No More "Calories In Equals Calories Out": Resistant Starches and the Gut Microbiota

by Cory Tripp MS, RDNNutrition
Produce at a farmer's market

Ever heard the phrase “calories in equals calories out”? It has been used among healthcare professionals and academics for decades to explain how the number of calories we consume is equal to the number of calories our body absorbs. The only problem is, it’s entirely incorrect. While “calories in, calories out” might seem like a common-sense concept, more recent research has found that this is not the case, that sometimes the amount of calories in food we eat is actually much higher than the amount of we absorb.

Several possible explanations for how this works have been proposed, such as genetic differences across individuals, how efficiently we each absorb calories, what hormones are produced and when, the kinds of fat we carry in our body, and the macronutrient makeup (carbohydrates, proteins, fats) of the different foods we may eat (1,2). However, a promising theory is emerging which suggests that the interaction between certain starches we eat and our gut microbiota (healthy intestinal bacteria) may be a significant factor in why the calories we consume do not always equal what we absorb (2).

The term ‘starch’ refers to carbohydrate from plants, primarily in vegetables and legumes, but also in grains, seeds, and other carbohydrate rich foods. Some starches are easily digested and absorbed by the body as calories, while others are more resistant to digestion and absorption due to their shape and enzymatic activity. These are called resistant starches (3).

Resistant starches are either not fully digested or completely undigested, meaning that the small intestine, which is the primary site of absorption, cannot take them in to body to convert to calories (4). Because of their limited absorption, resistant starches provide only 2.5 calories per gram instead of the normal 4 calories per gram for carbohydrate foods (5). The thing is, this is not always taken into account when determining the caloric content of foods, so total carbohydrates and calories per serving as listed on nutrition labels are often incorrect. Essentially, the labels may list more calories than the food provides, if the food is high in resistant starch (6).

So, what happens to these resistant starches that are not absorbed? Well, they pass through the small intestine and into the large intestine, where they come into contact with the gut microbiota (4). These bacterial species play a massive role in the digestion and absorption of nutrients that were not taken up by the small intestine (7). As the resistant starches come into contact with our microbiota, some of the starches are consumed by the bacteria, meaning that these starches act as prebiotics. Different from the probiotics that we often take as over-the- counter supplements or eat as fermented foods, prebiotics are indigestible foods that increase growth or activity of healthy bacteria in our large intestine (8). In other words, prebiotics are food for our gut bacteria.

Keeping our microbiota healthy and fed is beneficial to our bodies in many ways. For one, by increasing the numbers of good bacteria, the number of harmful bacteria that can sometimes take root in our colon declines due to lack of nutrients (9). Also, good bacteria can regulate metabolic process in and outside of the large intestine, such as maintaining gut motility and preventing kidney stones from forming (10,11). In terms of weight loss, there is some debate as to whether short-chain fatty acids, produced by resistant-starch fed bacteria, cause an increase or decrease in caloric absorption (12,13). Finally, resistant starches on their own as well as the healthy gut bacteria that eat them are able over time to increase insulin sensitivity, which could greatly benefit the millions of people in the US who are obese or have Type 2 diabetes (14).

Now that we know several of the health benefits of resistant starches—reduced caloric intake, prebiotics for our microbiota, increased insulin sensitivity—what kinds of foods should you be eating to include these amazing resistant starches? Resistant starches tend to be found in plant foods that are starchy and fibrous. Think legumes (such as beans, lentils, soybeans), whole grains (such as unrefined rice, oats, maize), and tubers (such as sweet potatoes and yams.) (15) Mix these foods into a predominantly plant based diet that’s comprised primarily of non-starchy vegetables, leafy greens, other colorful produce, lean proteins, healthy fats, and fermented foods for the absolute best nutrition for gut and overall health.


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