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".

If you've read yesterday's first part of the Potato Manifesto, you should by now be aware that the common notion of the pro-diabetic high-glycemic regular potato is another of the numerous black-or-white nutrition myths that do not become right, no matter how many bloggers and forum posters reiterate them. In today's second part of the series I will try to elucidate, whether the sweet potato, of which I would venture to say that she is the "en vogue tuber of the year", is not still the "safer starch alternative". So bear with me while I am using my scientific peeling knife to check whether there is a bitter truth hidden beneath skin of the sweet potatoes ;-)

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)

A brief note on the fallacy of common arguments for "traditional diets" (=sweet potato diets): I recently started listening to archived episodes of Jimmy Moore's Living La Vida Low Carb show and encountered time and again the 99% nonsensical argumentation that the XYZ thrived on whatever diet and that by just mimicking their diet(s), we would thrive, as well. To me this is like saying: "Look, the Panda bears thrive on a 100% bamboo diet! So, the ice-bears in our zoo should get along pretty well, if we stopped feeding them meat." Now, this is obviously a provocative comparison, but if I look at myself and then look at an Aborigine, the difference may not be as significant as the one between panda and ice-bear and yet, according to the results of a 1986 study by Anne W. Thorburn et al. (Thorburn. 1986) our glucose responses to the ingestion of 50g of carbohydrates from either the sweet-potato'esque bush potato and a "normal" western potato would be completely different (cf. figure 1). While the poor Aborigine would encounter the typical blood sugar ups and downs that are so characteristic for the ingestion of "high GI" carbs, my body would probably not care whether I eat the 100% paleo bush potato or its readily available, cheap cousin. The futility of common "neo-paleolithic" reasoning aside, this example also shows that genetic individualities are another major determinant of the glycemic index.

Three yeas later, in 1994, Thomas Wolever and his colleagues from the University of Toronto tested the glucose response of diabetic patient to 102 complex carbohydrate foods (Wolever. 1994). "Complex", in this case, indicates that they tested combinations of foods like they could appear on someone's dinner table and lo and behold, the "low GI" sweet potato took a close second to the "high GI" boiled white potato, when the both were boiled and served to 16 diabetic patients along with tomatoes. The 1 GI point difference between sweet and white potato (60 vs. 59) is yet well within the statistical margin.

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)

And in fact, as my juxtaposition of data from a 2010 study on the relation between amino acid and sugar content of commercially available regular potatoes and its effect on the formation of acrylamite during heating (Zhu. 2011) and data from the previously mentioned (cf. Part 1) study by Cuneyt Dincer and his (or her) colleagues from the Akdeniz University in Ankara, Turkey (Dincer. 2011) shows (cf. figure 2), there is a non-negligable difference in the amount and composition of "free sugars" (vs. sugars in the form of starch). Although I am not 100% about the exact nature of the exotic Chinese potatoes, it is quite obvious that, despite large varieties in the absolute and relative sugar content of "regular" potatoes, the "classic" potato has a lower absolute free sugar content and contains more fructose and glucose than her sweet sister, 90% of the free sugar content of which is plain table sugar (=sucrose = glucose + fructose).

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)

This "sugary" pattern does yet broaden, when you expose the sweet potato to heat in the form of either boiling or baking (cf. figure 4). In that, the cultivar-dependent slight reductions in glucose and fructose carry little weight compared to the sudden appearance of significant amounts of maltose, which had been beyond the detection before the sweet potatoes were boiled or baked (cf. figure 4).

Figure 4: Free sugar content (mg/g dry weight) of fresh and cooked sweet potatoes (data adapted from Dincer. 2011)

From a chemical perspective, the latter is not really surprising and its formation in response to heat treatment has been reported as early as 1923, when H.C. Gore published a paper in the Journal of Industrial and Engineering Chemistry (Gore. 1923), in which he makes the important observation that the formation of sugars from starch takes place within the first minutes of heating:

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)

If we disregard the differences between the 10 cultivars and focus on the overall pattern, it is quite obvious that on the "less to worst thing you can do with your sweet potatoes if you are concerned about GI"-scale boiling is at the most benign end of the continuum, while baking and roasting compete for the position at the other end - a pattern, those of you who have already read Part 1 of the Potato Manifesto should be familiar with: Contrary to common believe new boiled (regular / sweet) potatoes are in fact a low GI food. In our kitchen (and even more so in the industrial processing machinery of the food industry), where the harmless tubers are mashed, pureed, dried, instantized, fried, baked, roasted etc., both, regular, as well as sweet potatoes do yet acquire at least some resemblance to the "pro-diabetic frankenfood", as which they have gotten labeled within the blogosphere, all along.

Figure 5: Comparison of regular and sweet potato glycemic indices after boiling, frying, baking and roasting.

If we now take a final look at the comparison of the average GI (figure 6 is based on data from both installments of the Potato Manifesto) of regular and sweet potatoes after boiling, frying, baking and roasting, it seems as if the main differences were that on average the regular potato has a slightly higher baseline GI (I do not need to remind you that according to Soh. 1999 boiled Desiree potatoes have a GI of 10!, do I?) and tends to be more resistant to heat-induced deteriorations in her starch / free sugar composition.

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

Just as an aside, the Wolever study also examined the glucose response to another of the funky foods that are resurfacing on the Internet as "health foods", these days: Tapioca - also known as cassava, manioc, aipim, bitter-cassava, boba, mandioca, macaxeira, manioca, tapioca plant, yuca. With a GI of 84 it easily outperforms, corn chips (GI 76) and even waffles (GI 79).

Note: Wherever this was necessary I converted GIs that were given with white bread as a reference to the glucose as a reference.

Whether you could benefit from the (again, on average and in this case specifically referring to the white variety of the regular potato) higher beta-carotene and vitamin C content of sweet potatoes would depend on the rest of your diet. If the latter is "99% paleo" ;-), the likelihood that you would not get enough of these antioxidants is close to zero and it should thusly not really make a difference whether you go for the "paleo-" or the "neolithic" potato, as long as you don't (over-)process her into another of the innumerable frankenfoods the average member of the neolithic convenience society is so fond of.

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. menshealth.co.uk)

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. BBC.co.uk)

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 ;-)

New Results From the "Test Tubers": Paleolithic Men Could Have Been Healthier Had They Microwaved Their Potatoes.

Image 1: Potato roasting caveman-style - probably not the best way to "cook" your potatoes

As a non-native-speaker, I must admit that the first time I heard someone talk about "tubers" was on Robb Wolf's famous podcast, back in the day, when I was "listener #6" (or seven ;-). Contrary to common (foreign) belief, not all Germans subsist on potatoes and sauerkraut and, what's more, even those who do, don't really care that a potato is a "tuber", i.e. a "Knolle" in German - I suppose the idea that it grows in the dirt is not too appealing to some, while the large majority probably just doesn't care as long as those "tubers" make a good addition to their Schweinebraten... yet, whatever the reasons may be, my first encounter with "tubers" has ingrained the link of these "underground structure[s] consisting of a solid thickened portion or outgrowth of a stem or rhizome, of a more or less rounded form, and bearing ‘eyes’ or buds from which new plants may arise" (OED.com), as the venerable Oxford English Dictionary would have it, to the paleo style of eating so deeply into my brain that I have been seeing cavemen with roasted sweet potatoes on their sticks in front of a fireplace in my mind's eye, ever since. Now, if that really was the way life went, back in the paleolithic days, our ancestors did probably miss about 32% of the antioxidant magic of the tuberous roots - at least this is what the results of a recent study on the effects of different cooking methods on polyphenols, pigments, and antioxidant activity in potato tubers from the San Luis Research Center at the Colorado State University  would suggest (Perla. 2011).

In their experiment, Venu Perla, David G. Holm, and Sastry S. Jayanty

• boiled - 1h in a sieved double-boiler,
• microwaved + cooked - 10min in a commercial microwave oven at max. highest level +10min boiling, and
• baked  - 1h at 204°C in a commercial pre-heated oven

six months old stored potato tubers of 5 cultivars and 9 advanced selections of Colorado state (skin-color: 4x russet; 6x red; 1x white; 3x purple) and analyzed the samples for total phenolics, total flavonoids, total flavonols, and DPPH (2,2-Diphenyl-1-pikryl-hydrazyl) radical scavenging activity.

Figure 1: Loss in total polyphenol content of 5 cultivars and 9 advanced selections of Colorado state potato tubers due to cooking, microwaving, and baking (data calculated based on Perla. 2011).

As the data in figure 1 shows, all three preparation methods led to profound reductions of the potatos' total polyphenol counts. With Purple Majesty being most and Russet Nugget being least perceptible to the heat induced reduction in total polyphenol count. The cultivars CO97222-IR/R and CO97226-2R/R exhibited the highest total polyphenol counts (+77% and +137% above average in the raw state), with the former being particularly resistant to cooking (+119% above average) and the latter being particularly resistant to microwaving and baking (+154% and +169% above average, respectively).

Figure 1: Loss in polyphenol, flavenoid and flavenol content of 5 cultivars and 9 advanced selections of Colorado state potato tubers due to cooking, microwaving, and baking (data calculated based on Perla. 2011).

The superior resistance of these red-fleshed potato cultivars to cooking treatments is yet relative polyphenol-specific, due to their extraordinary high flavenoid and flavenol content in the raw state, CO97222-IR/R and CO97226-2R/R do yet retain a 241%, 155%, 199% and 243%, 331%, 395% higher flavenoid and 78%, 34%, 17% and 80%, 98%, 181% higher flavenol content  than the average potato (in the study) even after cooking, microwaving and baking, respectively.

The scientists also observed that, contrary to the white and yellow fleshed tubers, where the major pigment was lutein, the "dominant pigments in the red and purple fleshed tubers were anthocyanins", the antioxidant activity of which has been implicated in the prevention (by some even the treatment) of obesity, cardiovascular disease, diabetes, hyperlipidemia and even cancer. Unfortunately, these pigments are just as susceptible to cooking, microwaving and baking as the polyphenols, flavenoids and flavenols, so that the total anti-oxidant activity (as measured by DPPH free radical scavenging assays) of the tubers that were tested in this study was reduced by -26%, -32% and -38% by boiling, microwaving and baking, respectively.

Please note: While it is unlikely that you will die from eating a single raw potato. The solanine that can be found in all parts of the plant, including the leaves, fruit, and tubers, is a natural fungicide and pesticide the plant produces to protect itself from insects, the ingestion of which can potentially be fatal!

Now, despite the fact that microwaving is only slightly less damaging that baking, I assume that putting the potatos directly into the fire, like the paloelithic men and women in my daydreams use to do it, probably is the worst way to prepare your tubers. Yet while even microwaving would have been a healthier option, the best and, from an evolutionary perspective, more natural solution would be to boil your potatoes.

Metformin & Sweet Potatoes - What Do They Have in Common? Both Reduce Blood Glucose & Increase Insulin Response, Decrease Total Cholesterol & Increase HDL

Image 1: Leaves of wild sweet
patatoes from the Phillipines
(photo by Obsidian Soul)

"Nature knows best!" Who would have thought that this motto of mine applies to the #1 paleo-pasta-replacement, the sweet potato, as well? While the tuberous roots do of course contain many of those nasty carbohydrates, which, in reminiscence of the fat-phobia of the closing 20th century, could be called the "bad fats of the early 21st century", the whole plant also comes with a whopping dose of a Metformin-esque anti-diabetes drug that would undo potential damage secondary to carbohydrate overload., uf... yes if we consumed its heart-shaped or palmately lobed leaves (they are edible and non-toxic!)

In the VNU Journal of Science, Natural Sciences and Technology, Do Ngoc Lien and his Vietnamese colleagues published some very interesting data on the effects an ethyl acetate extract from the leaves Ipomoea batatas (L) Lam (sweet potato). had on glucose, insulin and lipid levels in a streptozotocin treated mouse-model of human diabetes type II (Lien. 2011):

When obese-diabetic mice [were] administrated orally daily by ethyl acetate fraction of 1000mg lyophilized powder/kg for 21 days, we showed that maximum hypoglycaemic (36.77%) and hypolipidemic effects, such as TC (35.18%), TG (29.17%), and LDLc (51.97%) were proven in treated mice compared to the control (untreated mice)

The results become particularly interesting, if we have a closer look at the effect size compared to what the current gold-standard in pharmacological diabetes management, Metformin (cf. figure 1):

Figure 1: Relative effect of 21 days on n-hexant (EtOH), chloroform (Chlf) and ethylacetate concentrates of sweet potato leave extract or Metformin on blood glucose and insulin levels compared to untreated control
(calculated based on Lien. 2011)

If we now calculate the ratio of glucose / insulin and take that as an (unreliable) measure of insulin sensitivity (cf. figure 2), we see that 1g/kg (human equivalent 81mg/kg) of an ethylacetate extract from the leaves sweet potato was ~90% as effective as the current diabetes drug of choice, Metformin, in restoring insulin sensitivity and thusly reducing blood glucose to acceptable levels.

Figure 2: Effect of 21 days on n-hexant (EtOH), chloroform (Chlf) and ethylacetate concentrates of sweet potato leave extract or Metformin on "insulin sensitivity", approx. by glucose/insulin ratio (calculated based on Lien. 2011)

The underlying mechanism, the scientists believe, is a compensation of the existing imbalance between hexokinase and glucose-6-phosphatase activity in the liver of the diabetic mice. I am not sure, though, whether this restorative effect on the formation and breakdown of glucose fully explains all the beneficial effects of the sweet potato leave extract, which also include a -29.17% reduction in the nasty triglycerides, of which many scientists think that they are rather causative of, than corollary to the insulin resistance that is at the heart of type II diabetes, and a reduction of the total cholesterol/ratio from 19.02 to 8.14 (MDs still want their patients in the &amp;amp;amp;amp;lt;4.0 regions, today). In view of the "pharmacological" (I need these "" in order not to get sued by the FDA ;-) potency of the leaves of a root vegetable, probably no-one would know of, if it had not been heavily promoted by Robb Wolf (whom I regard very highly) and the rest of the Paleo Crowd (some of whom have unfortunately turned into fanatics),  I guess you will probably understand why I foresee an increasing demand for the formerly worthless foliage of Ipomoea batatas.