The Potato Manifesto - Part 2/2: The Sweet Potato, Is It More Than Just the "En Vogue Tuber of the Year"?
|Image 1: A mixer like this would be one of the best choices to turn your healthy low-GI sweet (or regular) potato into a high GI "nightmare".|
Come on sweety, show me what's beneath your skin!
Now, if we take a look at the literature, a review of the glycemic index of 33 commonly available foodstuffs from the 1990s lists sweet potatoes with a glycemic index of 49 as #17 (with #1 peanuts and a GI of 11 and #32 cornflakes and a GI of 83 as the "extremes"; Nishimune. 1991). It is "outperformed by spaghetti (GI 47) and closely followed by Buckwheat, Yam (both GI 51) and you guessed it the good old "regular" potato, to which Takahiro Nishirnune and his colleagues assign a GI of 54, which - as you should know from yesterday's installment of the Potato Manifesto is nothing but an average on a scale that ranges from 10 to 110!
|Figure 1: Difference in plasma glucose concentrations (expressed as the increase due to the "high GI" normal potato vs. the "low GI" sweet potato) in response to the ingestion of 50g of carbohydrates in form of bush potato or regular potato in 7 Aborigines and 7 Caucasians (data adapted Thorburn. 1986)|
Identical glycemic index, but differing free sugar compositions
Although, I hope that I should by now have convinced you that the glucose response is no convincing argument in favor of the sweet potato. I am even convinced that, if people abused their sweet potatoes in similar ways as their "regular" potatoes, i.e. mashed, fried, pureed, powdered and instantized it, we would soon see the (contemporarily) sexier sister of the regular potato on the list of "foods to avoid if you want to lose wait or just live a long and healthy life". Nevertheless, taste and name of the sweet potato, leave no doubt that there must be a sugary difference between her and her white brethren.
|Figure 2: Free sugar composition (in mg/g) of 16 "regular" and 3 sweet potatoes cultivars (comparison based on data from Zhu. 2011 and Dincer. 2011)|
|Figure 3: Mean free sugar composition (in mg/g) of fresh "regular" and sweet potatoes (comparison based on data from Zhu. 2011 and Dincer. 2011)|
|Figure 4: Free sugar content (mg/g dry weight) of fresh and cooked sweet potatoes (data adapted from Dincer. 2011)|
The raw potatoes contained 0.34 per cent of reducing sugar calculated as maltose, the cubes cooked for 5 minutes in boiling water contained 8.32 per cent, and the cubes cooked for 1 hour, 7.82 per cent. Thus, no sugar formation occurred at the boiling point.In view of this immediate heat-induced increase in the high GI free sugars, maltose (maltose has a ~10% higher GI than glucose), it should not surprise you that a study that was conducted by Jamaican scientists (Bahado-Singh. 2011), only a few months ago, found statistically significant increases in glycemic index and the area under the incremental glucose curve after boiling, frying, baking or roasting 10 sweet potato cultivars, which are commonly consumed in Jamaica (cf. figure 5):
|Figure 5: Glycemic indices and glucose AUC of 10 common Jamaican sweet potato cultivars after boiling, frying, baking and roasting (data adapted from Bahado-Singh. 2011)|
|Figure 5: Comparison of regular and sweet potato glycemic indices after boiling, frying, baking and roasting.|
So, are boiled sweet and baked and roasted regular potatoes the way to go?
In view of the fact that probably 99% of the average and maybe 70% of the health conscious consumers don't even know the name of the potato cultivar they are using, let alone their age, storage temperature, the amount of phosphate in the soil on which it was grown, and all the other countless variables that have an impact on the "basal" GI of a potato, I honestly doubt that switching from one of the waxier, new regular potatoes (low basal GI) to a random (I mean, how many sweet potato cultivars are available at your grocery store?) sweet potato cultivar, would make you healthier, help you lose weight or offer any other GI-related benefits.
|Image 2: Tapioca, another purpoted "health food" with a surprisingly high glycemic index GI of 84|
Note: Wherever this was necessary I converted GIs that were given with white bread as a reference to the glucose as a reference.