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Resistant Starch - Why You Can't Get It From Diet Alone

Starch 101

Too much fat, too much salt, and too much sugar: These usual suspects often come up when talking about how to improve your diet. The sugar conversation also typically involves starch, which is a long chain made up of many, many links of sugar molecules. But starch molecules are too big to be absorbed by our body, so starch itself isn't a problem. In order to use starch as a source of energy, enzymes in our digestive tract must first cut the chain down into individual sugars, acting like molecular scissors. Once these sugars have been cut from the starch molecule, they can be absorbed by the body and enter our circulatory system.

Starch as a source of energy

Each molecule of starch contains thousands of sugar units, which sounds scary given all we hear about the evils of sugar, but this is actually the reason why we have adopted starch-heavy diets. The truth is that our bodies run on sugar. All of the energy-providing foods we eat are digested, absorbed, and ultimately converted to sugar because our bodies have evolved to use sugar as the 'currency' for energy. Organs like our brain rely almost exclusively on sugar for energy, which explains why confusion is often a side effect to low blood sugar levels.

What is resistant starch?

Metabolism of starch into sugar requires that our body's enzymes be able to access and cut the starch molecules. The starch molecules are readily available in most of the foods we eat, with digestion of the starch starting in the mouth and finishing in the small intestine. Many of our cooking methods break up granules of starch, helping our digestive enzymes get better access to the starch. Ripening of fruits like bananas can also make the energy in starch more accessible. But a small amount of starch in our diet remains inaccessible to the body's enzymes and is never digested – this is known as resistant starch.

Resistant starch, prebiotics, and the gut microbiome

The gut microbiome is a complex ecosystem of bacteria and other microbes that normally inhabits the large intestine. Emerging scientific research is demonstrating that the gut microbiome plays an important role in many aspects of our health, in everything from the immune system, metabolism, and even mental health. Many people are familiar with healthy gut microbes by another name: Probiotics. Often found in unpasteurized yogurt and other fermented foods, these healthy microbes pass through our digestive system and colonize the large intestine. But because these bacteria are living things, they need food called Prebiotics. Prebiotics include certain dietary fibres not digested by the body that make it to the large intestine where they are fermented and used as energy by the healthy microbes there. Resistant starch is a preferred energy source for healthy bacteria in the Gut Microbiome, including healthy Bifidobacteria. Insoluble resistant starches are not FODMAPs, unlike more common prebiotics, and are fermented slowing in the large intestine due to their large molecular size. The large size is related to the fact that there are thousands of glucose sugar molecules in each starch molecule, representing an energy-dense prebiotic food source for the healthy bacteria in your gut.

It's difficult to get resistant starch from diet alone

We've all heard that processed foods are bad because they strip vital nutrition from our diets for the sake of production, packaging, and customer taste preference. This is certainly true when it comes to traditional dietary fibre. Take white bread for example – much of the fibre has been extracted or chemically removed during the milling process. But the 'processing' that destroys resistant starch is much less scary: Simply cooking foods will destroy the resistant starch granules. Even the natural ripening of a banana destroys resistant starch. Our ancestors, unable to shop at the supermarket, would have consumed much higher levels of resistant starch because they could not afford to allow foods to fully ripen or may not have had the ability to cook their food. The bacteria in our ancestors' gut microbiome came to rely on this important source of prebiotics. Today, modern diets provide only a small percentage of the resistant starch expected by the gut microbiome, and this lack of prebiotic resistant starch has been linked with negative changes in the microbial ecosystem and related health problems.

How can I get more resistant starch in my diet?

Eating raw starchy ingredients or unripe fruit like bananas can boost your resistant starch intake but it's certainly not appealing. Minimally cooking foods like beans, lentils, and chickpeas may help preserve resistant starch but the total amounts will still be low, requiring you to consume relatively large servings of these foods. And while cooking then cooling foods can create something called 'retrograded starch', which can also resist digestion, the health benefits of this type of starch are not well established. This is also the case for starches that have been chemically modified to resist digestion. Importantly, the FDA only recognizes granular resistant starch from potato and banana as dietary fibre.

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