Monday, December 26, 2011

The Potato Manifesto - Part 1/2: A (Re-)Evaluation of the Contemporary Discrimination of "the" Ordinary Potato

Image 1: The grunty regular (left) and the cute sweet potato (left), which one would you commit your health to? (img.
This is a blogpost, which eventually turned out to be the first part of a series, is a post with a history, a rather complex one, to be precise. It is rooted (not tubered ;-) in my amazement over the contemporary craving for sweet potatoes within the ever-growing neo-paleolithic community on the Internet and was sparked by the recent publication of a study on sweet potatoes, I stumbled upon on my daily tours of the most recent scientific literature. To make long story short, instead of immediately summarizing the data, drawing some graphs and commenting on the real-world implications of this study, I decided to use the holiday to descend into the archives and take a closer look at what science has to say on the bitter truth about the grunty regular and the starchy promises of cute sweet potato (cf. image 1 ;-)...

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.
A note to my dear American friends: Don't crow too soon, the obesity rate in the US tops the one in the UK by more than just a margin. According to a 2010 paper by Flegal et al. that was published in the Journal of the American Medical Association, the estimated obesity rate in the US amounts to 35.5% among the women and 32.2% among the men (Flegal. 2010). And a constantly increasing percantage (currently 4.7%) of the American population is already "extremely obese" and has a BMI > 40! Just to put that into perspective. A man or woman with a BMI beyond 40 and a height of 5 foot and 7 inches (170cm) would weigh at least (!) 254.85lbs (115.6kg) - this is already "Kig-Size Homer"-territory (cf. Intermittent Thoughts).
If we follow the current dietary paradigms and ignore the frying procedure, the answer to this question does not appear to be very difficult. I mean, when wheat was devil's excrement, then regular potatoes would be his horns or even nastier body parts, I do not want to mention here... and though a reasonable explanation for the widespread vilification of potatoes still escapes me, the contemporary nutritional paradigm within the health and nutrition blogosphere suggests that regular white potatoes have an awfully high glycemic index, will spike your blood sugar levels and have your neolithic body pump out tons of insulin - even if the rest of your diet is 99% paleo, as many of the listeners of Robb Wolf's podcast like to describe the way they are eating after having read his (widely read, yet controversial - esp. wrt to starches / carbs, fish oil and a few other topics) book ;-)
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)
If we take a look at the data in figure 1, which shows both the incremental area under the glucose curve (AUC) and the GI of eight commercially available and commonly consumed British potato cultivars, it should however be quite obvious that the concept of the bad high-GI regular potato is about as misguided as the racist or religious prejudices some of our fellow human beings are still harboring against other members of the human race. With glycemic indices that range from 56 for the "waxy" Marfona to 94 for the "firm" Maris Peer, the "bad" regular potatoes cover the exact same GI range as their "healthy" sweet cousins (come back tomorrow for Part II of this series with more information on sweet potatoes).

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)
On the left = "better than white bread"-side of figure 2, we have cold, boiled red potatoes (GI 56.2) and, surprise, frensh fries (GI 63.6) and roasted Californian white potatoes  (GI 72.3, but lower AUC than white bread). On the right = "worse than white bread"-side, instant mashed potatoes (GI 87.7) and the hot variety of the "low GI" red potatoes (GI 89.4) are competing for the red lantern.

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)
Did you know that fructose, of all, is able to reduce the postprandial increase in serum glucose in nondiabetic adults (mean age: 26)? In their 2002 study, Patricia M. Heacock and her colleagues were able to show that pre-ingestion of 10g of fructose 60min and 30min before the ingestion of a 50g carbohydrate meal (from potatoes) reduced the area under the glucose curve (glucose AUC; cf. figure 3) by 25% and 27%, respectively (Heacock. 2002). The immediate co-administration of 10g of fructose with the potato meal (figure 3, 0 min), on the other, did not induce any statistically significant changes in the glucose AUC of the 13 male and 19 female study participants whose blood glucose levels were measured by finger-prick capillary blood (Accu Check) and glucose oxidase analyzers (YSI).
Furthermore, a 1999 study by Soh and Brand-Miller from the University of Sidney (Soh. 1999) shows that the real-world glucose responses of different individuals to differently processed and stored potato cultivars cover an even broader spectrum (especially on the low GI side) than the results of the previously cited studies suggested. The GI values, the Australian scientists calculated based on the glucose response to a 50g carbohydrate portion of eight different potato meals (three varieties, four cooking methods, two states of maturity) differed by as much as +/- 55pts, with canned new potatoes (GI 65) at the upper and boiled Desiree potatoes (GI 10) at the very lower end of the spectrum. And as if things were not complicated enough, already, Soh and Brand-Miller also introduce yet another variable into the equation - the size of the tuber, which showed a statistically significant correlation with the glucose response of the study 10 healthy participants (correlation between GI and tuber size: r=0.83, p < 0.05).

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):
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)
[...] 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).
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.

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 ;-)