeys for safekeeping. The genebanks will also be evaluating and characterizing the samples they grow to add to the sum of human knowledge about them.

How’s about one of the foundations specifically interested in nutrition or health stepping up to ensure that the GI of those samples is also measured?

P.S. If you’re stuck with only a high GI food and want a low GI response, don’t chew; swallow.

  1. Wild rice from Saskatchewan is in there too, which suggests that the nutritionists need a botanist’s help. []
  2. Same for sweet potatoes. Seriously people, “proper” names would really help proper researchers. []

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