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Ripe or as in this case overripe bananas have little to no starch, green ones almost only starch and only little sucrose / fructose.

It is well-known that people with higher intakes of resistant starches have a reduced risk of becoming obese, developing diabetes and hyperlipidemia. Studies also indicate that starchy carb lovers have overall lower insulin concentrations and an increased insulin sensitivity compared to the average junk-food eater (McKeown. 2002; Liese. 2004), whose low dietary fiber consumption is linked to a reduced in insulin sensitivity and inflammation (Parillo. 2004).

What distinguishes whole grains and co from plain sugar, cornflakes and white bread is that they contain dietary fiber in its non-digestible form that is fermented in the colon, where the production short chain fatty acids (SCFA | learn more) triggers many of the well-established health benefits by direct and indirect effects on the gut-endocrine axis (Topping. 2001).
One of the best dietary sources of resistant starches are bananas... well, green bananas or rather the high resistant starch flour that's produced from green bananas. Although, it takes some processing to extract it from the bananas, it's classified as "RS2", i.e. naturally occuring resistant starch, because it is after all a native (vs. man-made) starch granule that is naturally highly resistant to digestion by α-amylase enzymes in the small intestine (Englyst. 1992).

Figure 1: Proximate compositions of the edible portion of bananas at different stages as classified by the color of banana peel (Lii. 1982) - only a small fraction of the original starch is already RS2, it takes processing to isolate the "natural" resistant starch from the bananas and the amount of RS2 in banana flour is variable 50%-84% (Lehmann. 2002).

Since it's a natural product, banana starches from different sources may have diverse structures and may thus also have a different degree of resistance to enzymatic hydrolysis. It is thus no wonder that previous studies by researchers from the Universidad Juá rez Autónoma de Tabasco has shown a large variety in terms of the amylose-content and resistance to digestibility among Banana starches from different sources. In general, though, all of them share the important property of being largely indigestible and being able to trigger a reduction in the glycemic and insulinic response compared to regular starches in both healthy and diabetic subjects (Ble-Castillo. 2010).

Figure 2: Effects of either 24 g of NBS dissolved in 240 mL of water per day or 24 g/day of soy milk dissolved in the same volume of water for 4 week (data represents within subj. comparisons | Ble-Castillo. 2010).

In fact, Ble-Castillo et al. were already able to show that within only 4 weeks on 24g/day of native banana starch (NBS) both the body weight and insulin sensitivity of obese type 2 diabetics significantly improved - and that not "on average", but on a "within-subject" basis as it can only be observed in cross-over study.
In a more recent study, Guadalupe Jiménez-Domínguez repeated the Ble-Castillo study in a different, more diverse group of subjects:

Forty five subjects were screened for obese and lean participants. Anthropometric indexes and basic laboratory examinations were carried out. Participants for the obese group were included if they were healthy persons between 18 and 45 years of age, were obese, and had maintained stable weight during the three months prior to experimentation. Subjects in the normal-weight groups (lean group) were included if they were between 18 and 45 years of age, had 18.5–24.9 BMI values, had [normal fasting glycemia and]HbA1c values [...] and had maintained stable weight during the three months prior to experimentation. Subjects not included in this study were those with a previous diagnosis of diabetes, with fasting glycemia >126 mg/dL (> 7.0 mmol/L) or glycated hemoglobin > 6.5%, with digestive disorders, chronic diseases such as renal or hepatic, being pregnant, under psychiatric treatment, receiving medical or naturist treatment to reduce BW, practicing intense physical activity (> 90 min/week), receiving immuno-suppressants, or with a history of cigarette smoking or alcoholism. Thirty two participants were eligible for the study, 15 for the obese group and 17 for the lean group. However, only 10 subjects in each group completed the whole protocol" (Jiménez-Domínguez . 2015).

In said study, the subjects received a higher dose of 38.2 g of native banana starch (NBS) or the same amount of corn starch twice daily yet for only four days, before, on day 5, a 3-h meal tolerance test (MTT) was performed to evaluate the acute effects glucose and insulin responses that may explain the benefits Ble-Castillo et al. had observed (see Figure 2).

"All of the subjects received low-fiber diets at breakfast and lunch from the Nutrition Department’s restaurant during days 1‒4 of the treatment period. The low-fiber diet consisted of avoiding whole-grain/wholemeal breads and cereals, legumes, nuts, and seeds, and of removing the skins from fruit and vegetables" (Jiménez-Domínguez. 2015).

All subjects had to stick to the same baseline diet. Adherence to the diet and the randomly assigned supplements,

• the NBS beverage, which contained 38.39 g of NBS, 31.82 g of soy milk, and 240 mL of purified water, and 
• the DCS beverage, which contained 38.39 g of DCS, 31.82 g of soy milk, and 240 mL of purified water,

Figure 3: The 4 days on natural banana starch had quite significant beneficial effects on the glucose and insulin response to a standardized test-meal. It is yet also obvious that the obese saw greater benefits (Jiménez-Domínguez. 2015).

The most significant results in Figure 3 are not really surprising, though. As you may have expected, the native banana starch has a significant advantage over the corn starch, when it comes to its acute and chronic effects on glycemia in both, lean and obese individuals. An advantage that may well explain the significantly different metabolic effects in the Ble-Castillo study.
References:

• Ble-Castillo, Jorge L., et al. "Effects of native banana starch supplementation on body weight and insulin sensitivity in obese type 2 diabetics." International journal of environmental research and public health 7.5 (2010): 1953-1962.
• Englyst, Hans N., S. M. Kingman, and J. H. Cummings. "Classification and measurement of nutritionally important starch fractions." European journal of clinical nutrition 46 (1992): S33-50.
• González-Soto, R. A., et al. "The influence of time and storage temperature on resistant starch formation from autoclaved debranched banana starch." Food research international 40.2 (2007): 304-310.
• Hernández-Nava, R. G., et al. "Development and characterization of spaghetti with high resistant starch content supplemented with banana starch." Food Science and Technology International 15.1 (2009): 73-78.
• Jiménez-Domínguez, Guadalupe, et al. "Effects of Acute Ingestion of Native Banana Starch on Glycemic Response Evaluated by Continuous Glucose Monitoring in Obese and Lean Subjects." International journal of environmental research and public health 12.7 (2015): 7491-7505.
• Lehmann, Undine, Gisela Jacobasch, and Detlef Schmiedl. "Characterization of resistant starch type III from banana (Musa acuminata)." Journal of agricultural and food chemistry 50.18 (2002): 5236-5240.
• Lii, Cheng‐Yi, Shuh‐Ming Chang, And Ya‐Lan Young. "Investigation Of The Physical And Chemical Properties Of Banana Starches." Journal Of Food Science 47.5 (1982): 1493-1497.
• Liese, Angela D., et al. "Whole-grain intake and insulin sensitivity: the Insulin Resistance Atherosclerosis Study." The American journal of clinical nutrition 78.5 (2003): 965-971.
• McKeown, Nicola M., et al. "Whole-grain intake is favorably associated with metabolic risk factors for type 2 diabetes and cardiovascular disease in the Framingham Offspring Study." The American journal of clinical nutrition 76.2 (2002): 390-398.
• Parillo, M., and G. Riccardi. "Diet composition and the risk of type 2 diabetes: epidemiological and clinical evidence." British Journal of Nutrition 92.01 (2004): 7-19.
• Topping, David L., and Peter M. Clifton. "Short-chain fatty acids and human colonic function: roles of resistant starch and nonstarch polysaccharides." Physiological reviews 81.3 (2001): 1031-1064.

Walnuts, cacao and beans, 3 superfoods that are super only in super-high quantities.

Today's waredownloadsoft Food Science Research Update is all about three so-called "superfoods". It's an article from which you cannot just learn that walnuts, beans and cacao are "superfoods" as they protect you from cancer, improve your gut health and ameliorate diabetes.

It's yet also an article that puts the (rodent) science into perspective. A perspective you won't see taken very often, because it has the "superfoods" look much less "super" than they appear to be without a reality check.

• Dietary walnut suppression of colorectal cancer in mice: Mediation by miRNA patterns and fatty acid incorporation - As a waredownloadsoft reader you are well aware of the beneficial health effects of regular walnut consumption. As far as the mechanisms are concerned, many of you may have speculated that it is related to omega-3s, but do walnuts even have enough omega-3s to make a difference? A recent study from the Beth Israel Deaconess Medical Center at the Harvard Medical School (Tsoukas. 2015) seems to have found the answer is "yes", but it takes some serious walnut eating.
  
  To investigate the protective effects of a high walnut diet, the scientists injected rodents with HT-29 colon cancer cells. 7 days after the injection, the mice were randomized to either control or walnut diets for 25 days of diet treatment. After the study period, thirty samples of tumor and of omental adipose were analyzed to determine changes in lipid composition in each dietary group in response to diets containing either no or 10% (of weight) walnuts.
  

  Figure 1: Gas chromatography analysis of fatty acids in colorectal tumors (left) and omental adipose tissue (right) in 30 mouse samples (Tsoukas. 2015). *P<.05.

  In the tumors of the walnut-containing diet, the scientists found significant increases in α-linolenic, eicosapentaenoic, docosahexaenoic and total omega-3 acids, and a decrease in arachidonic acid, as compared to the control diet.
• More importantly, however, the final tumor size measured at sacrifice was negatively associated with percentage of total omega-3 fatty acid composition (r=−0.641, P=.001). MicroRNA expression analysis of colorectal tumor tissue revealed decreased expression of miRNAs 1903, 467c and 3068 (P<.05) and increased expression of miRNA 297a* (P=.0059) in the walnut-treated group as compared to control diet... and YES! This is the epigenetic "magic" everyone is talking about.
• Beans are SFCA-precursors and colon protectors - Short-chain fatty acids are all the rage these days. SFCAs are the fatty acids that are produced by our bacterial subtenants when we eat resistant starches.
  
  Beans are an excellent source of resistant starches and, according to a recent study from the Agriculture and Agri-Food Canada (Monk. 2015), both cooked white (WK) or dark red kidney (DK) bean flour can, when it is added at 20% to a rodent diet  for 3 weeks, can protect rodents from the pro-colitic assault of dextran sodium sulfate.
  

  Figure 1: Effect of kidney bean diets on (A) SCFA concentrations, (B) cecum size and (C) colon histomorphology in healthy mice + representative images (40×) of  stained colon sections stained highlighting the increased crypt height in the bean-fed mice are shown (scale bar=50μm | Monk. 2015).

  Not only did the "bean preload" (a) reduce the disease severity. It did also ameliorate (b) the colonic histological damage and (c) increase the mRNA expression of antimicrobial and barrier integrity-promoting genes (Toll-like Receptor 4 (TLR4), MUC1–3, Relmβ and Trefoil Factor 3 (TFF3)) and reduce proinflammatory mediator expression [interleukin (IL)-1β, IL-6, interferon (IFN)γ, tumor necrosis factor (TNF)α and monocyte chemoattractant protein-1], which correlated with reduced colon tissue protein levels.
  
  In conjunction with their beneficial effects on IL-17A, IFNγ, TNFα, IL-1β and IL-6, there's little doubt that bean floor can enhance the microbial-derived SCFA metabolite production, gut barrier integrity and the microbial defensive response in the healthy colon, this making it more resilient to artificial and natural assaults.
• Cocoa-rich diet ameliorates hepatic insulin resistance by modulating insulin signaling and glucose homeostasis in Zucker diabetic fatty rats  - While the latest "eat chocolate to lose weigh" paper has been busted, findings of a recent study from the Ciudad Universitaria suggest that cocoa, not chocolate, has the potential to alleviate both hyperglycemia and hepatic insulin resistance in type 2 diabetics - in rats, it already works (Cordero-Herrera. 2015).
  
  In the study, male Zucker diabetic fatty (ZDF) rats which are a common model for "lifestyle" diabetes were fed a control or cocoa-rich diet (10%), and Zucker lean animals received the control diet. ZDF rats supplemented with cocoa (ZDF-Co) showed a significant decrease in body weight gain, glucose and insulin levels, as well as an improved glucose tolerance and insulin resistance.
  

  Figure 1: Effect of the cocoa-rich diet on glucose tolerance and insulin resistance in ZDF rats. (A) Time-course changes in the basal level and after glucose loading (1 g/kg) of blood glucose. (B) AUC calculated from GTT data. (C) Time-course changes in the basal level and after glucose loading (1 g/kg) of serum insulin. Each point represents the mean±S.D. from 8 determinations. (D) HOMA-IR was determined as described in Materials and Methods (p < 0.05 | Cordero-Herrera. 2015).

  The cocoa-rich diet also ameliorated the hepatic insulin resistance by abolishing the increased serine-phosphorylated levels of the insulin receptor substrate 1 and preventing the inactivation of the glycogen synthase kinase 3/glycogen synthase pathway in the liver of cocoa-fed ZDF rats - or put simply, it kept the glycogen storage (and thus another way to store glucose and normalize the blood glucose levels) in the liver intact.
  
  The authors believe that this anti-hyperglycemic effect of cocoa must at least partly be mediated through the decreased levels of hepatic phosphoenolpyruvate carboxykinase which will have the liver pump out more glucose (made mostly from amino acids) although the glucose levels in the blood are already in the red zone while increasing the values of the previously mentioned glucokinase and glucose transporter 2 in the liver which will allow cacao aficionados to store the superfluous glucose as glycogen, not fat. The additional suppression of the c-Jun N-terminal kinase and p38 activation which is usually elevated in response to insulin resistance may also protect you against cancer.