With the ever-increasing epidemic of obesity in North America, more than 70 percent of adults and almost 20 percent of children are overweight, which increases risk for heart attacks, diabetes, strokes, arthritis, and some types of cancers. A recent review of the world’s scientific literature suggests that obesity is determined to a large degree by the types of bacteria that live in your colon (Front Pediatr, Mar 19, 2021;9:657020).

The food you eat passes through about 20 feet of small intestines and five feet of colon that is loaded with more than a hundred trillion bacteria. Your gut bacteria influence how you respond to your own insulin and bile, which affect how much of the food that you eat is converted to fat in your body. Changes in the colon bacteria can lead to obesity, diabetes, asthma, and atherosclerosis (J Obesity, 2016:10.1155:7353642). Overweight children have more of the types of gut bacteria that convert carbohydrates into fats. so they absorb more calories from their food than thinner children do (J Clin Endo & Metab, Sept. 20, 2016).

What you eat determines which types of bacteria thrive in your colon (Nutrients, 2015 Jan; 7(1): 17–44). A diet loaded with sugar added foods increases the numbers of colon bacteria associated with insulin resistance and diabetes (Nutrients, Jun 2018;10(6):761). These bacteria affect how high your blood sugar rises after meals and how many calories you absorb from the food you eat. The highest rises in blood sugar come from sugared drinks, and a review of 30 studies of more than 242,000 adults and children found that sugar-sweetened beverages are associated with obesity (Obes Facts, Feb 2018 Feb;10(6):674–693).

Changing Colon Bacteria May Help to Control Weight
A review of several studies showed that eating a diet based on whole plant foods (vegetables, fruits, beans, whole grains, nuts and other seeds), and restricting refined carbohydrates, appears to be a safe and effective way of changing colon bacteria to help control weight and high blood sugar (Gut Microbes, Jan-Feb, 2012;3(1):29-34). These studies show that people who take in lots of sugared drinks and sugar-added foods, foods made with flour (ground-up grains) and other refined carbohydrates are more likely to be overweight, and that those who eat lots of fruits, vegetables, nuts and grains that have not been ground into flour tend to be thin. Currently, the best way to change your colon bacteria to favor the types that do not increase absorption of calories is to eat a diet that includes lots of high-fiber foods that are not quickly absorbed.

Fiber, a Benefit from Unprocessed Vegetables, Fruits and Seeds
Fiber is the type of carbohydrate formed from long chains of sugars that human intestines cannot split into single sugars. All plants we eat contain fiber, much of which cannot be absorbed and passes from your body contributing no calories whatever. The soluble types of fiber are broken down by bacteria in your colon. These bacteria convert the soluble fiber into beneficial short-chain fatty acids (SCFAs) that are easily absorbed through the colon. SCFAs reduce inflammation, which helps to lower high blood pressure, high cholesterol and high blood sugar levels.

How Refined Carbohydrates Can Increase Obesity
When food manufacturers strip fiber away from plant parts, the resulting foods are more easily broken down in the intestines and are quickly absorbed. These foods are called refined carbohydrates. The process of refining grains to make flour removes the outer coating of the seeds so most of their beneficial fiber and other healthful components of the seeds are lost. Even if some of the fiber is added back in (to make “whole wheat” flour), the grinding process separates the starch from the fiber so it is absorbed much more quickly, which causes higher rises in blood sugar and makes you hungrier so you eat more and gain weight.

My Recommendations
Emerging evidence suggests that obesity is driven to a significant degree by the types of bacteria in the colon. To a large extent, diet determines the type of bacteria that live in a person’s colon.
• Foods that encourage the growth of bacteria that help to prevent and treat obesity include vegetables, fruits and seeds (beans, nuts, and whole grains that have not been ground into flour).
• Foods that result in growth of the types of colon bacteria associated with obesity and diabetes include sugared drinks, sugar-added foods, foods made from flour (ground-up grains) and all other refined carbohydrates.
Calories from Foods Vary with Preparation Method and Your Gut Bacteria
What You Eat, Not Your Genes, Determines Your Microbiome
How Soluble Fiber Promotes Good Gut Bacteria
Anti-Inflammatory and Pro-Inflammatory Foods