|Image 1: The grunty regular (left) and the cute sweet potato (left), which one would you commit your health to? (img. menshealth.co.uk)|
When wheat was devils excrement, would regular potatoes then be his horns?
According to the official obesity statistics of the European Union, British women are the fattest in Europe (Eurostat. 2011): 23.9% were classified as being obese (BMI > 30) in the year 2008 to 2009. This certainly raises the not altogether serious question, whether the British obesity problem (the obesity rate among the men was 22.1% and thusly topped only be the Maltese with 24.7%) is due to the fried fish or the fried (regular) potatoes in the unofficial British national dish, fish & chips.
|Image 2: Obese British woman's rear view (img. BBC.co.uk)|
|Figure 1: Names and characteristics of eight common potato cultivars in the British diet (left) and experimentally evaluated area under the glucose curve and glycemic indices (right; data based on Henry. 2005)|
Cultivar, processing, serving temperature and more have profound influences on the GI
The type (=cultivar) of the (classic) potato, is yet neither the only, nor the most important determinant of the glycemic index of a potato meal. The processing method and, to my own surprise, even the food temperature have considerable influence on the glucose response to otherwise identical test meals, as well:
|Figure 2: Incremental areas under the curve (AUC) and glycemic index values for 50 g available carbohydrate portions of white bread and seven potato meals tested in a cohort of 12 healthy subjects (data adapted from Fernandes. 2005)|
With regard to the unexpected differences between cold and hot red potatoes, it is important to note that the results of a 2011 study by Kinnear et al. confirm that the latter is not an artifact of the Fernandes study. In their trial, the scientists from the University of Toronto found an average GI reduction of -37% (mean GI for the tested cultivars ~47), when the freshly boiled potatoes were refrigerated at 4°C for 24–28h before they were served to the 10 healthy study participants (Kinnear. 2011). As far as the reasons for this temperature-dependence of the glycemic index is concerned, Kinnear et al. speculate that it is an effect "due to starch retrogradation", which is a process that takes place in gelatinized starch, when the amylose and amylopectin chains realign themselves and thusly causes the liquid to gel. This is quite interesting, as it stands in line with the low GI of the Marfona potato (cf. figure 1), the texture of which is described as "waxy". The long-established relation of the phosphate content and the degree of starch gelatinization (and thusly digestibility and GI), on the other hand, could explain difference between crops and differences between identical crops grown on soil with different phosphate contents.
|Figure 3: Areas under the glucose curves (AUC) in 32 healthy volunteers to 50g carbohydrates from mashed potatoes with or without 10g fructose administered 0, 30 or 60min before the meal. Measure by Accu Check finger-prick glucometer and YSI glucose oxidase analyzer (data adapted from Heacock. 2002)|
Black-and-white thinking and ineradicable prejudices
If we base our argumentation solely on the glycemic index, which is in fact the main argument that is brought forward against "regular" potatoes in the public debate, it is quite clear that the poor (regular) potato is another victim of the human propensity to black-and-white thinking and the public's stubborn adherence to convential nutritional wisdom. With reference to the "unjustified generalization" that "all potatoes have a high glycemic index", Anette E. Buyken and Anja Kroke, two researchers from the Research Institute of Child Nutrition in Dortmund, Germany, write in their letter to the editor of the British Journal of Nutrition (Buyken. 2005):
Buyken and Kroke support their argument by the means of an illustration of the broad range of the glycemic responses (and respective GI) of study participants to 46 different potato meals (cf. figure 4) and emphasize that there are significant differences in both the preferred potato cultivars, as well as the respective cooking / processing methods between European and US customers. While the former "prefer potato varieties characterised by a lower GI", the prevailing potato varieties in the US are mature and exhibit significantly higher glycemic indices. This trend toward higher glycemic indices in US potato meals is reinforced by the average American's preference to fry, bake, mash, roast or microwave his potatoes, so that it would be "thoughtless" for any European, or American who selects less mature, low GI cultivars and refrains from frying, baking, mashing, roasting or microwaving his potatoes to follow the grossly over-generalized recommendation to eat less potatoes.
[...] as with all GI data, the GI values of potatoes may depend on cooking method, processing, variety and the composition of the meal. This fact deserves attention since mashed potatoes, French fries, baked potatoes and potatoes cooked in a microwave are characterised by GI values mostly exceeding the upper limit for a high GI value of 70; whereas conventionally boiled potatoes appear to have a GI value on average below 70. The values of conventionally boiled potatoes do vary considerably though, so it may also be that some potato varieties have an inherently low GI what-ever the cooking method (Najjar. 2004; Fernandes. 2005). In this context, it should be considered that most currently available GI values are based on mature potato varieties (Ontario, Prince Edward Island, Desiree, Pontiac, Sebago). The starch of more mature potatoes is, however, easier to digest, presumably due to increased amylopectin branching and hence lower resistance to gelatinisation, which in turn results in a higher GI (Soh. 1999).
Figure 4: GI values (glucose reference) for potatoes by different cooking methods; the horizontal bars indicate the minimal and maximal glycemic index; the dotted vertical lines mark the conventionally accepted "low GI" <55 (left) and "high GI" >70 (right) cut-off points (the figure was taken directly from Buyken. 2005)
Ok, not all regular potatoes are made equal, but sweet potatoes are still king, right?
Against the background that my grand father who lived a 100% healthy, diabetes-free life into his late 90s, competed in track and fields and swam laps until about 6 months before he died, had regular potatoes with every dinner, I may be somewhat biased as far as the "bad potatoes" are concerned. This does yet not compromise the value of the rational arguments and scientific evidence against the unjust and, above all, over-generalized vilification of regular potatoes I have brought forward in this first installment of the Potato Manifesto. If you are interested in how the "holy" sweet potato which is currently hailed as the savior of the neo-paleolithic race compares, come back tomorrow for Part II of the Potato Manifesto ;-)