By Dr. Mercola
While fiber is crucial for optimal gut health, what makes certain types of fiber even more important than others is its potential for fermentation. Unripe tropical fruits such as banana, papaya and mango contain digestive-resistant starch1 — indigestible low-viscous fibers that slowly ferment in your large intestine. These resistant starches feed healthy bacteria, essentially acting as prebiotics.
They also bulk up your bowel movements for easier, timelier disposal without making you feel bloated or gassy. Best of all, they don’t spike your blood sugar the way completely ripened fruit and other starchy foods do, so they actually help improve rather than worsen insulin regulation.2,3 In many ways, resistant starch could be considered a third type of fiber (in addition to soluble and insoluble fiber).
Unripe fruits aren’t the only foods with this ability, however. Researchers have discovered even high net-carb foods such as potatoes,4 rice, bread and pasta become more digestive-resistant when prepared in certain ways.
Specifically, the process of cooking, cooling and reheating these foods appears to trigger this beneficial change in composition, so the leftover mentality can be quite useful in more ways than one. Not only will you save money by eating yesterday’s leftovers, starchy leftovers will actually be healthier and less calorie-dense.
Health Benefits of Digestive-Resistant Starches
Starches are made up of glucose, the primary building block of carbohydrates. While carbs are a source of cellular energy, glucose is not an ideal fuel for your body. Healthy fats are far better, as fat creates fewer reactive oxygen species (ROS) than glucose when burned.
When you eat a high-starch meal such as pasta with a side of bread, your blood sugar will spike. When this happens regularly, such as several times a day, your body becomes progressively more resistant to insulin, which is released in response to elevations in blood glucose.
Insulin resistance, in turn, is at the root of most chronic diseases, including Type 2 diabetes,5 heart disease, cancer and dementia. Resistant starches, on the other hand, pass through your digestive system without being broken down; hence, they don’t raise your blood sugar and insulin. Instead, resistant starches end up fermenting and feeding beneficial bacteria in your gut.
Byproducts of this fermentation process in your gut are short-chain fatty acids that help reduce inflammation, improve immune function,6 normalize blood pressure7,8,9,10 and lower your risk of heart disease and heart attack.11
Short chain fatty acids produced through fiber fermentation also serve as substrates for your liver to produce ketones that efficiently fuel your mitochondria and act as powerful metabolic signals, and science suggests resistant starch may play a role in the prevention of colon cancer12 and inflammatory bowel disease.13
Starch Retrogradation Improves Nutritional Quality of Starchy Foods
Although this is an interesting and novel insight on digestive resistant starch in breads and other carbohydrate, it is my belief that most will still need to avoid these foods even if they create more resistant starch because of their impact on insulin resistance. Also, most wheat has lectins and is contaminated with glyphosate, which will impair tight junctions in the gut.
If you are metabolically flexible, then healthier carbs like sweet and purple potatoes cooked in the methods described below make a great addition to your diet and will likely move your microbiome in a healthy direction. I personally use recooked sweet potatoes twice a week and purple potatoes four to five times a week as I love the texture they provide in my salad.
While some starchy foods are naturally digestive-resistant, others become more resistant to digestion through cooking and cooling — a process known as starch retrogradation.14,15 Other research shows resistant starch content is further increased when cooked and cooled foods such as rice, pasta and potatoes are later reheated.
As noted in one such study,16 “[C]ooling of cooked white rice increased resistant starch content. Cooked white rice cooled for 24 hours at 4 degrees C (39.2 F) then reheated lowered glycemic response compared with freshly cooked white rice.” The cooked and cooled rice had 2.5 times more resistant starch than cooked rice, and when consumed, the former led to lower blood glucose response in test subjects.
Animal research17 has also found that eating rice that has been heated and cooled multiple times resulted in less weight gain, better gut function, increased stool output and lower cholesterol levels compared to eating the same amount of common rice powder. Similar results have been found with cooked and chilled potatoes, wheat, barley and legumes. For example:
- In one study, cooking and then cooling potatoes overnight increased their resistant starch content by 280 percent.18
- Research has also confirmed that eating potatoes with higher amounts of resistant starch results in far lower blood sugar response than eating carbohydrate-rich foods with no resistant starch in them.19
- Cooking and cooling wheat commonly used in bread making has been shown to more than double the resistant starch content, raising it from 41 to 88 percent.20 The same effect is believed to occur in wheat pasta.
- Cooking and chilling barley, peas, lentils and beans also results in higher resistant starch content. As noted in one study,21 “The mean [resistant starch] contents of the freshly cooked legumes, cereals and tubers (4.18 percent, 1.86 percent and 1.51 percent dry matter basis, respectively) increased to 8.16 percent, 3.25 percent and 2.51 percent, respectively, after three heating/cooling cycles with a maximum increase of 114.8 percent in pea and a minimum of 62.1 percent in sweet potato.”
Cooking Rice With Coconut Oil Increases Resistant Starch Conversion
In another study, cooking traditional nonfortified rice with a teaspoon of coconut oil added to the water and then cooling the rice for 12 hours increased resistant starch tenfold, which reduced calories by as much as 60 percent. The addition of coconut oil was found to be a key strategy here, not just the cooling itself. As explained in a press release:22
“The oil enters the starch granules during cooking, changing its architecture so that it becomes resistant to the action of digestive enzymes. This means that fewer calories ultimately get absorbed into the body.
‘The cooling is essential because amylose, the soluble part of the starch, leaves the granules during gelatinization,’ explains [team leader Sudhair] James. ‘Cooling for 12 hours will lead to formation of hydrogen bonds between the amylose molecules outside the rice grains which also turns it into a resistant starch.'”
How Freezing and Toasting Impacts Glycemic Impact of White Bread
Interestingly, even bread can be made healthier through heating and cooling. In a 2008 study published in the European Journal of Clinical Nutrition,23 10 healthy test subjects were given homemade and commercial white bread that had been prepared in four different ways:
- Frozen and defrosted
- Fresh, toasted
- Frozen, defrosted and then toasted
Incremental blood glucose and peak glucose response were measured after randomized repeated feedings. Compared to fresh bread — whether homemade or commercial — both freezing and toasting resulted in lower postprandial blood glucose measurements. Compared to fresh homemade bread:
- Frozen and defrosted homemade bread lowered blood glucose from an average of 259 millimole per minute per liter (mmol min/L) to 179 mmol min/L
- Toasting homemade bread lowered blood glucose from 259 to 193 mmol min/L
- Toasting following freezing and defrosting resulted in a blood glucose level of just 157 mmol min/L
Similarly, compared to fresh commercial white bread, which resulted in an average glucose level of 253 mmol min/L, toasting lowered it to 183 mmol min/L, while frozen, defrosted and toasted commercial bread resulted in a glucose level of 187 mmol min/L. According to the authors:
“All three procedures investigated, freezing and defrosting, toasting from fresh, and toasting following freezing and defrosting, favorably altered the glucose response of the breads.
This is the first study known to the authors to show reductions in glycemic response as a result of changes in storage conditions and the preparation of white bread before consumption. In addition, the study highlights a need to define and maintain storage conditions of white bread if used as a reference food in the determination of the glycemic index of foods.”
Unripe Tropical Fruits Are a Good Source of Digestive Resistant Starches
As mentioned earlier, green bananas and mango are great sources of digestive resistant starches. They also contain a number of valuable vitamins, and all three make for tasty “green” fruit salads. Unripe mango, for example, is exceptionally rich in vitamin C. A single green (unripe) Langra mango contains as much vitamin C as 35 apples, nine lemons or three oranges.24 In India, green mango is used as a natural remedy for:
- Gastrointestinal (GI) disorders: Green mango, eaten with salt and honey is used to treat a range of GI problems, including diarrhea, dysentery, piles, morning sickness, indigestion and constipation.
- Liver problems: The acids in unripe mango increase bile secretion and act as an intestinal antiseptic. It also helps purify your blood and acts as a liver tonic. Green mango with honey and pepper is used for stomach ache due to poor digestion, hives and jaundice.
- Blood disorders: The high vitamin C content of unripe mango helps improve blood vessel elasticity and increases formation of new blood cells. It also aids absorption of iron and decreases bleeding. According to the Indian magazine Deccan Herald,25 “Eating an unripe mango daily during the summer season prevents … infections, increases body resistance against tuberculosis, cholera [and] dysentery …
It tones the heart, nerves and cures palpitation of the heart, nervous tension, insomnia and weakness of the memory … Eating raw mango with salt quenches thirst and prevents loss of sodium chloride and iron during summer due to excessive sweating. It tones up the body and helps one to tolerate the excessive heat.”
There’s one caveat, however: Avoid eating more than one unripe mango per day, as it may cause throat irritation and/or indigestion when eaten in excess. Also avoid drinking cold water immediately afterward, as it coagulates the sap, thereby increasing the risk of irritation. Recipes for green banana salad, green mango salad and green papaya salad can be found in my previous article, “The Surprising Health Benefits of Unripe Banana, Papaya and Mango.”
Fiber Differentiates ‘Good’ Carbs From the ‘Bad’
Bread, rice, pasta, potatoes, fruits and vegetables are all carbohydrates. However, from a health standpoint they’re not created equal, and it’s primarily the fiber content that differentiates “good” carbs from the “bad.” Most vegetables and certain fruits are very high in fiber, which means they’re very low in net carbs, and when it comes to carbs, it’s the net carbs you need to pay careful attention to.
To determine the net carb content of a food, simply subtract the fiber amount in grams from the total carbohydrate amount. Vegetables typically top the list in terms of high fiber content, but certain unripe fruits score high as well, while adding novelty to your diet. As for rice, pasta, potatoes and bread, which are common staples among carb addicts, remember that cooking, cooling and reheating can rather dramatically improve their nutritional profile by increasing the amount of resistant starch they contain.
Potato salad would be one way to indulge in potatoes rather than eating them hot, either cooked, roasted or baked. Alternatively, you could whip up a batch of roasted potatoes, chill them overnight, then reheat them in a pan. Purple potatoes are my new favorite and a great addition to most of my salads.
Reheating cooked and chilled rice is also far preferable to eating fresh cooked rice. With regard to bread, the greatest benefits were seen when the bread was frozen, thawed and then toasted. Just beware of the fact that toasting creates harmful acrylamide, a carcinogenic substance, and the more burnt the bread, the more acrylamide is created. So, if toasting, take care not to brown it excessively.
Overall, most people do not get enough fiber in their diet. Boosting your fiber intake by eating more soluble and insoluble fiber from vegetables and organic psyllium will benefit your health. And, by preparing high-starch foods like rice, potatoes and pasta in such a way as to boost the digestive resistant starch content in the food, these may at least be rendered less harmful. While there are individual differences, as a general rule, most people could benefit by:
- Restricting net carbs to less than 50 grams per day (if you exercise a lot or are very active, you might be able to increase it to 100 grams. However, this is a general recommendation and once you are metabolically flexible it would be wise to increase this level a few times a week, especially when you’re doing strength training
- Increasing fiber to approximately 50 grams per 1,000 calories